GROWTH AND REPRODUCTION OF STENELLAATTENUATA IN THE PACIFIC COAST OF JAPANTOSHIO KASUYA, NOBUYUKI MIYAZAKI
Ocean Research Institute, University of Tokyo, TokyoAND

WILLIAM H. DAWBIN
School of Biological Sciences, University of Sydnry, Sydney

ABSTRACT

Data collected from 7 schools of S. attenuata were analyzed. Body length
at birth is 89 cm. Sexual maturity is attained at 8.2 years in females and10.3 years in males. Physical maturity is attained soon after the attainmentof sexual maturity at a length of 194.9 cm (females) and 203.3 cm (males).
The pigmentation and size of the animals slightly differ from the corresponding species in the eastern North Pacific. Mating seasons are in February andMarch, July, and November. Gestation lasts for 11.2 months, and lactationfor 29 months on average. The mean length of reproductive cycle is about
50 months, but changes with the age of the animal. The annual mortality rate
is 0.074 in the mature female and about half of it in immature animals. Some
females live for more than 40 years, and have an average of 8.3 calves. The
mortality is higher in males than in females. Schooling behavior is discussed.
INTRODUCTION

Stenella attenuata (Gray, 1846) is distributed widely in tropical and subtropicalwaters of the ludo-Pacific Ocean. In the eastern tropical Pacific the probably
conspecific separate population (Perrin, 1973) S. grajfmani is often caught by tunaseine, and its biology is being studied (Perrin, 1969, Perrin and Roberts, 1972).
In the southern Pacific some biological study is reported by Dawbin (1966) basedon animals caught at Malaita (Solomon Islands). Though this species is alsocaught at Taiwan by hand harpoons for human consumption (Kasuya, unpublished),
it has not been studied biologically.
In Japanese coastal waters this species is common along the southern coastfrom Kyushu (Mizue and Yoshida, 1962) to Onahama (36°56'N, 139°55'E) (Kasuya,1971), and has been caught sporadically by fishermen with hand harpoons or thedriving method. On the east coast of Izu Peninsula S. attenuata is usually caught
in October and early November slightly before the peak of the season of S. coeruleoalba (Tobayama, 1969), and on the west coast of Izu Peninsula (Nishiwaki, 1965)
slightly after the spring season of S. coeruleoalba. However, on the coast of Kii
Peninsula a few animals have been captured with hand harpoons in all seasons ofSci. Rep. Whales Res. Inst.,

No. 26, 1974, 157-226

158

KASUYA, MIYAZAKI AND DAWBIN

the year, and recently fishermen have started to catch this species by the driving
method in both winter and summer seasons. This suggests that S. attenuata lives
in warmer waters than S. coeruleoalba, and in early autumn they move to the south
along the Pacific coast of Japan. As they are found in waters at more than 16.8°C(Miyazaki et al., 1974), they surely migrate up to 40°N but the information on
northbound migration is scarce (Kasuya, 1971, Miyazaki et al. 1974). The distribution of this species in the Japan Sea is also unknown.
The biology of S. attenuata was partially studied by Nishiwaki et al. (1965).
The present study is intended to clarify differences between the life history of thisspecies from that of a similar species, S. coeruleoalba, and to provide some information on relationships between ecology, environment, and certain species of delphinidae.
MATERIALS AND METHODSAll the materials used in this report were obtained, with a few ancillary materials,
from 7 schools caught by the driving method at Kawana or Futo on the east coast
oflzu Peninsula, or at Taiji on the coast of Kii Peninsula (Fig. 1, Table 1). One
48 cm fetus was collected in February from a female caught by hand harpoon at
Taiji, and 2 of the 4 sets of data on stomach contents of calves were obtained separately at Kawana.

~-~--~--~--~--~~:·-,~-~33"
Fig. 1. Map showing the positions of sampling, A indicates Arari, K Kawana, F
Futo, and T Taiji. Dotted lines indicate 200 m depth.

As shown in Fig. 2, school 1 was composed of adult males, females and newborn calves, but was lacking in calves of from 128 cm to 156 cm in body length.
The pregnant females had small fetuses. The length frequencies of schools 2 to
5 are more or less similar with calves and adult dolphins of various length, but the
fetuses were slightly bigger than those observed in school 1. School 6 was different
from schools 2 to 5, in the larger size of fetuses. In school 7, full term fetuses andnewborn calves were observed. It is difficult to know the exact composition of theSci. Rep. Whales Res. Inst.
No. 26, 1974

STENELLA ATTENUATA

159

larger individuals because of the limited material, but it may not be very different
from that of school 3.
Body length was measured in 1 cm intervals in a straight line from the tipof the upper jaw to the bottom of the notch of the tail flukes. In some cases 4
length measurements shown by 4n~L~4n+3 were grouped, where n is an integer
and L is body length in cm.
The mammary gland was observed in the field and classified into lactating
and nonlactating. In some cases its thickness was measured in a longitudinal cut
made by the fishermen, because they did not welcome transverse section of the
mammary gland. These data were used for reference.
7

..A

15 6
10
5
-~

20 4
Ill 15
c 10
a. 5

:c
0

"O

.....0 5

3. 25 2

~ 20

15
10

.

/\

,,;>.,\

5
15 1
10
5

l-.

0

.. A

. . -~!::. "
20

0

40

60

80

100

120

140

160

180

200

220

Body length in cm

Fig. 2. Length frequencies of S. attenuata in the 7 schools studied. Numbers at
the left indicate the school, open circles females, closed circles males, dotted lines
fetuses, and solid lines postnatal animals. The full term fetuses and newborn
calves in school 7 are not separated.

The ovaries were collected from most of the adult and immature females, andfixed with 10 % formalin solution. The number of corpora albicantia and lutea
was counted by Kasuya in the laboratory by slicing the ovary into about 1 to
2 mm thickness. The three diameters of all the corpora lutea and albicantia andthose of the largest follicle were measured. The volume index of the corpora orfollicle obtained by multiplying the three diameters was used in the analysis ofovarian activity. But in this report the mean diameter obtained as the cube rootof the volume index has usually been used.
Both testes were collected from the males in schools 1 and 2, but only the left
one in other schools. They were preserved in the same manner used for ovaries.
Sci. Rep. Whales Res. Inst.,
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KASUYA, MIYAZAKI AND DAWBIN

The weight was measured in the laboratory. As there is no significant difference
expected in the weights of right and left testes, usually the left side was used forstudy and the right was used only when the left testis was not available.
Several maxillary teeth were collected from the center of the tooth row witha pair of gardening scissors. The tooth section for age determination was preparedafter the method of Kasuya (1972) slightly modified to use a grinder. The growthlayers in dentine were observed by Kasuya under transmitted light with a binocular
microscope (x50).
TABLE 1.
School
no.
l

2
3
4
5
6
7

Dates of

MATERIALS USED IN THIS REPORT.
No. of dolphins*
~

catch

kill

caughtstudied23, X, '70.
10, XI, '70.
4, XI, '72.
13, XI, '72.
16, XI, '72.
12, II, '73.
2, VII, '73.

25, x.
15, XI.
4, XI.
13, XI.
18, XI.
12, II.
2, VII.

264
1381
189
192
67
146
Ca. 60

131
273
48
117
23
122
36

Locality

Researcher

KawanaKasuya and Miyazaki

"

"
Fu to
Taiji

Miyazaki"Dawbin* : Fetuses not included.

Some of the selected testes were studied histologically after staining with haematoxylin and eosin. In these cases samples were collected from the center of thecross section made at the midlength of a testis. In order to check the presence of
spermatozoa in a wider range, smears were taken from most of the testes on thesame cross section, and observed after staining with methylene blue. These testes
observations were made by Miyazaki and Kasuya.
AGE DETERMINATION AND GROWTH

Age determination
The growth layers in dentine were used to determine the age characteristics
as the cementum of S. attenuata is too thin to count the growth layers. The natureof the dentinal growth layers of this species does not differ much from that of S.
coeruleoalba (Kasuya, 1972), but counting is slightly more difficult because of theirregularity of the layers. The poorly calcified secondary dentine is observed inteeth with about 12 or more layers. This may indicate that the growth of the dentine becomes slower in these ages, when physical maturity of the animal is attained.
To find the annual accumulation rate of dentinal growth layer, the condition of
the last layer was checked on teeth which had 10 or less opaque bands and in which
the thickness of the layer was not too thin to estimate the relative thickness. Though
the relative thickness of the layer was estimated in comparison with the former layer
taking into consideration the gradual decreasing size of the layer, this observationis less reliable than the identification of opaque or translucent layers. As shownin Fig. 3, most of the animals killed in July, October and November were forming
Sci. Rep. Whales Res. Inst.,
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STEJVELLA ATTENUATA

161

opaque layers, but almost all the animals killed in February were accumulatingtranslucent layers. In the present study it has been impossible to decide the datewhen the formation of the two kinds of layers alternate, but it seems probable thatthe alternations occur in December or January and in April or May, and thateach opaque and translucent layer represent about 6 months respectively.
In this study the number of opaque layers in dentine, excluding the fetal layer,
was counted together with observation of relative thickness and condition of thefirst and last layers. The layers were usually counted along the line set from the80......

~
60
u
c

a.

:J

g.... 40IJ...

X XI XIIII Ill IV

V VI VII VIII IX

X

Month
Fig. 3. Seasonal change of the formation of opaque and translucent dentine layers.
Black with white spots indicates thin opaque layer, the black thick opaque layer,
the white with small spots thin translucent layer, and the white with large spots
thick translucent layer.

TABLE 2.

CRITERION FOR CALCULATING THE AGE FROM THE
NUMBER OF OPAQUE LAYERS (n).

First and last layers

Possible range of age

Approximate age (x)tt-tt

n+0.50~x>nx~n+0.25

tt- t

n+0.25~x>n-0.25

x~nt- t }

n~x>n-0.50

x~n-0.25

n-0.25~x>n-O. 75

x~n-0.50

n-0.50~x>n-l

x~n-0.75

oo-tt
oo- t }
o-tt
00-00}'

o- t
oo- 0
o- 0

n-0.75~x>n-l.25

x~n-1

n-l~x>n-1.50

x~n-1.25

o : thin opaque, oo : thick opaque, t : thin translucent, tt : thick translucent

Sci. Rep. Whales Res. Inst.,
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162

KASUYA, MIYAZAKI AND DAWBIN

middle of the neonatal line to the central portion of the pulp cavity, because thestructure near the distal tip is sometimes discontinuous especially in the young layers.
When the number of opaque bands (n) is counted the possible range of real age(x) is generally shown by the following formula.
n-1.5<x<n+0.5
In case of animals at 3.5 years or older, the approximate age was represented by
the value at the center of both extremities, orx:;=n-0.5
However, in the case of the younger animals, it was calculated from the criterion
shown in Table 2.
As indicated in the latter section the counting of the dentinal growth layers
is not reliable after the accumulation of secondary dentine, but the age of animals
obtained from dentinal growth layers by the above method has been used in thisreport without special comment.

Length frequencies
The length frequencies of fetuses and juveniles varies between schools. This
reflects the special schooling behavior related to the maturity and reproductive
cycle. These problems are dealt with separately. There is no significant difference observed between schools in the shape of the peaks for animals more than
184 cm in females or 196 cm in males. When all schools are combined, the highest
frequency is found in the length group 192 cm to 195 cm in females, and 204 cm
to 207 cm in males. The difference of the modal lengths is about 12 cm. Most
females are shorter than 212 cm and males less than 220 cm, but exceptionallylarge records of a female and a male were 220 cm and 234 cm respectively.
The above modal lengths seem to be slightly bigger than that of S. grajfmani
reported by Perrin (1969).
Petal growth and length of gestation
The mean body length at birth is estimated roughly from the 5 full term fetuses
or newborn calves in school 7. In this school 85 cm female, 85 cm male, and 88 cm
male were considered to be definitely prenatal because they were found with large
umbilical cord attached, or taken out of the uterus by Daw bin. The 89.0 cm male
was considered to be a newborn calf because it had no trace of umbilical cord,
but the umbilicus was the shape of an oval hollow with thin epidermis and thesize was larger than that of a fully healed umbilicus. Its stomach contained milkof about 13 g in weight. One 88 cm female animal was found in the pile of viscerawith the thin fragment of the external layer of the umbilical cord. However, thesize of the fragment was much smaller than that of the 3 specimens first mentioned.
The dorsal fin was not bent completely to one side as in the case of the fetus, butstood obliquely. On the lateral side of the body of the animal there were observed
Sci. Rep. Whales Res. Inst.,
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STENELLA ATTENUATA

163

several transverse shallow grooves which are often found on the fetus. It had nomilk in the stomach. According to the observation of Nakajima et al. (1963) ona Tursiops truncatus born in an aquarium, it takes several hours for the dorsal fin
to stand vertically. Our observation on a newborn S. coeruleoalba, 100 cm in bodylength, shows that it had plenty of milk in the stomach in spite of features quite
similar to the above 88 cm S. attenuata in the umbilicus, dorsal fin, and grooves on
the flank. This suggests that the 88 cm animal was postnatal. However, we havesome query on the last specimen of S. attenuata as to whether it was delivered by theartificial action given to the mother in the period from the start of driving to thetime of kill. However, the present best estimate of the mean body length of S.
attenuata at birth comes between 88 cm and 89 cm. In this paper 89 cm has beenused as the tentative mean length at birth. Even if it is 88 cm in reality, the lengthof gestation is changed by only 3 days, which is negligible in relation to our tentative
estimate of the mean length of gestation obtained below.

140
E 120

u

c
~ 100

Oi
c
(!)

--;:_ 80
-0
0

CQ

60
40
20

Month
Fig. 4. Growth of fetus and juvenile calves. Length frequencies are grouped intomonths. The dotted area indicates fetuses, the white postnatal, and closed circles
the mean body lengths of fetuses in each school. A school caught in May (Nishiwaki
et al, 1965) is shown by solid line.

The length of gestation and fetal growth were calculated from the seasonalchange of fetal length. In Fig. 4, the mean fetal length in each school was plotted
Sci. Rep. Whales Res. Inst.,
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164

KASUYA, MIYAZAKI AND DAWBIN

against the date of kill. In school 2 the fetal length is bimodal, but it was notseparated because the deviation from the main mode is not large. In other schools
the fetal length is unimodal. In school 7 all the full term fetuses and newborn
calves were combined in calculating the mean length. In addition to the 7 schools,
one record of a fetus caught at Taiji in Feburary is also plotted in Fig. 4. All theabove 8 points fall on a straight line, but the fetuses reported by Nishiwaki et al.
(1965) come on quite a different position. The latter are considered to represent
other mating seasons, and are dealt with separately. Laws (1959) showed inodontoceti that, except during the very early stage of growth, the relation between
fetal length and the elapsed time after conception is shown by a single straightline. If a straight line is calculated by least squares for the above 8 points, thegrowth of the fetus is shown by the following formula,y = 0.30094x- 76. 74
where y is the length of the fetus in cm and x is the number of days counted fromthe last lst of January. This formula shows that the daily increment of length at
the straight part of the growth is 3.0 mm. The extended growth line cut theaxis of time at day 255.01 or 12th of September, and attains the mean length atbirth, 89 cm, on day 550.73 or 5th of July. Accordingly the length from the datewhen the extended straight line crosses the axis of time to the date of birth is295.72 days.
If 0.135 X gestation period is tentatively used here for Lto of Laws (1959),
which is the time from the start of gestation to the date when the extended straightline of the fetal growth cuts the axis of time, the length of gestation (L) is shownby the following formula
L-Lto = 295.72 days
Lto=0.135L
Then Lis 341.87 days or 11.24 months and Lto is 46.15 days or 1.52 months.
The date of conception of the growth line in Fig. 4 comes on 28th of July. The
fetal growth rate of this species at the straight part of growth is close to that of
Tursiops and Phocoena reported by Fraser and Huggett (1973).

Postnatal growth
The relation between body length and age of 195 males and 239 females isshown in the scatter diagram of Figs. 5 and 6.
The growth during the first three years is rapid. The mean growth curveof this part was drawn by eye based on the mean lengths in each 1/4 year intervaland considering the distribution of each point in Figs. 5 and 6. As no sex differencein these age classes is observed, both sexes were combined.
The growth of males between the age of 3 years and 12 years, and of females
between the age of 3 years and 7 years seems to be almost straight. The growthline of this age range was calculated by least squares based on the mean body
Sci. Rep. Whales Res. Inst.,
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STENELLA ATTENUATA

165

lengths, weighted by the sample size. As the sample is small and no significant
sex difference is observed in the age groups between 3 and 6 years, the data for

200

.

180

•

§

.

.£ 160·/:t:l-~~-1~: _i_:_~::J..::--f-• 'fr ·r.
~f. ·r "
I

~".:

·~
;

~

~

·!'
:

+

•--',------:

.

.

/" .

l.

140

~ 120tm

100.
800~~~~~-5~~~~~_.__,__._~____...---"~"---'--L--...L--i--'--1--'-----'--'

1015
Age in years

20

Fig. 5. Mean growth curve of female S. attenuata. Age was estimated from thedentine growth layers. Each spot indicates one animal and rod the mean bodylength.

!

*________________._
t_ _ :

200

...

_,,_

i.

E 150
u

.£

_;,

..... 160

.J::
01

c

~

140

>-

"O

-;-

.}
..

m 120 f
0

100!
80

0

10

20

Mean growth curve of male S. attenuata.

For symbols see Fig. 5.

5Fig. 6.

15
Age in years

females were included. Then, the the following relation is obtained between theyear after birth (x) and the body length (y) in cm.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

166

KASUYA, MIYAZAKI AND DAWBIN

y = 4.20x+ 153.8,

3.5~x~ 11.5

This indicates that the mean annual increment of body length is 4.20 cm
The increase of the mean body length finishes in males at the age of 11.5 years.
This is the age when most of the animals attain physical maturity. The asymptotic
length of males at 203.3 cm is obtained as the mean of 49 males with ages of 12.5
years or more.
The mean growth of females becomes slower after 6.5 years of age and thedifferences of length between the sexes increases. The growth of females seems tostop at the 1 lth year. The mean asymptotic length of females obtained from 137
females which are at the age of 11.5 years or more, is 194.9 cm. This is 8.4 cm
smaller than that of males. This difference results from the earlier slowing downof growth and the earlier attainment of physical maturity in females. Females
seem to attain physical maturity soon after the attainment of sexual maturity asin the case of males. This is confirmed by the fact that the mean body length atthe ovulation of 1, 2, 3, and 4 is 189.5, 191.4, 192.5, and 192.4 cm respectively,
and attains at 5 ovulations 195.1 cm which is approximate to the asymptotic length
194.9 cm. Then the mean lengths do not show the increase but fluctuate in therange between 192 cm and 198 cm. The mean body length of the sample in each
TABLE 3.

GROWTH OF STENELLA ATTENUATA.
Body Length (cm)Age(year)

mean of sample (sample size)

6
0
0.25
0.50
0.75
1.00
1.25
1.50
l. 75
2.00
2.25
2.50
2.75
3.00
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5;;;;

Q

89.0 ( l)113.4 (17)124. 6 (18)
133 .1 ( 8)140.5 ( 2)
150.8 ( 4)149.0 ( 2)
160.2 (10)158.3 ( 4)165.3 ( 4)166.0 ( l)175.7 ( 3)168.8 (10)172.6 ( 8)177 .3 (13)182.6 (11)179.4 ( 7)185. 7 (16)186.2 ( 9)186.8 (18)187 .6 ( 20)193. 3 (16)191.5 ( 22)198.1 (19)194.0 ( 18)204.4 ( 9)194.9 ( 22)203.3 (49)194.9 (115)

on growth curve

6

Q

89.0
111.0
124.6
134.5
142.7
148.3
152.3
155.8
158.9
161.2
163.1
164.7
166.0
168.5
172. 7
176.9
181.1
181.1
185.0
185.3
189.5
188.3
193.7
191.5
197.9
194.0
202.1
194.9
203.3
194.9

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167

STENELLA ATTENUATA

age group and the corresponding body length on the growth curve obtained in theabove are shown in Table 3.
Kasuya (1972) tried to adopt exponential formulae for the mean growth
curve of S. coeruleoalba, but the fit was never good. In the present study we could
not find a suitable formula to express the mean growth of the present species. For
comparison, the mean growth curve of S. coeruleoalba is shown in Fig. 7. This was
based on the 881 animals obtained by the end of 1972 including the data used by
Kasuya (1972), in which age determination was made by the same author. The
y=234.4

220 __2!~~---------------~C>~~----- -~t.P~~'l.i..

200 _JnQ__ _______ 0___...E-----0--·"--"--------'~10y"223.5:'y-203.3.

%- ------~------~--~-o-o-0-~-o--~-':

180 _J~'i,Q ___ -r--:~~~\<;,~~--:---,. ·_1· ..

y•194.9

r:: f/~.
Oi

c

~

120

"8>- 100
i::o

40

'

'

20

:
''

'
'

'
N: CO:~~
oo: oj:~:' ''O"'---'----'---'--'---'----'---'--'--...L...'~!,__-'-'-'---'---'--'---'----'---'--'---'--'
-1 0
2
4
6
8
10
12
14
16
18

Time in years
Fig. 7. Comparison of the mean growth curves of Stenella coeruleoalba (upper pair)and of S. attenuata (lower pair). The largest circles indicate the mean length represented by 25 or more animals, the intermediate from 11 to 24 animals, andthe smallest 10 or less animals. In both species closed circles and the upper line
indicate males and open circles and the lower line females. The age of female S.
coeruleoalba at sexual maturity was cited from Kasuya (1972). The slow growthstage at early pregnancy is not shown.

straight part of the male growth curve was calculated by least squares, but that of
the female was drawn by eye. The feature of this part is its strong resemblance
Sci. Rep. Whales Res. Inst.,
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168

KASUYA, MIYAZAKI AND DAWBIN

to that of S. attenuata, except for the slight between sexes difference of length. If
enough data is accumulated, a similar pattern of growth seems likely to be observedin S. attenuata. The age at physical maturity of S. attenuata seems to be about 1
year earlier than that of S. coeruleoalba. The difference of asymptotic lengths between two species is mainly affected by the difference of growth in young stage.

Weaning
Direct information on the start of the weaning is scanty (Table 4). Ourdata suggest that some animals start feeding on solid food at the length of 115 cm
and the age of 1/4 year, but most of the animals start at about 130 cm, or about
half a year after birth.

~10

2 3 4, 3

~

~ .8

3 4 2

0

I- .2
00-

2

3

4

5

6

7

8

9

Age in years
Fig. 8. Relation between age and the thickness of postnatal dentine of S. attenuata.
Open circles indicate the mean of females, the closed males, and vertical andhorizontal rods the range and mean for both sexes. Numbers at the top indicate
sample size. The line was drawn by eye.
TABLE 4.
Body length
115cm
122cm
128cm
129cm
168cm

STOMACH CONTENTS OF JUVENILE STENELLA ATTENUATA.
Age
1/4
1/2

Sex

6
6
6
6
~

Milk

+

Food remains

Nematoda

+

+

+
+

+

The thickness of postnatal dentine measured in cross section at the level ofthe proximal end of prenatal dentine was compared with the growth of the animal.
As shown in Fig. 8, the increase of the thickness of dentine after 1 year of age showsa similar pattern to that of body length, but it shows a rapid increase at the age
between 0.5 and 1.0 year. When it is compared with the body length (Fig. 9), theSci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

169

growth of dentine changes from a low rate to a higher rate at a body length ofabout 133 cm, which corresponds to the age of 0.7 year.
It is generally known that the baleen plates of mysticeti grow rapidly at theperiod of weaning. A similar feature is also expected in the growth of teeth,
because teeth would start to have a mechanical function in catching food after
the start of weaning. Probably the weaning of S. attenuata starts, in the average,
at an age between 0.5 and 0.75 year or at the length of 130 cm. However, theweaning period seems to last long in the case of toothed whales, and the start offeeding on solid food does not necessarily mean the cessation of suckling. The
duration of suckling is dealt with separately.
3 12

5

1 2

E t.4
E
.i;;; 1.2

12

g 5

(])

c

:.;;; 1.0

&;

"'C
-

0

3

.8

m.6

6

1
•

2

4

t

(])

c

i5 .4

:.c

I-

.2
0

2

100

1

120

140

160

180

Body length in cm
Fig. 9. Relation between the thickness of postnatal dentine and body length of
S. attenuata. The line was drawn by eye. For other symbols see Fig. 8.

The calf of Tursiops truncatus starts to take solid food at an age of from 6.3
to 9 months, or 0.5 to 0.8 year (Tavolga and Essapian, 1957, Nakajima et al, 1963).
This coincides well with the information obtained on S. attenuata.

Age at the attainment of sexual maturity of the female
As the left ovary of this species matures earlier than the right (Ohsumi, 1964),
it is reasonable to expect earlier development of the left ovary even in immature
animals.
Fig. 10 shows the difference of weight between the two ovaries of immature
females of S. attenuata. In young animals the weight of the left ovary is nearly
equal or slightly smaller than that of the right, but the growth of the left ovary
starts to exceed that of the right at a body length of about 170 cm. The age corresponding to this length is about 4 years. This may indicate the approach ofpuberty even if development of the follicle has not started. This age is about one
year later than the age of the male when the formation of spermatozoa in theSci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

170

testis starts.
Fig. 11 shows the relation between age and the diameter of the largest follicleof mature and immature females of S. attenuata. The observed largest diameter
was between 10 and 11 mm. This size is considered to be close to the size at
ovulation. The development of the follicle in immature females is observed onlyin animals at the ages of 6.5 years or more. This means that puberty of females
starts at this age which is very close to the age at the attainment of sexual maturity,and that the growth of follicles is rapid to attain the size at ovulation.
The youngest mature female is found in the age class of 7.5 years and theoldest immature in that of 9.5 years. The percentage of newly mature females at

1.2

en .s
,£;
......... .6
0:
I

~ .3

-.3 ,_1_,_20_ __,____14.LO,---L---:-'.16~0:---'---;::18:.:;-0-~~

Body length in cm
Fig. 10. Relation between the difference of the weights of both ovaries and bodylength in immature S. attenuata.

TABLE 5.

MATURITY OF S. ATTENUATA BY AGE GROUPS.
Male*

FemaleAge
immat.
<1<2;;::;3
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5

12
2
2

pub.

3
1
2
3
9
12
13
9
7
2
3

mat.

2
3
11

6
8
8
10

mat. (%)

immat.

mat.

mat. (%)

0
0
0
0
0
0
10.0
0
13.3
25.0
61.2
75.0
72.8
88.9
100

15
4
10
8
3
7
7
6
7
2
0
0
0
0
0

0
0
0
0
0
0
0
2
12
18
18
21
27
15

0
0
0
0
0
0
0
25.0
63.2
90.0
100
100
100
100
100

11

*: Based on testis weight when other information was not available

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

171

the age of between 6.5 and 7.5 years, 7.5 and 8.5 years, 8.5 and 9.5 years, and
9.5 and 10.5 years are calculated from Table 5 as 25.0 %, 38.2 %, 26.8 %and 10.0 %respectively. This indicates that most of the animals attain maturity at an age
between 7.5 and 8.5 years. But as there are observed some females which are con10

E
Ee
,!;;;
.... 6

OJ

Q;

E4
Cj

i5 2
0 W-6.ll.5--6.l-5

105

125

145

165

165

20.5

22.5

Age in years
Fig. 11. Relation between the diameter of the largest follicle and age.
squares indicate immature females and black the mature.

Male

120

->.ae

White

100

u

80

c
~ 60
0-

....(!J

LL 40

20

I

"'g
8

10

_J
12

14

7

9

11

Age in years
Fig. 12. Age at the attainment of sexual maturity in S. attenuata. Open circles
indicate the cumulative frequency of immature animals, and closed circles thatof mature animals. For explanation see text.

sidered to be in the second cycle of reproduction in the age classes of 7.5 and 8.5
years, a few females seem to attain sexual maturity at an age between 5.5 and6.5 years (Table 19). The mean age offemales at the attainment of sexual maturity
was obtained after the method of Kasuya (1972) as the age where the cumulative
frequency of immature animals in older age classes is equal to that of mature
animals in younger age classes (Fig. 12). It gives the mean age of females at theSci. Rep. Whales Res. Inst.,
No. 26, 1974

KASVYA, MIYAZAKI AND DAWBIN

172

attainment of sexual maturity as 8.2 years.
on the mean growth curve.

The body length at this age is 187 cm

Age at the attainment of sexual maturity of males
The bilateral difference of testis weight was checked on 44 animals collectedfrom schools 1 and 2.
If the mean of the left weight minus right weight is equal to zero it is concluded that there is no bilateral asymmetry. As shown in Table 6, the mean difference in mature animals is small enough to consider that there is no significant
asymmetry of the testis weight. In testes with 68 g or less weight, the observed
deviation of the mean from zero is large enough to be expected at the probability
between 2 to5 %. But at present we consider that this does not prove asymmetryof testis weight, so the weight of the left testis was usually used in the following
discussion.

20.......·:·:
104

108

1.12
us
Ratio

1.20

....
:·:·
....
1.24

Fig. 13. Frequency distribution of the ratio of the weight of the heavier testis tothe lighter. Dotted area indicates the pairs where the right testis is heavier or
equal to the left.
TABLE 6.

T-TEST OF THE SYMMETRY OF THE TESTES WEIGHT.

Weight(left, g)

Range(L.-R.)

Mean
difference

SamplesizeStandard
error

Probability

68>-10.9- 1.4-30.0-30.0-30.0-30.0-1.2720+1.9053+0.100025
19
44

0.58083.6574
1.60750.05>p>0.02
0. 70>p>0.50
p>0.9068~

Total

The ratio of the heavier testis to the lighter comes between 1.00 and 1.28
(Fig. 13). However, about 93% of all the pairs observed show a ratio of less than
1.20. This means that if the left testis is weighed, the weight of the right side is
expected with a probability of more than 90 % to be between 1.20 and 1/1.20(0.83) times that of the left.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

173

The maturity of the testis was checked by two methods. One is histologicalexamination of testis tissue taken from the center of the cross section at the midlengthof the testis. When spermatozoa were observed in this part, the testis was considered to be in full maturity. The smallest testis in this condition was 66 g, andthe largest immature testis was 71 g. Accordingly the mean weight of testis at
the attainment of full sexual maturity is considered to be about 68 g, and any
testis of this weight or heavier was classified as mature.
600
400
300

•

,f •

,f

8

o I oo J
oJ-ooooJ

200

8

• ~
•

I

..

.

g

• g·

I

:J> o 8

J,.... 100
.9 80
Ill

·~

~

60

-

40
30

0

20

2

\
\

8
6

\

\o

.
..

.... . \, .

~

\

I

0

L\1~6
~

•• ...
1:" \

~ ...

. . • ~. . t8 ~ ~:EOl·a; 10
3

\

~

0

\

••

:

\

o\

4
3 \ •\

"··:

2 ...
1

0

2

4

6

8

10 12

14

16

18 20 22

Age in years
Fig. 14. Scatter diagram of the weight of testis and age. The weight of testis
is plotted on logarithmic scale. Large closed circles indicate spermatozoa absent
in the smear, open circles spermatozoa present in the smear, small closed circles
no observation of the smear, rod at the upper left immaturity of the tissue at thecenter of testis, rod at the upper right the maturity, and solid line mean weightof testis at the attainment of sexual maturity.

Even if a testis is identified as immature by the above method, when the smear
taken from the cross section at the midlength of the testis is observed some spermatozoa are usually found. This probably means that even when the core of thetestis is not mature, some part, most likely the tissue at the periphery has alreadystarted the formation of spermatozoa. This phenomenon is the reverse of theobservation on sperm whales reported by Best (1969), and will need further confirmation. An animal which has the testis at this stage is considered to be in
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

174

KASUYA, MIYAZAKI AND DAWBIN

puberty. This stage of testis was commonly found among those with weight between 6.0 g and 68 g, so any animal with a testis of between 6.0 g and 67.9 g wasclassified as puberal.
Among animals which have a testis weight corresponding to the criterion ofpuberty, there are some which have no spermatozoa in the smear. The ratio ofthese to all puberal animals varies between schools. This can be related to thereproductive season or the school structure and will be dealt with separately.

600
JJ

400
300

0

0

200

1:
-~ 10
3 8
6

4

3

120

140

160

180

200

Body length in cm
Fig. 15. Scatter diagram of the relation between weight of testis and body length.
For symbols see Fig. 14.

As shown in Table 5 and Figs. 14 and 15, all males younger than 2.0 years
or 160 cm in body length are immature. Puberal animals first appear in the ageclass 2.0 years and are observed until the age of 13.5 years. Most animals in age
classes between 2 and 8 years or length between 160 cm and 187 cm belong to the
puberal stage.
Fully mature animals start to appear at the age of 6.5 years and all animals
attain sexual maturity by the age of 14 years. After this the weight of testis does
not increase with age.
The mean age at the attainment of sexual maturity obtained by the same
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

175

method used for females is 10.3 years. This is about one year higher than that ofS. coeruleoalba, in spite of the larger weight of testis and smaller diameter of seminiferous tubules at sexual maturity of S. attenuata. The mean body length corresponding to this age, 197 cm, is obtained from the mean growth curve.
Fig. 16 shows the relation between mean diameter of seminiferous tubules
and weight of testis. The histological samples were taken from the center of asection at midlength of the testis. The mean of the measurements of 15 tubules
measured on a photograph was used as the mean diameter of tubules of the animal.
The relationship between weight of testis and the mean diameter of tubules isshown by two lines as calculated below.

D=24.94 W 0 · 22528, W <68
D=ll.11 wo.44692, W~68
In the formulae, D is the mean diameter of tubules in µ, and W the weight oftestis m g. The mean diameter of tubules at the attainment of sexual maturity
is between 60 and 70 µ, which is smaller than that of S. coeruleoalba (Hirose andNishiwak, 1971).
250
200

..

150
Cl>

:E 1003

0:__ .... ....:.··.....,, · .... --

70

,'

L..

,

2Cl>

.. tf>:".

E 50
0
i5 40

.

.30

..

.

...............
500

20L---L.:..-1..--1..:...1..L.Ll...Ll..-~l..--'-'-'-..L.U....LJ..L.~-'--"--'-

1.5 2 3.

5

10

20

50

100 200

Weight of testis (g)Fig. 16. Relation between the mean diameter of seminiferous tubules and the weightof testis, plotted on logarithmic scales. Lines are drawn by the least squares.

The weight is proportional to D 4 · 4 in immature animals but to D 2 · 2 in mature
animals. This suggests that the fluctuation of the weight of testis of mature animals
is affected by the change in diameter of seminiferous tubules.

The body length where half of the animals attain sexual maturiryThis length is defined as that where the number of immature animals above
this length is equal to that of mature animals below this length, and obtained by
the same method used in the former section in calculating the mean age at theSci. Rep. Whales Res. Inst.,
No. 26, 1974

176

KASUYA, MIYAZAKI AND DAWBIN

attainment of sexual maturity. It gives a length 181.9 cm for females and 194.3 cm
for males (Fig. 17). These values are slightly shorter than the body lengths corresponding to the mean age at the attainment of sexual maturity. The differenceis 5.1 cm in females and 2.7 cm in males. These discrepancies are reasonable,
because the growth of the animal stops soon after the attainment of sexual maturity
and the attainment of sexual maturity may be more dependent on age than on
body length. However, the absolute values of the differences obtained here are
not highly reliable. The body length where half of the animals are sexually mature
is strongly affected by the age composition of the sample or of the population.
If the ratio of sexually mature old animals decreases by the effect of exploitation,
the value may become higher. This value is useful only when the maturity must
be classified based on the body length frequency.
Female

160
<!J

120

Ol

0 80
......
cCl.I 40
u
,_
<!J
a. 0

172

180

188

196

204

212

<!J

>
:;;

Male

.Q 160
:J

E 120
:J

u

80
40
0

180

188

196

204

212

220

Body length in cm
Fig. 17. Body length of S. attenuata where half of the animals attain sexual maturity.
For explanation see text.

Body weight
Body weight of S. atteuata is known for 20 females and 14 males. These data
are plotted on a logarithmic scale in Fig. 18. As the data are scarce a difference
between the sexes was not demonstrated. The relation between body length (L,cm) and the weight (W, kg) seems to be expressed by two straight lines. If theformula W =aLb is calculated by least squares, the following constants are obtained.
For the samples between 86 cm and 130 cm
a 1 =0.0001901
bi=2.3638
For the samples between 110 cm and 207 cm
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

177

a 2 =0.00001876
b2=2.8504
The two regression lines cross at the point L= 116.6 cm. This means that the
body weight of the newborn calf is heavier than the weight obtained by extrapolating
the length-weight relationship of the older animals, and that after birth the calf
grows more in length than in circumference and therefore becomes more slender.
After the attainment of a length from 110 cm to 130 cm or an age of about 6 months,
the circumference increases more closely in parallel with that of the body lengththan at the earlier stage. However, as b 2 is still less than 3, S. attenuata continues
to become slender with increase of body length.

200
170
150

-----'-·~--'---'~~-~~~~-=':c-::'='"

6

7

8

10

15

20

25

30

40

50

60 70 80

Body weight in kg
Fig. 18. Relation between body length and body weight (top), and weight without
viscera (bottom). In the former each mark indicates one animal, and in thelatter each mark the mean for each length group. Open circles indicate female
and closed male. Weight and body length are plotted on logarithmic scale.

The weight of the animal without the viscera was measured by the fishermen.
The mean weight in each length group calculated from 179 females and 148 males
are shown in Fig. 18. The relation between body length and weight (w) without
viscera in kg was obtained combining both sexes as follows:
For the data between 84 cm and 131 cm in body length
w=0.0001237L2· 4217
For the data between 112 cm and 220 cm in body length
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

178

w=0.000009887L2· 9595
These two lines cross at a body length of 109.8 cm
If the ratios of the visceral weight to the total weight is calculated from the data
used in the top of Fig. 18, they are 17.5% (4 animals from 85 to 89 cm), 11.5%(10, 114-130 cm), 11.6% (9, 149-189 cm), and 9.9% (11, 190-207 cm). They seemto decrease with the growth of the animal, and the decrease is especially rapid at abody length between 89 cm and 114 cm. This feature is also presumed from thetwo sets of formulae in the above.
PIGMENTATION
The pigmentation of S. attenuata from the Pacific coast of Japan was studied on
100 animals in schools 1 and 2. The pigmentation was classified into 4 grades as
explained below, and compared with the body length or with the age determination based on the dentinal growth layers. However, the latter was used only
for reference, because age information is scanty. The body lengths were grouped
into nearest 5 cm units.
Males

.

Females. .

A. White tip of up. jaw

2••11111•1·11.tl•1·-.-j-!--'-•-•-r•iil· 11-1~-•Jl'lt•
I
I 11110 ·-:-1 •

I I

.

c....

.

B. White tip of lo. jaw

Cl.I

·•·"r
... .
. ........

•• 111•_!.!J~2
a.
·.-p.-:-f-1-tll 1 •.
I l-fttl
If-•~·• I 11 •c 0 ·-1-I• .111
0.Q

C. Pale spots in the dorsal

<ii

~2
E
.Qlo •-1-1
a...

•

•tlll

• • 11 .~.-.

•I~-~·· . 1l-•-·---f/I I I

•

2

I I

•I

~-·"

I

I spots in the ventralII• • & .+I t1-t+••y-1• t ~I ••of •fof-f-l I..D. Dark•·•••·

0 ·-·___..-/
150

of'.f

170

•.)1 I t-1-o"'. . v.
I I ..........

.

190 210150 170
Body length in cm

190

210

Fig. 19. Relation between body length and pigmentation of S. attenuata. Smaller
circles represent one animal and the larger 5 animals. Body lengths are grouped
into 5 cm intervals. Lines indicate the mathematical mean of the grades. For
further explanation see text.

Pigmentation of newborn calf
The pigmentation of a newborn animal is pale and the pattern is quite different
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

179

from that of an adult animal. The dorsal surface is bluish gray with a pale band
extending postero-dorsally at the base of tail peduncle. Both jaws and the dorsal
surface of the flipper is bluish gray. A dark band connects the eye and the baseof the upper jaw, and a paler band connects the insertion of flipper and lower jaw.
A very faint wide dark band extends from eye to the anal region. This band iscovered, in the adult, by dark spots and becomes inconspicuous. Other parts oflateral and ventral regions are white. The border of the dorsal dark area and ventral
white area is shaped like an arch at the part from the chest to the forehead. Thispattern of pigmentation strongly resembles that of Delphinus.

White area at the tip of upper jaw
The white tip was classified into 4 patterns. Satage 0 represents no purewhite area. In many animals classified into this stage, there is observed a small
area of paler pigmentation at the tip of the jaw. Stage 1 is represented by the
animal which has the white area but it does not extend posteriorly beyond theanterodorsal corner of the upper jaw, stage 2 by the animal of which the white area
extends between the antero-dorsal corner and the midlength of the rostrum, andstage 3 by the animal of which the white area on the upper jaw extends beyond
the midlength of the rostrum.
FemalesMales

A. White tip of up. jaw

.

2

•••••

I

~1-1-.--.-:i

0

• I

•

I •

I

.

•

···~

-~-·
I I I
• · •··•... .
·~. ·-· .'.~.~·-·-·-·-·~·. . . ..
•.•~·---·-•..,:~:·~.....
.
B. White tip of lo. jaw

..

I

•
• ••

I I I

•

•

C. Pale spots in the dorsal

.

' I

•-•

................

2

0 •?

I • ' •

D. Dark spots in the ventral. I ·i·..
.••• I : -.
/·--·""" ~·-.-.

... . ····- .

•.

..

....

0 2 L. 6 8 10 12 14- 2 L. 6 8 10 12 14Age in years
Fig. 20. Relation between the pigmentation of S. attenuata and the age of the animal.
For symbols see Fig. 20.

The animal of stage 0, or the animal which. has no white tip at the upper
jaw is found only among animals smaller than 185 cm (male) or 195 cm (female)in body length. The white tip starts to develop at a body length of 155 cm andSci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

180

the ratio of animals with a white tip increases with age. The body length where
half of the animals have a white tip is about 170 cm in both sexes. All animals havethe white tip at a body length of 190 or 200 cm. The length 170 cm mentioned
above corresponds to an age of about 3.5 years. According to age determination ofa few animals, the white tip of the upper jaw is formed at an age between 2 and 12
years. As stage 2 is found in animals of slightly larger length, it is supposed thatthe extent of the white area increases in some degree with the growth of the animal.

Females

Males
A.

c-

• •

2

I

•

• •

0 I

• I

I

• • I I• I • I
•

c.

•

I

•

D. •

• I

I

•

2

• I

•

I

I

I

""""
• .....

I

I

•

I

10 20 30 50 80200
Testes weightc-

I .. xII•-· - - -

• c-1

0 I
1

I

I

I

• I
•
• • I I I
I • •

c-

I

I

c-

•

c-

I

•

I I

.. • I . II

B.

I-·w ..

• I -6••-- - -- •

•
'
'
- -•
....•

a
1

-

1

I

I

I

0 1-2 3-4 5-910Ovulation number

Fig. 21. Relation between the pigmentation of S. attenuata and gonadal condition.
Open circles indicate 20 animals. For other symbols see Fig. 20.

White area at the tip of the lower Jaw
The 4 stages of the development of the white area on the tip of the lower
jaw were defined by the same method used above. The white areas on the tipof upper jaw and of lower jaw show good correlation, and both jaws showed thesame stage in almost all the animals observed (Fig. 19). Accordingly, the mean
body length where half of the animals have the white tip on the lower jaw is thesame as the corresponding figure for that of the upper jaw.
White spots on the dorsal surface
The stage of this character was classified into 4 categories of stage 0 of no
Sci. Rep. Whales Res. Inst.,

No. 26, 1974

STENELLA ATTENUATA

181

spot, of stage 3 of heaviest spot, and intermediate stages 1 and 2. The spot starts
to appear in the earliest individuals at a length of 165 cm (male) or 170 cm (female),
and all animals of more than 195 cm in body length have white spots on the dorsal
surface. The body length where half of the animals have dorsal spots is about
180 cm in males and 170 cm in females. According to age determination all animals
show the spots by the age of 9 years. The density of the spots increases withgrowth of the animal.

Male

Female

A

A

•

•

•

50 ----------•

•

c

-

•

•

••

c

Cl>

--- ·- -- -- ---~50 - - - --- -- --- -- -

Cl>

a..

•

.

50 --

:

•:

•

150

170

190

150

170

190

Body length in cm
Fig. 22. Body lengths where half of the animals start to develop the pigmentationof the adult. Lines are drawn by eye.

Dark spots on the ventral surface.
The density of dark spots on the ventral surface was classified by the same
method used for pale spots on the dorsal surface. The dark spots first appear on
the lower jaw and on the band connecting the lower jaw and base of flipper, thenon other parts of the ventral surface.
The age when dark spots start to develop shows a large sexual difference.
In males it starts to develop between 150 cm and 160 cm in body length or between
1 1/ 4 and 2 years of age. However in females all the animals show the spots at
175 cm or 5 years of age. The body length where half of the animals have dark
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

182

KASUYA, MIYAZAKI AND DAWBIN

spots on the ventral surface is about 153 cm in males and about 170 cm in females.
The former corresponds to an age of 1.5 years and the latter to an age of 3.5 years.
As shown in Fig. 19, the mathematical mean of the stages decreases after
190 cm in males and 180 cm in females. This does not mean the disappearanceof the spots, but indicates that the spots are so numerous as to have contact witheach other. In our method of classification this stage was not separated from theearlier stage because we checked mainly the density of independent spots. This
stage seems to corresponds to the fused stage of Perrin (1969).

Discussion
The typical development of pigmentation is shown schematically in Fig. 23,
and can be summarized as follows. The newborn male starts to develop dark
spots in the ventral and throat region at the age of 1.5 years. The female does not
Body
length Age

Sex(cm) (year)

120

0.5 M and F

153

170

>200
>190

>11
>9

M
F

Fig. 23.

Schematic figure of the pigmentation of S. attenuata at various growth stages.

have the spots at this age but show only slight darkening of the band connecting
the base of flipper and lower jaw. At an age of about 3.5 years the white mark at
the tip of both jaws of both sexes, and dorsal white spots and ventral dark spotsof females start to develop. The white spots on the dorsal region start to develop
in males at an age of about 6 years. Then both kinds of spots increase in density
to fuse with each other, and attain the final adult form at the ages of 11 years inmales and 9 years in females.
The age when the animal attains the pigmentation of the adult coincides wellwith the age at the attainment of sexual maturity. The formation of the white
tip on the jaws coincides with the age of the attainment of puberal stage, and with
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

183

the time when the animals start to leave the breeding school and join the schoolof immature animals. This suggests that they may be formed by the influence ofsome kind of sex hormone. If there is any function in the white tips of jaws andin the mottled pattern, it seems to be a social and ecological function to help thevisual identification of the species or of the growth stage of the same species assuggested by Yablokov (1963) and Evans and Bastian (1969). However, the probable effect as camouflage suggested for S. plagiodon (Caldwell and Caldwell, 1966)
is not denied in S. attenuata.
The reasons for the delayed formation of dorsal white spots or of earlier formation of ventral dark spots observed in the male are not analyzed here. It may berelated to the difference of endocrinological condition between the sexes.
Table 7 shows the frequency of the pigmentation patterns of females in various
reproductive stages. Though other characteristics of pigmentation do not show
any difference between the reproductive stages of mature females, the density ofdark spots is slightly lower in lactating females. If these data really reflect theratio in mature females, it is supposed that dark spots become less conspicuous inanoestrous females.
In the present data, 96.3 % of sexually mature females and 100 % of sexuallymature males have the white part on the tip of the upper jaw. Animals withwhite tips are scarce in S. attenuata in the eastern Pacific (Perrin, 1969). This difference seems to indicate morphological differentiation between these two populations. In this respect the animals reported from Malaita (Dawbin, 1966) are closer
to S. attenuata in the Pacific coast of Japan. If observations on pigmentation are

TABLE 7.

Characterand stages•l

A

B

c

D

ii

[l
!l

[l
ll

See Fig. 18.

2l

ImmaturePregnant2l

LactatingResting.----'-----..

~.----'-----..

~

no.

%

no.

%

no.

%

no.

%

22
14
2
0
22
13
3
0
20
11
2
0
14
3
7
8

57.9
36.8
5.3
0
57.9
34.2
7 .9
0
60.6
33.3
6.1
0
43.7

0
10
9
0
0
13
6
0
1
8
5
5
0
3
7
9

0
52.6
47.4
0
0
68.4
31.6
0
5.3
42.1
26.3
26.3
0
15.8
36.8
47.4

3
25
18
0
3
30
13
0

6.5
54.5
39.0
0
6.5
65.2
28.3
0
2.2
23.9
39.1
34.8
0
41.3
32.6
26.1

0
8
8

0
47 .1
47.1
5.8
0
58.8
41.2
0
31.3
0
31.3
37.4
0
17.6
35.3
47 .1

9.4
21.9
25.0

11
18
16
0
19
15
12

Pregnant and simultaneously lactating females included.

Sci. Rep. Whales Res. Inst.,

No. 26, 1974

PIGMENTATION OF FEMALE S. ATTENUATA BY
REPRODUCTIVE CONDITION.

0
10
7
0
5
0
5
6
0
3
6
8

KASUYA, MIYAZAKI AND DAWBIN

184

made on many animals along the Malaita coast, it will give some information on
the identification of populations.
The pigmentation of other spotted species S. plagiodon in the Atlantic (Caldwell
and Caldwell, 1966) is distinguished from that of S. attenuata in the Pacific by theabsence of the fusion of ventral dark spots and by the larger size of the spots.
FEMALE BREEDING CYCLE

Graafian follicle
The frequencies of the diameter of the follicle obtained as the cube root of
the multiple of three dimensions are shown in Fig. 24. The ovaries of pregnant
females are inactive having no large follicles in any school or in any season. The
ovaries of all the lactating and the resting females in school 7, which was caught in

10

2

A

s

0

N

D

J

F

M

Month

Fig. 24. Seasonal variation of the diameter of the largest follicle in mature femalesof S. attenuata. Black squares and thick solid lines indicate the resting females,
dotted squares and thin solid lines the lactating, and white squres and dotted lines
the pregnant or pregnant and simultaneously lactating.

July, show high activity. On the other hand in schools caught in November or in
February, there were observed many lactating or resting females with inactive
ovaries. As a result the mean diameter of the follicles of females in the two categories is smaller than that of the schooll killed in Juy. However, even in schools
killed in February or in November the size of follicles of active females is not small.
Accordingly it can not be concluded that all females in February and November
are sexually inactive. It will need observations on more schools in various seasons
to draw conclusions on the seasonal variation of the size of follicles.

Diameter of corpus luteum
Even when we got the ovaries of pregnant female it was not necessarily possible
to obtain the fetus, because the fetus or fetus and part of uterus were sometimes
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

185

STENELLA ATTENUATA

removed by the fishermen and piled elsewhere. - In these cases the pregnancy waschecked by a fragment of uterus or by the condition of ovaries. Table 8 shows
the relationship between the size of corpora lutea and the presence of a fetus.
This table shows that if the corpus luteum is larger than 6000 mm 3 in volume index
it may be correct to consider that the animal was pregnant. The scarcity of corpus
luteum of the size between 4000 and 6000 mm 3 in volume index may indicate thatwhen the ovulation is not successful even if the corupus luteum may grow to asize similar to that of corpus luteum of pregnancy, it may regress very rapidly.
Accordingly, when other information is not available, we tentatively consisderedthat a corpus luteum smaller than 4000 mm 3 in volume index or 15.87 mm indiameter as the corpus luteum of ovulation.
26

14

0

20

40

60

80

Body length in cm
Fig. 25. Relation between the diameter of corpora lutea and body length of fetuses.
Solid line is drawn by least squares, and dotted lines by eye. Open circle indicates a lactating female.
TABLE 8.

SIZE OF LARGE CORPORA AND THE SEXUAL CONDITION OF FEMALE.

Range in volume
index (mm 3)

Diameter(mm)

1001- 2000
2001- 3000
3001- 4000
4001- 5000
5001- 6000
6001- 7000
7001- 8000
8001- 9000
9001-10000
10001-11000
11001-12000
12001-13000
13001-14000
15001-21000

12.60
14.42
15.87
17 .10
18.17
19.13
20.00
20.80
21.54
22.24
22.89
23.51
24.10
30.00

R

L

PL

p

PF

3

Total

3
2

2

1
4
2

2
5

3
1
2
6
2
2
2
6

R: resting. L : lactating. PL : pregnant and lactating, fetus not confirmed. P : pregnant, fetus notconfirmed. PF : Pregnant, fetus confirmed.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

186

The relation between diameter of corpus luteum of pregnancy and the bodylength of fetus is shown in Fig. 25. The diameter of a corpus luteum of a pregnant
female with 1.0 cm fetus was so large as to be in the range of corpora with larger
fetus. Though the diameter of corpus luteum does not show a large change related
to the growth of the fetus, there seems to be a slight decreasing tendency. If theregression line is calculated by least squares for the 14 corpora corresponding to
the medium and larger fetuses, it gives the relation y= -0.01825x+23. 700, where
y is the diameter in mm and x is the length of fetus in cm. The mean diameterat the time of parturition is 22.08 mm on the above formula.

Regression of corpus albicans
Fig. 26 shows the frequency of diameters of corpora albicantia in each age
group. The modal diameter gradually decreases with the increase of the age ofthe animal until 14.5 years of age or the 7th year after the mean age at the attainment of sexual maturity. Then the highest peak of the mode stays between 2 and
2

13

11

13

16

12

B

9

4

5

0

17.5

19.5

0

0

14

12

E
E10
.!;
..... 8
<I.I
.....

E6

c
i5 4
2
9.5

115

13.5

15.5

215

235

Age in years

Fig. 26. Distribution of the diameter of corpora albicantia of S. attenuata. The
numbers at the top indicate the number of animals, and the dotted line the positionof the mode.

3 mm in diameter, and the ratio of the corpora number constituting the modeincreases with age. No corpora smaller than 1 mm in diameter were observed.
They indicates that the corpora albicantia attain a final size within several
years after the start of regression and may stay in the ovary all the lifetime. The
mean final diameter suggested by the shape of the mode is about 2.5 mm or slightly
more.
It is easily supposed that the size of corpora albicantia decreases rapidly after
the parturition. If the size of the largest corpora albicantia of lactating females
in each school are arranged by size, some modes are observed. These modes may
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

187

represent some parturition season. The mean growth curve and the mean modal
length of calves caught in the same school give the time after parturiation. The
length frequencies of juvenile calves and the size frequencies of the largest corpora
albicantia of pregnancy of lactating female in corresponding schools are shown in
Fig. 27. In this figure the few corpora albicantia of ovulation were omitted and
0

I

0.5
I

1.0I

LJ~

14

12

13

11

I

30
40
I I I

I

(year)

D~

D

10
9
8
7
Diameter in mm

2.0
I

6

5

4

I

3

Fig. 27. Comparison of the frequency of the diameter of copora albicantia of thelast parturiation of lactating females and the length frequency of juvenile calves.
Scale in years was calculated from the growth curve. Numbers at the left indicate
the school. The length frequency of school 3 was calculated from the weightdata.
TABLE 9.

LACTATING FEMALES WHICH OVULATED AFTER LAST PARTURITION.

AnimalDiameter of corpus (mm) of

schoolno.

largestlast parturition

3
3
3
3
4
4
4
4
4
4
5
6

3
24
35
38
5
19
47
48
96
119
48
102

4.88
12.35
8.82
12.12
5.37
15.14
10.69
5.24
5.01
8.51
6.46
4.66

4.38
3.29
4.95
3.21
5.30
4.79
5.28
4.78
4.72
6.50
6.12
4.28

*: based on the criterion used in this study.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

No. of ovulation
postpartum

postlactum *

1
3

1
3

1
3

188

KASUYA, MIYAZAKI AND DAWBIN

the probable corpus albicans of the last parturition was used. These exceptional
individuals are shown in Table 9. The mean modal diameters of corpora albicantiaof the last parturition of lactating females are plotted in Fig. 28 against the timeafter parturition. It shows that the regression of the size of corpora albicantia is so
rapid that it regresses from the mean diameter of corpus luteum 22.08 mm to13.6 mm within 0.2 months. Then the speed ofregression gradually decreases, andafter 16th month the diameter decrease is almost straight. If this line is extended,
it reaches the mean final diameter of corpora albicantia at about 70 months. This
coincides with the information described above.
The elapsed time after the start of regression of corpora lutea was calculated

16
14 7

E12

E

,£; 10.....

a;CIJ 8

4

•2

E

.Q 6
Cl

4
2
00

4

8

n

IB

W

~

W

~

~

~

~

~

~

MonthsFig. 28. Regression of the diameter of corpora albicantia obtained from the combinations in Fig. 27. Larger circles indicate reliable combinations, number by theeach circle the school.

from the regression line of the corpora albicantia drawn for volume index andvolume indices of the corpora, assuming that the size of corpus albicans of pregnancy
and of ovulation is not significantly different. This assumption should not be unreasonable, because the corpus luteum of ovulation seems to attain a size near tothat of pregnancy and the regression rate is rapid in the first month. The corpora
albicantia of pregnancy were then identified based on the following assumptions
and considering the reproductive conditions at the time of death.
1. During pregnancy the female does not ovulate.
2. There may be some postpartum ovulations or postlactum ovulations.
3. The time from a parturition to next conception is usually more than one year.
Thus the age of corpora albicantia which started the regression within recent 5
years were calculated and the corpora albicantia of pregnancy and those of ovulation were identified. The real date of ovulation of corpora albicantia of pregnancy
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

189

is roughly estimated assuming that the length of gestation is 11.2 months. The
date of ovulation of a corpus luteum of the pregenant female was calculated fromthe fetal growth curve and the fetal length. In a few cases where fetal length was
not available the mean feta] length of the school was used.

Postpartum ovulations
The postpartum ovulations were analyzed in relation to the time from thepreceding parturition based on the ovulation diagram prepared with regression
curve of the corpora. The source of information was classified into two. The
first (series A) is the ovarian activity after the last parturition ending at the death10'
f+-r~~~~-,-~~~~~~~~~~io~·_..,_~~~~~~~~~~~~~§ 30''.§:~r,---'.-~~~~~~~~~~~~~::J~101----Y-~_L__>.~~~~~~~-,_

10'o10r,,~L,-~~~~~~~~~~~~~~~~--or

Zd 10"

4~'"~"~--'-~-'--'--~~~--'H~~~~~~~~~~T-lf

Fig. 29. Seasonal variation of ovulation shown in relation to the date of the lastparturition. Females are grouped by the month of parturition (numerals at theleft). The females that are grouped into Jan., Feb., March, and Oct., show no
ovulation, but are included in the sum. Number of ovulations is shown permonth per 100 animals. Based on series A.
TABLE 10.

NUMBER OF POSTPARTUM AND POSTLACTUM OVULATIONS.
Postpartum ovulations

Postlactum ov.

No.
A
0
2
3
Sum
Mean

60
10
2
1
73

82.2%
13.7
2.7
1.4
100
0.234

36
6
9
0
51

B

Total

70.6%
11.8
17.6
0
100
0.471

96
77.4%
16
12.9
11
8.9
1
0.8
124 100
0.331

No.
2
3
4
Sum
mean

B
34
14
2

66.7%
27.5
3.9
2.0
51 100.1
1.412

of the animal, which is more reliable because the time after the parturition is usually,
but not necessarily, short and the reproductive condition of the female at the timeof death was taken into consideration in assessing the corpora albicantia of the last
pregnancy. The next (series B) is the ovarian activity from the parturition that
occurred one cycle before the parturition used in series A and ends at the last conception. The date of parturition of series B is usually earlier than that of series A.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

190

At the first step the analysis was made on series A. The samples were grouped in accordance with the month of parturition, then the dates of ovulations werearranged chronologically. and the frequency of ovulation per month per animal was
calculated (Fig. 29). This only shows that the ovulations after the recent parturition had occured in various seasons of the year.
To know the range of the time when postpartum ovulations may occur, the relation between the time after the last parturition and the number of ovulations thatso~~~~~~~~~~~'-'-'-'~'-'-"-'...L.LI..LI..L.'-'-'-'-"-'-'-.L.U

0

6

~

a
~
E
Q
~
Months from parturition

IB

~

W

Fig. 30. Relation between the time from parturition and the number of ovulations undergone by each female (numbers at the right). Figure at the top indicates the number of females observed. Based on series A.

occurred after the parturition was analyzed based on the materials of series A (Fig.
30). In some animals ovulation occurs soon after parturition and the number ofovulated females increases. As a result the percentage of unovulated females decreases untill the 12th month, then the percentage becomes rather stable during thenext 12 months. After this the number of unovulated females again starts to decrease. We consider that the former period corresponds to the period of postpartum
ovulations and the latter probably to postlactum ovulations.
Thus defining postpartum ovulations, the time and frequency were obtained
from series A and B (Tables 10 and 11 ). About 20 % of the females have a post
partum ovulation but they seems to be all infertile. The mean number of post
partum ovulations is 0.331 per reproductive cycle.
As shown in Table 11 and Fig. 31 the time of the postpartum ovulation measured from the date of parturition shows some regularity. The first peak is in
the 3rd month after parturition, then next peaks come with the interval of 4 months.
These peaks may, as mentioned in a later section, correspond to the mating seasonof this species. But it must be noticed that some postpartum females do not ovulate
in the first season but in the 2nd or 3rd seasons. The length of interval between two
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

191

postpartum ovulations was studied on the few females which have experienced two
or more postpartum ovulations. As shown in Fig. 31, if more than 1 ovulation are
made in one breeding season the minimum of the interval is about 1 month. The
longer intervals are approximately a multiple of 4 months.

Postlactum ovulation
The postlactum ovulation was tentatively defined as the ovulation occurring
after 12 months or ~ore from the last parturition, and analyzed only on the series
B because series A does not end at conception. It was not confirmed here whether
the animal has finished lactation by the first postlactum ovulation. However ourdata suggest that it is not necessarily so. As shown in Table 12 the intervil between
the last postpartum and the first postlactum ovulations, on animals where two or
more postlactum ovulations exist) is usually 12 months or more. This means thatthe ovarian activity usually stops for a long period after the last postpartum ovulation. However only in 2 cases among 73 animals the interval was 4.0 or 8.8 months.
In these cases the postpartum ovulation is almost continuous to postlactum ovulation. About 67 %of the females conceive by the first postlactum ovulation, and theremaining experience two or more postlactum ovulations. The mean number of
postlactum ovulations is 1.412 per one reproductive cycle (Table 10).
The peak of postlactum ovulations is observed in the interval of 4 months(Table 11 and Fig. 31). There are some females which do not ovulate at the 4thmonth before conception but do it at the 8th or 12th. The information on postpartum and postlactum ovulations are shown in the diagram of Fig. 32.
The cycle of postlactum ovulation of females which have two or more postlacTABLE 11.
Time in
month
(t)*
n=l
2
3
4
5

6
7
8
9
10
11
12
13
14
18

TIME OF OVULATION MEASURED FROM THE DATE OFPARTURITION OR OF CONCEPTION.
Postpartum ovulation

lst

2nd

2nd**

3

3

3

4
3

4
3

3

3
3
6
7

2

5

2

1
2

3

2
3

2

2

*: Time range (t): n-1 ;;;;t<n,* * : Nearest to postlactum ovulation of pregnancy or the first.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

4th

5
6

5

2
2
4
4
2

3rd

Sum

Sum

3
4

3rd

Postlactum ovulation

KASUYA, MIYAZAKI AND DAWBIN

192

3

2
>.

u

c

<I>
:J

g.... 3

2

lJ..

-1

2

0

3

4

5

6

7

8

9 10 11

12

Interval in months
Fig. 31. Intervals of postlactum ovulations (top) and of postpartum ovulations(bottom), in the animals where several of them are observed. Black indicatesunreasonable case, where the superficial date of ovulation comes after the date ofconception .

.09
.08

•

/\

» .07
:;; .06:.ad .05

{ i

.0
0 .04

....

a.. .03

.

.02
.01
op

2

4

6

8

10 12 14 16

, I:

!l
: !'ii
!\
\ !\:
\! \:
!~
20 18 16 14 12 10 8 6 4c2Time in months
Fig. 32. Diagram of ovulation in one reproductive cycle. The postpartum ovulation is at the left and the postlactum ovulation at the right. P indicates parturition, C conception, dotted lines postpartum ovulation nearest to parturition or
postlactum ovulation nearest to conception, chains the second, thick solid lines
the third, and thin solid lines the total of postpartum ovulation. Probabilityof ovulation is shown by the number of ovulations per animal per cycle. The
fertile postlactum ovulation, of which probability is 1, is not shown. The lengthof the reproductive cycle is set at 50.3 months, one of the estimate of mean reproductive cycle in S. attenuata.

TABLE 12.

INTERVAL BETWEEN LAST POSTPARTUM OVULATION AND
FIRST POSTLACTUM OVULATION.

Interval in month (n)*Frequency

4

9

11

12
2

13
2

17
1

20

24

2

1

29

31

*: Interval is shown by month, n-l<month~nSci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

193

tum ovulations is classified into two types. One is the cycle of 4 months or a multiple of it. This may be the cycle related to the mating season. The other is shortand is an ovulation cycle in one breeding season. This probable real oestrous cycle
is of about 1 month. These features are quite similar to the cycle found in postpartum ovulations.
Accordingly it is concluded that S. attenuata is polyoestrous and the oestrous
cycle is about 1 month, and this oestrous cycle occurs m some of the breeding
seasons which have an interval of 4 months.

DJFMAMJ

JASON

50

40
30

20
10
,__

J)

---~--=

oiJ:::::::r11:

14

E

~ 12

10

8
6
4
2
OJ

FMAMJ

J

AS

ONO

Month
Fig. 33. Breeding season of S. attenuata. Top indicates the monthly frequency ofparturition calculated from the length frequencies of fetuses and juvenile calves
younger than 2 years of age. Bottom indicates the frequency of the date of conception calculated from the ovulation diagram. The scales of date of conception(bottom) and that of parturition (top) are moved apart for 11.2 months, thelength of gestation. White areas indicate conception within 1 year before thedeath of the animal, the dotted that between 1 and 2 years, the black that between 2 and 3 years, and the shadowed that between 3 and 4 years.

Breeding season
The parturition seasons are calculated from the mean growth curve and length
frequencies of juvenile calves and of fetuses (Fig. 33). As the apparent mating
season differs between schools, it is necessary to observe many schools and a wide
chronological range in obtaining the information on the mating season of the species.
Sci. Rep. Whales Res. Inst.
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

194

However after the attainment of 2 years of age, it is impossible to estimate the dateof birth because the growth of the calf becomes too slow compared with the range ofindividual variation of the body length. By this method the reproductive activityof the females in the most recent 3 years can be estimated. The date of conception
was obtained by sliding the chronological scale of the parturition season for 11.2
months.
In all the 7 schools the peak of conception calculated from the fetal length isin July and August. This strongly suggests that one of the mating seasons of thisspecies is in these months. The result calculated from animals born within one
year is almost similar to that obtained from fetuses, but there appears another small
peak of conception in January. Though the information obtained from the calvesborn between 1 and 2 years before death is inaccurate, there seems to be peaks ofconception in February, in May and June, and in September. But when all aretotaled there remain only two peaks of conception one in July and August, and theother in January and February.
The bottom of Fig. 33 indicates the frequency of conception in the past 4 years
obtained from the size of the corpora albicantia of pregnancy. In this figure the
100
90

24

80
0.S 70

22

0

?: 60
.....

<II

lL

50
40
20

0

.

'
'' ' 'D Jy·','/ "'· ...

'o'

30

Vl

c

20
0
18 s>

'

'\

,''

\,.-

~·

: 16 00
14 zcii
12

FMAMJJASOND

Month
Fig.' 34. Seasonal change of frequency of ovulations (solid line) and of the ratio of
fertile ovulations (dotted line). Ovulations in the recent 4 years studied are
combined.

date of conception corresponding with the corpora lutea of pregnancy was calculated
from the length of fetus. There seems to be three peaks in the conception occurred
between the past 1 and 4 years. However it is also possible to read in the totalheight two main peaks of mating between May and September, and between
November and March, each of which having one accessory peak in it. This feature
coincides with the seasonal change of the ratio of fertile ova to the total number ofovulations (Fig. 34 ). The ratio of fertile ova is higher in the two mating season.
However the frequency of total ovulation shows another feature. As expected fromthe 4 months cycle of ovulation, there are observed three peaks in January to
Sci. Rep. Whales Res. Inst.,
No, 26, 1974

STENELLA ATTENUATA

195

March, July, and October and November.
The above three sets of data on mating seasons do not coincide well. Between
the first two methods (Fig. 33), the peaks around February and that in July coincide.
This first method is good in accuracy but covers too short period. On the other
hand the second method covers a longer period and suggests the other peak in December, but the accuracy of dating is less reliable. The third method or the frequency of ovulation, which is similar to the second but includes all the ovulations,suggests a mating seasons of four months interval. To increase the accuracy of thesecond method, if the dates of the conception at each end of the series B or the selected reproductive cycles used in the former section is considered, there is observed
three clear peaks of conception (Table. 13). Two of these mating seasons coincide
well with those obtained from Fig. 33. The third peak in November coincides approximately with the corresponding peak in Fig. 33 (bottom) and frequency ofovulation in Fig. 34. Then the interval of the peaks are 4 months which is suggested by the postpartum and postlactum ovulations.
As a conclusion, though more confirmation is necessary, we are inclined to expect three mating seasons with the peaks in February and March, in July, and in
TABLE 13.
MonthFrequency

MONTHLY DISTRIBUTION OF DATE OF CONCEPTIONOBTAINED FROM SERIES B.

Jan.

Feb.

Mar.

Apr.

May

Jun.

July

Aug.

Sept.

Oct.

Nov.

Dec.

1

7

4

1

1

5

14

5

1

4

4

4

November. The last two mating seasons coincide with the seasons of just after thenorth bound and south bound migrations, and the first just before the north boundmigration. However it is probable that the migration is affected by the breeding
cycle and there is a segregation of '.females in different reproductive conditions.
The parturition seasons seems to be about one month ahead of the mating peaki.e. in Jannuary and February, in June, and in October.

Reproductive cycle
The mean length ofreproductive cycle can be obtained from the ratio of various
reproductive conditions assuming the 11.2 months for the approximate length ofgestation. When the data of all the schools are combined the number of pregnant
females is 64 or 25.6 % of mature animals, including each half of the number of animals which are simultaneously lactating and pregnant into both categories (Table
27). The length of lactation 140+64x 11.2=24.5 months is calculated. By thesame method the length of resting 8.1 months, and the length of one reproductive
cycle of 43.8 months or 3.65 years are obtained.
If the samples are obtained evenly from various seasons in a year and covers
schools of various reproductive stages, this method gives good information on themean length ofreproductive cycle. However, as can be seen in Table 27, the samples
were collected mainly in October and November, and the frequency of mature females constituting the reproductive stages shows wide between school variation. In
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

196

KASUYA, MIYAZAKI AND DAWBIN

this condition we cannot expect the accuracy of the method. Another problem
is the presence of females which are sexually mature but in which the reproductive stage was not identified. This kind of imperfect observation is usually madeon animals whose reproductive organs have been removed by the fishermen andprecise study was not made. On this case the identification of pregnant females isusually possible based on fragments of uterus. However it is very difficult to distinguish the resting animal from the very early stage of pregnancy. Accordingly
the real ratio of resting animals, which represent the sexually mature females other

100

9

21

22

16

15

21

14

4

7

5

5

7

4

7
9
11
No. of ovulations
13

7

a

5

3

3

17

13
3

2

o o

0

~ ~t~t~ Hlil\~l\ ~llfa@

1·::·:·:·:·:·:·····:·:·:·:·:·:·:

11.5

9.5

23.5

21.5

19.5

15.5
13.5
Age in years

Fig. 35. Change of the ratio of sexual conditions by age in mature females of S.
attenuata. Black indicates pregnant females, black with white spots those pregnant and simultaneously lactating, white with black spots those lactating, andlines those resting. The number at the top indicates sample size.
TABLE 14.

No. ovulations

1- 6
7-12
13-18
Total
Age (years)

7.5-11.5
12.5-17.5
18.5-23.5
Total

RATIO OF REPRODUCTIVE CONDITIONS AT THE TIME OF DEATH
AMONG MATURE FEMALES OF VARIOUS AGE GROUPS.
Pregnant

Preg. & Lact.

Lactating

Total

no.

%

no.

%

no.

%

no.

%

no.

%

Reproductive
cycle(Months)

24
8
2
34

23.1
19.0
20.7

5
0
0
5

4.8
0
0
3.1

55
30
8
93

52.9
71.4
44.4
56.7

20
4
8
32

19.2
9.5
44.4
19.5

104
42
18
164

100
100
100
100

40.2
58.8
100.8
50.3

15
13
0
28

24.2
16.9
0
19.4

3
2
0
5

4.8
2.6
0
3.5

33
44
5
82

53.2
57.1
100
56.9

11
18

17.7
23.4
0
20.1

62
77
5
144

100
100
100
100

38.6
57.5

Resting...------.. ---------... ---------... ---------... ---------...

II. I

0
29

52.9

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

197

than pregnant or lactating, may be higher than that observed. Accordingly thereal mean reproductive cycle must be longer than 43.8 months.
Table 14 and Fig. 35 shows the change of the pregenant ratio accompaniedwith increase of age. They show that the ratio of pregnant females decreases andlength of reproductive cycle increases in the older animals. The length of reproductive cycle calculated by adding all the ovulation classes or age classes are 50.3
and 52.9 months respectively. As these calculations are based on observation ofreliable materials, the figures obtained are more reliable than the 43.8 months cycle
obtained from Table 27. Furthermore as the ovulation data used in Table 14 represent 96.5 %of all the available ovulation data, the figures obtained from it seems
to be the best estimate of the mean reproductive cycle of the present population of
Stenella attenuata in Japanese coastal waters. It gives the length of gestation 11.2
months, lactation 29.3 months, resting 9.8 months, and i:otal length of reproductive
cycle 50.3 months or 4.19 years.
The mean length of reproductive cycle is obtained separately from the frequency of ovulations of conception. It is generally observed in the ovulation
diagram that the presumed dates of ovulation are not very reliable in the case of 5
years or older corpora albicantia, and that it is almost impossible to distinguish thecorpora of pregnancy and of ovulation on the diagram of that part. This is clearlyshown in Table 15, where numbers of corpora albicanta of preganancy and of ovuTABLE 15. REPRODUCTIVE PARAMETERS CALCULATED FROM OVULATIONDIAGRAMS IN RELATION TO THE INTERVAL FROM THE DATE OF DEATH.
Year

lst

2nd

3rd

4th

5th

6th

A. Sample size (No. X Month)
B. Fertile ovulationc. Total ovulation
D. Pregnant ratio (BX 11. 2 7 A)
E. Mean annual ovulation (C7AX 12)

1159
21
40
0.20290.41421464
22
380.16830.31151424
24
440.18880.37081312
31
670.2646
0.61281162
37
1040.35661.0740673
27
89
0.4493
1.5869lation are shown together with the number of animals (no. X months). In thistable the pregnant ratio and mean annual ovulation rate show smooth fluctuation
until the 4th year from death, but they show higher fluctuation after the 5th years.
The mean of the pregnant ratios in the recent 4 years in Table 15 is 0.2062, which
gives the length of reproductive cycle as 54.3 months or 4.63 years. This figure isvery close to the figure 50.3 or 52.9 months obtained from the ratio of various reproductive conditions of individuals of known age.
The individual variation of the length of reproductive cycle is obtained,though it is unsatisfactory, from the ovulation diagram. Fig. 36 shows the relation
between the time after the last partutition and reproductive stage at the time ofdeath. It shows that some individuals finish lactation at about the l 7th monthafter parturition but some animals continue lactation for about 60 months or 5 years
after parturition. If the length of lactation means the length of the suckling
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

198

KASUYA, MIYAZAKI AND DAWBIN

period, the calf may continue to take both milk and solid food until the age of 17months in the shortest case or of 60 months in the longest case after starting to feedon solid food at the age of about 9 months. It is highly probable that the lactation
is finished in the usual shortly before the conception, and that the lactation lasts
longer on an animal of which interval between conceptions is long.
The length of lactation can be presumed from a direct comparison of thenumber of juvenile calves and that of lactating females caught together (Kasuya1972). This method does not give the mean length of lactation but the longest
range. At first step, the body length where the cumulative length frequency ofjuvenile animals coincides with the number of lactating females (Table 27) was obtained as shown on Fig. 37. Schools 5 and 7 were excluded here because the number
2
2(/'I 3.£1..c. ~"""'~"""'"""".......~~"--"'"'-'~~~~~~~~~
00.1 l----"""------=---=---=----'=---~..,.__ _ _---:i6-0 3

0 1~~_.E-l;,;;;;;;;____r""""L-"'"""'-...Efil-=""""'"-=---'""l,,,,---..,....,_---",,,_---1

zd 1 f"....."--.L'."'------"=q=---"'"'--=-- --"'"---""'"""'""'--------I
8

6
4
2

0

6

12

18

24

30

36

42

48

MonthsFig. 36. Relation between the sexual condition at the time of death and the lengthof time after last parturition, obtained from the ovulation diagram series A.
Black dots indicate lactating females, lines the resting, white dots the pregnant,and black the females lactating and simultaneously pregnant. Numbers at theright show the school.

of investigated lactating female was too few. On Fig. 35 the results obtained fromschools 2 and 6 give a very small length (correspond to 1.0 and 1.75 years respectively), so they will not give the real length oflactation but will show only that there
were no large suckling calf in the school. Accordingly the informations obtained
from schools 1, 3 and 4, or the length 169 cm, 178 cm and 175 cm respectively, wereused here. Then the modal lengths of the peak which includes the above bodylengths were obtained on Fig. 2. They are shown in parentheses in Fig. 35. The
ages corresponding to these modal lengths read on the mean growth curve are 4.0,
5 1/4 and 4.0 years respectively. For the comparison, same method was applied
for the age frequencies of the schools 3 and 6. In these schools the ages of all theSci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

199

young animals of known length were determined. The length of lactation obtainedby it was 5.5 years (school 3) or 1.5 years (school 6), showing a good coincidence withthat obtained from the body length frequencies. These result indicates that in someanimals the lactation lasts for 4 or 5 years, even if there are many females whichfinish the lactation within a shorter period.
The date when the female starts the next pregnancy, or that enter into thenext cycle after weaning the calf, is calculated by estimating the date of conception
from the length of fetus. Fig. 38 indicates that some females enter into the next
pregnancy at about 18 months after the last parturition. After this date, the numberof females which enter into the next pregnancy increases rapidly. At 28th monthafter parturition there begins to appear the females which give birth to the next

100
<II
01
ctl

'E 80
<II
....
<II

'' ''

0. 60
<!J

''
''

~

.' ''

l)

I

.:::

:; 40

E
:::J
u

o

''
'I
'I

I'

20

,....,1

--.,1

QI

01

0) 1

~:

0

I~

1 00

B: t:.• .§?:
100

120

140

160

U")

1

I(()

:::: :;::::
180

Booy length in cm
Fig. 37. To ascertain the maximum length of lactation, the cumulative frequency
of calves is compared with the number of lactating females. Numbers at the top
indicate schools. For further explanation see text.

calf and complete one reproductive cycle. The time when all animals complete
the reproductive cycle is about 68 months. This information on the length of reproductive cycle coincides approximately with that obtained by other methods. In
this method, as the abundance of lactating female is underestimated comparedwith that of pregnant females, it is incorrect to presume the mean length of reproductive cycle as the time when 50 % of females finish the cycle.
Fig. 39 shows the length frequency of reproductive cycles based on 67 cyclesof 64 animals obtained from the series B. As the ovulation diagram covers only
about 65 months, the frequency of long reproductive cycles is underestimated andthe cycles longer than 60 months are entirely neglected. However this can give a
Sci. Rep. Whales Res. Inst.,
No. 26, 1971:

KASUYA, MIYAZAKI AND DAWBIN

200

C!>

g'50

.....c
C!>

u

Q; 40
a.
20

Fig. 38. Length of reproductive cycle calculated from the data in Fig. 36 andlength of fetuses. The dotted area indicates the females which have finished thecycle after giving birth to the calf. Scale at the top indicates the number ofmonths from last conception, and that at the bottom months from last parturition. L, lactating. R, resting. P, pregnant. PL, pregnant and lactating.
16r-----;=-----J.---------==i100
90
14

80

12

70

gi 10

60

c<II 8

50

C!>

u

40

~ 6.

a..

30

4

20

2

10

·:·:·:·:·:·:·:-::·:·:·:·:
56

60

Fig. 39. Frequency of the length of reproductive cycle in months obtained as theinterval between the dates of two ovulations of pregnancy (left scale), and its
cumulative frequency (solid line and right scale). Arrow indicates the positionof mean length of the cycle.

rough idea on the range of the length of reproductive cycle. An exceptionallyshort reproductive cycle is about 23 months, but most of the cycles are more than
29 months. This means some few females start the next pregnancy at the 12thmonth after the former parturition but more usually at the 18th or later months as
indicated in Fig. 38.
As the length of reproductive cycle changes with age, the reproductive parameters must be obtained in relation to the age of the animal. At first step theSci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

201

number of animals, number of fertile ovulations, and total number of ovulationsin the recent 4 years were calculated on the ovulation diagram for each group of
females classified by the number of ovulations at the time of death. Then themean values of various parameters in the past 4 years were calculated (Table 16).
The constants of the regression lines calculated by least squares are shown in thesame table, and by the lines in Fig. 40. It shows that the mean length of a reproductive cycle increases rapidly from 3.6 years of the young animal to 6.5 years forthe old animal. This prolongation of reproductive cycle in old females is not produced by the cessation of ovulation, but by the increase of unsuccessful ovulations.
The fertility ratio of ova decreases from 64 %of young animals to 32 %of old animals.
On the other hand the number of annual ovulation shows a very slight increase
with age. Anyway such a long mean reproductive cycle of old females suggests
Calculated age

No. of ovulations
Fig. 40. Relation between age and reproductive parameters obtained from theovulation diagram. 1 indicates pregnant ratio (left scale), 2 annual ovulation
rate (left scale), 3 fertility ratio of ovum (left scale), 4 number of ovulations in a
reproductive cycle (right scale), and 5 length of reproductive cycle in years (rightscale).
TABLE 16. REPRODUCTIVE PARAMETERS CALCULATED FROM OVULATIONDIAGRAMS OF THE PAST 48 MONTHS IN RELATION TO THE
NUMBER OF OVULATIONS AT DEATH.
Number of ovulations at death
Sample size, No. X Month
Fertile ovulationc. Total ovulation
D. Pregnant ratio
E. Mean annual ovulation
F. Ovulation per reproductive cycle
G. Length of reproductive
cycle, Month
H. Fertility of ovumA.
B.

* y=ax+b.

13-15

16-18

1606
959
733
565
1474
23
21
12
12
36
50
67
33
26
28
0.2735 0.1604 0.2453 0.1834 0.2379
0.4071 0.5006 0.4129 0.4256 0.5947

288
3
10
0.11670.4167

1-3

4-6

7-9

10-12

b*

-0.00584 0.2584
0.00327 0.4286

1.389

2.913

1.571

2.167

2.333

0.333

0.0817

1.518

40.94

69.83

45.67

61.08

47.08

96.00

2.119

39.97

0.72000.34330.63640.64150.42860.3000-0.01923

0.6643

D=BX 11.2-;.-A, E=C-;.-Ax 12, F=C-;.-B, G=A-;.-B, H=B-;.-C.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974a*

KASUYA, MIYAZAKI AND DAWBIN

202

that their participation in reproduction is almost negligible.

Accumulation rate of corpora albicantia
The corpora albicantia with clear yellow or yellow brown colour and haveno internal structure typical of usual corpora albicantia nor typical external scar
were considered to have been derived from atretic lutealized follicles (Harrison
and Ridgway, 1971), and were not included in the number or corpora albicantia.
The number of corpora atretica was counted, if found, when the number of corpora
albicantia and lutea was counted. The relation between number of ovulations andof corpora atretica is shown in Table 17. However if they had been searched formore intensively, the number might have been more than that shown here.
No corpora atretica are observed in immature animals. However, in themature animals, corpora atretica ranging from 0 to 13 in number were observed.
It does not seem to increase with the age of animals. This may be because the sizeof corpora atretica decreases rapidly and many of them have been overlooked.
But at present the possibilities are not denied that corpora atretica are formed frequently in young animals or that very few corpora atretica are erroneously included
as corpora albicantia.
Ohsumi (1964) showed on S. attenuata that the left ovary is precocious and thenumber of ovulations in the left ovary usually predominates that in the right.
Among the present 171 pairs of mature ovaries, 162 pairs or 95. 7 % of the total females had the higher number of corpora in the left ovary. This supports his result.
TABLE 17.
No. ofovulations
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18'
Total

0
126
10
18
18
16
13
18
10
6
4
2
6
4

RELATION BETWEEN NO. OF OVULATIONS AND NO. OF
CORPORA ATRETICA OBSERVED.
2

3

4

5

6

7

8

9

10

2
3

2
3

2

2

3
2
2
260

20

6

0

2

3

2

0

0

13

mean
0
0.09
0.77
0.74
0
0.47
0.41
0.47
0.57
0
2.00
0.14
0.20
2.50
0.60
0.33
6.00
1.33
0.50
0.33

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

203

However, the presence of 8 pairs (4. 7 %) of ovaries, in which the number of corpora
in right ovary exceeds that in the left, indicates that there may be few females which
do not show the precocity of left ovary. The same number of corpora in each ovary
was observed only in one (0.6 %) female. These few females should be dealt withseparately in analyzing the ovulation activity of the females with precocious leftovary.
Fig. 41 shows the mean number of ovulations in one ovary plotted against
the number of ovulations in both ovaries. It is considered from this figure that theright ovary of some of the females with a precocious left ovary starts to ovulate at
the age of 6 to 10 ovulations. But most of the left ovaries cease the ovulation at 13
or 14 ovulations when the right ovary take the place of left ovary. The mean number of ovulations at this stage calculated from 13 females with 13 or more corpora
is 13.5. The thick solid and dotted lines in Fig. 41 indicate the mean ovulation activity of the female with a precocious left ovary. On the other hand the bilateral
18

16

>- 14
~

i; 12
(!)

c 10

0

.!: 8

0"' 6
e-

8 4

-

1

a s m n~-~·ll-+--+---t--1

a

~

m

IB

n

IB

Total no. of corpora

Fig. 41. The asymmetry of corpora number in the ovaries of S. attenuata. Blacksquares indicates the frequency of corpora in left ovary, the white that in rightovary, closed circles and thick solid line the mean number of corpora in the leftovary of the female with precocious left ovary, and open circles and dotted linethat in right ovary in the same animal. Three thin solid lines indicate the casewhere all the corpora are in the left ovary or in the right ovary, and the cases
where both ovaries have the same number of corpora.

asymmetry is not clear in females in which the corpora number in the left ovary
does not exceed that in right ovary. This may mean that the right ovary of these
individuals starts to ovulate at a younger age, or that both ovaries ovulate equally.
Possibly both cases occur in these individuals.
The accumulation rate of corpora albicantia in one reproductive cycle wasshown to change from 1.6 of youngest animals to 3.0 of oldest animals (Fig. 40).
The mean accumulation rate of corpora in all the age groups is obtained by totalling the mean number of postpartum ovulations and that of postlactum ovulations(Table 10). This method gives the value 1. 743 for the mean number of ovulations
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

204

in one reproductive cycle. This value corresponds in Fig. 40 to the value at the ageof 3 ovulations or 12.8 years of age calculated by the method mentioned in a later
section.
The number of ovulations experienced before the first pregnancy is obtainedby two methods. One is to use the ovulation diagram. In some recently matured
females the number of ovulations undergone before the first pregnancy is directly
counted on the ovulation diagram (Table 18). The mean of these ovulations is
1. 784, which is fairly close to the number of ovulations per one reproductive cycle
obtained as above and to the value obtained on Globicephala melaena (Sergeant,1962). The second is the method of Sergenat (1962). If the female attains thesexual maturity at age between 5.5 and 10.5 years of age and minimum length of
reproductive cycle is 2 years, most of the young pregnant or lactating females may bein the first cycle of reproduction (Table 19). In this table the animals shown by
italics were excluded from the calculation because they are considered to be in thesecond cycle. Then the mean number of ovulation 2.08 was obtained from 13 animals, which is too high compared with the values obtained by the above two
methods. This is possibly because some postpartum ovulations, or females in thesecond cycle are included.
Accordingly we conclude that, though it may change with the age of the animal, the mean number of ovulations in one reproductive cycle is between 1. 7 and
1.8, and that it will not be very different from the number of ovulations undergone
before the first pregnancy.
The relation between the number of ovulations and the age determined by
the dentinal growth layers is shown in Fig. 42. The highest annual accumulation
TABLE 18. NUMBER OF OVULATIONS UNDERGONEBEFORE THE FIRST PREGNANCY(FROM OVULATION DIAGRAM).
Frequency
No. of ovulations
1
2

3
4

TABLE 19.

no.

%

19
9
7
2

51.4
24.3
18.9
5.4

NUMBER OF OVULATIONS IN RECENTLY MATURED PREGNANT
OR LACTATING FEMALES.
No. of ovulations

Age
2
7.5
8.5
9.5
Total

3

4

5

6

7

8

9

10

1
3
3
4

2
1

2

5

3

l+l

1
1
2

1
1

1
1

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

205

STENELLA ATTENUATA

rate of the corpora is 3.0 on the figure and the lowest 0.50. The relation between
the number of ovulations (y) and the age (x) of the mature females, y=0.821 x-:3.967, was obtained by least squares. If this annual accumulation rate 0.821 15
applied to the mean length of the reproductive cycle 4.19 years, the number of ovulations in one reproductive cycle must be 3.44. Even if the shorter and less reasonable estimate of reproductive cycle 3.65 years is adopted the ovulation in one
cycle is 3.00. These values are higher than any other values obtained in this section, and considered to be unreasonable.
The open circles and dotted line in Fig. 42 indicate the mean age corresponding to each number of ovulations. This dotted line is almost vertical after the attainment of 12.5 years of age. As the annual ovulation rate does not change muchwith the age of animal (see Fig. 40), this tendency must indicate that the age of theanimals older than that point is underestimated. This age corresponds roughly ~othe time when the poorly calcified secondary dentine starts to be accumulated in
!
..-··

16

••• .114

l!! 12
0

B10
:;
>
oa
0

+
+

d6

z

4
2

0

I
7.5

9.5

115

13.5

15.5

17.5

Age in years

195

215

235

Fig. 42. Relation between number of ovulations and age of S. attenuata estimatedfrom the dentinal layers. Open circles and the dotted line indicate the mean age
in each ovulation number, closed circles and thick solid line the mean number ofovulations in each age class, thin solid line the relation between ovulation andage calculated by least squares, and cross marks the number of ovulations estimatedfrom the ovulation diagram.

the pulp cavity and age determination becomes difficult. As physical maturity ofthis species is attained by the age of 11.5 years, it will be concluded that after the attainment of physical maturity the secondary dentine is accumulated and reliable
age determination becomes impossible by the present technique.
A similar feature is expected even in Stenella coeruleoalba. Though K.asuya(1972) obtained the number of ovulation-age relationship y=0.69x-5.16, theSci. Rep. Whales Res. Inst.,
No. 26, 1974

206

KASUYA, MIYAZAKI AND DAWBIN

fitness is poor in younger animals. And the relationship between mean ages andnumber of ovulations is almost vertical after the age of 14 years. This is an indication of underestimation of the age. If 13 years of age or younger 15 females selected from his data (Kasuya 1972, Fig. 14), excuding one 10 years old female whichshows too high count of the corpora, are used, 0.325 is obtained as the annual
pvulation rate. This value is less than the half of the annual ovulation rate estimated by Kasuya (1972).
It is impossible to use a similar method to estimate the real ovulation rate ofS. attenuata, because reliable age determination is possible only for a shorter period.
The real annual ovulation rate of S. attenuata was calculated from the relation between the annual ovulation rate and the number of ovulations (Table 16).
Yt = 0.00327xt+0.4286

where Yt is the mean annual ovulation rate between the age t and t-4 years and Xt
is the number of ovulations at the age oft years. This formula means that the annual ovulation rate is approximately 0.432 at the first year after the attainment ofsexual maturity and 0.487 at the age of 18 ovulations which is the oldest animal
observed. These values are higher than the corresponding value obtained in S.
coeruleoalba, but coincides with our observation that simultaneous ovulation is rarein S. coeruleoalba. A Tursiops truncatus matured and died in captivity had undergone 7 ovulations in 15 years (Sergeant et al, 1973) showing an ovulation rate,
0.465/year, similar to that of S. attenuata.
The number of corpora albicantia and lutea at age t is obtained from the above
formula. If the constants of the above formula are shown by a and b, it is rewritten as follows.
or1b
Xt+l = - --xt+-- 1 -a
1 -aHere, if the age at sexual maturity is shown by m, the number of ovulations at n
years after sexual maturity is shown by the following equations,
Xm

=1
n

Xm+n

= rn+b 1:; rnn=lwhere n is interger and r is (l-a)- 1 •As a and bare obtained from Table 16, the number of ovulations at age m+n is
calculated from this formula. The age corresponding to each ovulation number was
obtained on the graph and shown in Table 24. The oldest animal observed had
undergone 18 ovulations corresponds the age of 45.2 years. However as an animal·with many corpora may have undergone an annual ovulation higher than the average, the observed real maximum age is considered to be less than 45 years.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

207

LIFE TABLE
Litter size
Among 59 confirmed pregnancies, twins were observed only in two females inschool 1., the other 57 pregnancies were single. Accordingly the ratio of twins in
total pregnancy is 3.39 %, and the mean litter size is 1.034.

Sex ratio
The sex ratio is shown in this paper by the number of males per one female.
It is shown in Tables 20 and 21, and Fig. 43 that the number of males exceeds thatof females in the fetus and newborn animals. Then the ratio of male constantly
decreases to attain the sex ratio of 0.579 in the age groups of more than 10 years ofage where animals of both sexes have attained sexual maturity. As males attainsexual maturity 2 years later than females, the sex ratio of mature animals is very
low. When calculated from length frequency it is 0.380, and when calculated from
10 years or older males and 8 years or older females it is 0.440. Probably the former
figure is close to the truth because it is based on much more data. A similar ten-

TABLE 20.

Sex

Fetus

Juvenile 1l

Puberal2 l

Mature 3l

Total

2
J

5
3
0.600

10
0.700

13
14
1.077

58
29
0.500

86
53
0.616

2

2
J

14
19
1.357

38
33
0.868

22
44
2.000

105
31
0.295

179
127
0.709

3

2
J

0
2

3
7
2.333

2
5
2.500

18
11
0.611

23
25
1.087

4

2
J

0
2

3
10
3.333

11
16
1.455

45
32
0.711

59
60
1.017

5

2
J

0
3

3
4
1.333

1
4
4.000

11
0
0

15
11
0. 733

6

2
J

3
2
0.667

12
14
1.167

21
2.100

59
15
0.254

75
52
0.693

7

2
J

2
2
I.OOO

0
5

2
11
2.200

17
1
0.059

21
19
0.905

Total

2
J

24
33
1.357

69
80
1.159

52
115
2.212

313
119
0.380

458
347
0.758

SchoolIJ

SEX RATIO OF S. ATTENUATA BY SCHOOLS AND LENGTH RANGES.

7

up to 165 cm, 2> 166cm to 181 cm (2), or 194cm (J), 3> more than 182cm (2), or 195cm (J).

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

208

dency of the ratio in relation to the age of animal is also found in Globicephala melaena(Sergeant, 1962).
There are three possible reasons for the fluctuation of sex ratio. One possibility is the segregation of adult males. Though it may exist in the puberal stage,
there is observed no rapid change of sex ratio at the ages corresponding to thesexual maturity of the animal. Accordingly this possibility cannot explain all thephenomena. The other possibility is the difference of catchability between sexes.
As males are always single, they might find more chances to escape from the chasing boats. However, the fact that the sex ratio continuously decreases with age
even before the attainment of sexual maturity denies this hypothesis.
The only certain reason is the higher mortality of the male. As physical maturity is attained slightly earlier in females and age determination by the dentine

200
1.60

.g0 120
.
....

.80

x

(!J

(j)

.40

0
30
(/)
c
:.c 20
.Q.
0

"0

Males

10
0
Females

zd 20
10
0

0

4

8

12

16

20

25

Age in years
Fig. 43. Relation between sex ratio and age of the animal presumed from thedentine layers. Crosses indicate the mean sex ratio in Table 21.

TABLE 21.

SEX RATIO OF S. ATTENUATA BY AGE DETERMINEDBY DENTINE LAYERS.

Age
range
0>
3;;:;,
8>,
10>,
13>,

> 0
> 3
> 8
>10
>13
Total

mode
-0.5
1.5
5.5
9.0
11.5

Female
no.

Male
no.

Sex ratio

24
29

33
45
39
34
31
46
195

1.375
1.552
1.114
0.810
0.517
0.630
0.816

35
42
60
73
239

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

209

layer becomes difficult, if the mortality rate of both sexes is equal there must be observed more old males than the females. However as shown in Fig. 43, the numberof males of the age more than 16 years is only 6, whereas the corresponding numberof females is 34. This indicates that the mortality of the male is much higher thanthat of the female.
If the mortality of the male can be higher than that of female in any stage ofthe life, it is reasonable to expect that the sex ratio may change even in the fetal
stage. But this problem was not analysed in this paper. When the mortality
coefficient of male is shown by m, that of female by µ, and the sex ratio at birth bya, the sex ratio at the age oft years can be shown by the formula in the below.
Sex ratio = a·e-<m-µlt
The a and m- µ are obtained from the 5 mean sex ratios in Table 21. The data on
animals older than 12.5 years were not included because age determination bydentine growth layer is in some case unreliable. The a and m- µ calculated by theleast squares are 1.568 and 0.08348 respectively. This indicates that 38.94 % of
total number of newborn calves is female. The age when the both sexes are equalin number is 5.4 years of age. In other species of mammals such a high sex ratio
at birth is observed in red deer (Asdel, 1964).

JV!ortality and recruitment
Fig. 44 shows the age frequencies of the 239 females and 195 males based on theage determined by the dentine growth layers. The age is grouped into nearest
n+0.5 years, where n is an integer. Each half of animals at the age of 1.0, 2.0,
and 3.0 years were divided into age groups of younger or older animals.
The number of animals between 3 and 8 years of age is small. This categoryof animals may have left the breeding schools which we had the chance to study.
The highest frequency of females is in the age group of 12.5 years and that of males
10.5. If the mortality is constant after this year and the age determination is correct,
there is expected to be the following equation,
It = loe-µt

where t is the age of animal in years, It the number of animals at age t, and µ the
mortality coefficient. The mortality coefficient of the animals of both sexes morethan 12.5 years old were calculated by the least squares. It gives the mortality
coefficient,µ = 0.3545

or the annual mortality rate,
M = 1-e-µ = 0.2985

The mortality coefficient of mature females can be calculateed by anothermethod, based on the annual ovulation rate. The age frequency calculated with
the Number of ovulation-Age relationship is shown in Table 22 and Fig. 45. The
mortality coefficient and mortality rate of mature female, µ and M, calculated forSci. Rep. Whales Res. Inst.,
No. 26, 1974

210

KASUYA, MIYAZAKI AND DAWBIN

Fig. 44. Age frequency of S. attenuata based on the dentine layers. Open circles
and dotted line indicate females, and closed circles and solid line males. Crosses
show the total of both sexes including animals of unknown sex. Straight line wascalculated by least squares for all animals of more than 12.5 years of age.

TABLE 22.

No. of ovulations
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
No. of the mature

FREQUENCIES OF NO. OF OVULATIONS AND OF AGE
CALCULATED FROM NO. OF OVULATIONS.
School no.

Calculated
age
8.2
10.5
12.8
15.0
17.3
19.6
21.8
24.0
26.2
28.4
30.5
32.6
34.8
36.9
39.0
41.1
43.1
45.2

Total
22
1
9
4
4
5
6
3

2

3

4

5

6

64
4
2
5
5
2
3

5
2

15
3
2
4
3
5
3
2

5

13

2

4
4

5
5
3

7

3
3
2

1
2

2

5
2
2
2

5
2

1
2
2

2
35

29

20

2
36

6

31

13

124
11
22
23
16
15
22
15
7
4
5
7
5
4
5
3
3
2
170

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

211

the ages corresponding to 2 or more ovulations are shown below.
µ = 0.07705

M = 0.07416

The catch of S. attenuata by means of the driving method is, on the coast of lzu
Peninsula, sporadic and small in number (Table 23). The catch of school no. 7 at
Taiji by the same method was the second case at least in recent several years.
Though the tuna seine is operated in the western North Pacific, as schools of tunaare usually found by the floating wood or flocks of birds in these waters, the catch

1 ~~~~~-'-'--'-'--'-'--'-L..l-1-1-L.L.L.Jl-LJ'--'-l_L..l__l....l...L.L..L.W
8~
IB
W ~ W ~ E~"

Age in years
Fig. 45. Age frequency of female S. attenuata calculated from the number of ovulations and estimated accumulation rate of corpora. Line was calculated by leastsquares excluding a point of the youngest animal.
TABLE 23.
Year
Drivingno. of school
no. of dolphin
Harpoon2>

NUMBERS OF S. ATTENUATA CAUGHT AT IZU AND KII PENINSULA.

1963

'64

'65

'66

'67

'68

'69

'70

'71

'72

'73 1>

1
37

l

470

0
0

2
322

2
373

0
0

437

7
2907
14

0
0
37

4
660
12

5+
1366+
17+

1963-1968: Tobayama (1969), 1l : Includes 206 animals in two schools caught at Taiji. 2l : Taiji.

of S. attenuata by tuna boats seems to be negligible (Ohsumi, 1972, Dr. Suda, personalcommunication). Catch by means of hand harpoon is operated off Taiji on the
coast of Kii peninsula. But the number caught is small.
Accordingly, the population of S. attenuata in the western North Pacific is considered to be close to virgin stock. In the stationary virgin stock, there must be an
equation shown below.
S·r- 1 ·f·l·e-µ't = S·(l-e-µ)where S is the number of sexually mature females, r length of reproductive cycle inyear, f ratio of females at birth, 1 the mean litter size, t the age at the attainment ofsexuall maturity, µ' the mean mortality coefficient before the attainment of sexual
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

212

KASUYA, MIYAZAKI AND DAWBIN

maturity, and µ the mean mortality coefficient of sexually mature females. Assuming t=8, r=50.3 months (=4.19 years), l=l.034, and f 0.3894, the following
values are obtained.
µ' = -0.14312, and M' = -0.13335, when µ = 0.3545µ' = 0.03240, and M' = 0.03188, when µ = 0.07705.

The values at the top are unreasonalbe, indicating that the mortality coefficient,µ=0.3545, obtained from the age frequency based on the dentine growth layer is toohigh. This is easily expected because the age of physically matured animal is often
underestimated by this method. Accordingly, one of the estimates of the meanmortality coefficient of immature females is 0.03240 when that of mature females
is 0.07705. The mean length of reproductive cycle 50.3 months used here is the best
estimate. Even when the less reasonable under- or over- estimates 43.8 months or
54.3 months are applied, the mean mortality coefficients of immature females are
0.04964 or 0.02264 respectively and still certainly lower than that of mature females.
Possibly the best estimate of the mean mortality coefficients of immature females
seems to be between the above two estimates or 0.023 and 0.049. This is about
half of the value for mature females.
The rough estimate of the morality of the male is calculated from the mortalityof female and the sex ratio at the age t. As m- µ=0.08348 was obtained in page
209, the mean mortality coefficient of the male at the age between 0 and 8 years is
m = 0.08348+0.03240 = 0.12588

and that at the age over 8 years is
m = 0.08348+0.07705 = 0.16053

The corresponding annual mortality rates are 0.118 and 0.148 respectivelty.
The present result shows that the mortality of immature animals is lower
than that of sexually matured animals. Brodie (1969) suggested that the long lactation of toothed whales will be effective as the period of education. This seems tobe true in S. attenuata. This species starts feeding on solid food at the age of about
3/4 years, but it continues to take both solid food and milk at least by the age of 1.5
years. This weaning period seems to work as the education or training period of thecalf, and work to lower the mortality of the young calf. The immature animals ofboth sexes aggregate after weaning, to have probable cooperative life. Of course
other than these special mode of life, the life of immature female free from thereproductive activity must keep the mortality lower than that of sexually mature
females.
The probability of survival in Fig. 46 and Table 24 is drawn based on thelowest mortality coefficients in the above and assuming 8 years as the age at theattainment of sexual maturity. It indicates that the number of females at attainment of the maturity is 83.4 % of that at the birth. The mean life expectancy of
newborn females is 14.6 years, and that of sexually mature females is 17.0 years.
The probable number of gestations experienced by a female is obtained by inSci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

213

tegrating the relation between fertility rate of ova and number of ovulations (x) in
Table 16.

J~ (-0.01923x+0.6643)dx
The known highest number of ovulations, 18, suggests 8.3 gestations in the past.
The number of gestations, corresponding to the mean number of ovulations of adultfemales, 6.16, is 3.2.
The number of female calves (Rt) produced annually by the mature females(St) of the age t is shown by the following formula, assuming the start of conception
at the beginning of the year.
Rt = St ·r;;-1 • l ·f. P11.2112

to
0.8

()'
~ 0.6
:J
0-

~ 0.4
ll0.2

0

.... . . . . . . . . . . . . . .
0

8

4

12

16

20

24

28

32

36

40

Year after birthFig. 46. Calculated survival and reproduction in the female of S. attenuata.
dots at the bottom indicate the annual recruitment.

Small

TABLE 24.

LIFE TABLE OF FEMALES. ATTENUATA AT SELECTED AGES.

Year
after
birth

Probabilityofsurvival1 1

Total
ouvlation

Annualovulation
rate

0
4
8
12
16
17
20
24
28
32
36
40
44
48

1.00000.91340.83430.61300.45040.41700.33100.24320.17870.1313
0.09650.07090.05210.0383

0
0

0
0
0.432
0.437
0.443
0.445
0.449
0.455
0.461
0.467
0.473
0.480
0.486
0.492

2.7
4.5
4.9
6.3
8.1
9.9
11.8
13.7
15.6
17.5
19.5

Fertility
ratio ofovum

Ratio ofpregnant
femaleAnnualTotal
Reproductive cycle, reproduc- reproduc2
tion
tive rate >
year

0.645
0.612
0.578
0.570
0.543
0.509
0.474
0.437
0.401
0.364
0.3'32
0.290

0
0
0.253
0.243
0.232
0.230
0.222
0.211
0.201
0.189
0.178
0.167
0.156
0.145

0
0
0.10680.09840.09080.08930.08430.07870.07380.06920.0652
0.0616
0.05840.0553u lt=e-o.022Mt, 1 ;;;;t;;;;8. lt=0.834335e-o.onorn-sl, t;s8.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

2l RtfSt.

3.51
3.81
4.13
4.20
4.44
4.76
5.08
5.41
5.75
6.09
6.42
6.77

0
0
0.089
0.370
0.560
0.597
0.690
0.778
0.839
0.881
0.910
0.930
0.944
0.954

214

KASUYA, MIYAZAKI AND DAWBIN

where p is the annual survival rate, rt the length of reproductive cycle in year atage t, and 11.2/12 is the length of gestation in year. Other symbols are same withthe formula in page 211. rt is calculated from the relation between the number ofovulations and age (page 206), and that between length of reproductive cycle andnumber of ovulations (Table 16).
The annual recruitment thus calculated, and the total recruitment are shownin Fig. 46. Though the total recruitment must be 1 when the probability of survival
is 0, lt of the formula of survival, lt= 10e-µt, can be 0 only when t is infinite. In
Fig. 46 the probability of survival at the age 48 is 3.83 %, but the probability of total
recruitment is 95.40 %, and the recruitment seems to be slightly lower to keep the
population stable. But as this model is based on several assumptions, the above
defect is considered to be negligible at present.
If the population is depleted as the result of exploitation, the total reproductive rate ~Rt/~St will increase as the results of the increase of the ratio of young
mature female with high reproductive potential, and of the shortening of reproductive cycle of each age classes. The increase of the ratio of the female in the newborn
animals is also expected. This change of sex ratio works to increase the reproductive rate.

SCHOOLING BEHAVIOR

School size
The information of the number of dolphins constituting one school of S. attenuata
was obtained from the records of the catch by the driving method in the fishermen's
cooperative unions at Arari, Futo, Kawana, and Taiji. The records cover the years
from 1963 to 1973. As fishermen hope to catch all the animals they find, they may
sometimes drive to combine schools found at close distance. However, it is also
probable for them to lose a part of the school. However these problems are notdiscussed here, because the information at the time of driving is insufficient and thedefinition of a school is also unsatisfactory.
Table 25 shows the frequency of the size of the school. The smallest school
was composed of 37 animals, and the largest, 1,381 animals. The mean school size
is 281.0 animals. There are 4 modes observed in the frequency, or from 37 to 98,
from 120 to 437, from 509 to 617, and 1,381. The modal means are shown in thetable and they suggest that the basic unit of the school is less than 100 animals, andthat larger schools might be formed as the result of annexation of the smaller schools.
Immature animals
The age frequency of 7 shcools shows that the number of juvenile animals of
the age between 2 and 7 years is few (Figs. 44 and 4 7). It is lowest at the age of
3.5 years (male) or 4.5 years (female). Though we had no chance to confirm it, it
is suggested that the calves of the above age classes may have left the school of themother as in the case of S. coeruleoalba (Kasuya, 1972). The age when they leave theSci. Rep. Whales Res. Inst.,
No. 26, 1971:

215

STENELLA ATTENUATA

25
20
15
10
5
(/)

c

:c

6

.9-10
0
"'O 5

'5

5
.. o..,,,

d

z

4

5

3
2

5
0

1

2

3

5.5

105

15.5

205

Age in years
Fig. 47. Age frequency of S. attenuata by schools and sexes based on dentine growthlayers. Open circles and dotted lines indicate females, and closed circles andsolid lines males.

TABLE 25.

SCHOOL SIZE FREQUENCY OF S. ATTENUATACAUGHT BY DRIVING METHOD 0 •School size

No. of schools

Percent

1- 24
25- 4950- 7475- 99100- 124
125- 149
150- 199
200- 299
300- 399
400- 499
500- 599
600- 699
1300-1399
Total

0

0
4.8
9.5
4.8
4.8
4.8
23.8
19.0
9.5
4.8
4.8
4.8
4.8
100

2

I

5
4
2

l

21

Mean

)

65.5

)

235.9

}

563.0
1381. 0lJ 4 schools reported by Tobayama (1969) and Nishiwaki (1965)
included.

Sci. Rep. Whales Res. Inst.
No. 26, 1974

216

KASUYA, MIYAZAKI AND DAWBIN

mother's school is not directly known, but it may be soon after the weaning. It was
indicated from the length of lactation that calves of S. attenuata are weaned at theage between 17 and 60 months with an average of 29.3 months (2.44 years). As
the latter age is slighly before the age of lowest frequency, the above suppositionis supported. The age when the calf leaves the mother's school might be slightly
earlier in males than the female.
After the age of 3 or 4 years, the number of immature animals in the breeding
school increases. In females, the age of 4 years corresponds to the time when theweight of left ovary start to exceeds that of the right indicating the approach of the
puberty. And in males the age of 3 years is the time when the production of spermatozoa, an indication of the arrival of puberty, is started in many animals. Accodingly it is highly probable that the calves of both sexes which have left the mother's
school gradually return to the school of mature animals after reaching puberty,
and all the animals will return by the attainment of sexual maturity. However
some of the few calves which are accompanied by the lactating female for more than
4 years might attain puberty or sexual maturity in the same school.
The school of immature animals of S. coeruleoalba reported by Kasuya (1972)
was composed of the males between 1.50 years and 12.5 years of age (228 samples),and of the females between 2.75 years and 7.75 years of age (62 samples). The
modal age of that school was 4.5 years in males and 3.5 years in females. This feature strongly resembles the schooling behavior of juvenile animals of S. attenuata
presumed here.
The school of mature females usually contains puberal animals in it. The
only exception among 7 schools is school 5. In this school the animals of the age
corresponding to the puberal stage are only 2 animals. As the age determination
was made on only one third of the total number of the animals in the school, it is not
TABLE 26.

PRESENCE OF SPERMATOZOA IN TESTIS SMEAR.
Testis weight (g)School
no.

3

4

6

Spermatozoa

(
(
(

none
few
mediummanymean weight (g)nonefew
mediummanymean weight (g)nonefew
mediummanymean weight (g)

0-5.9
3

6-67.9
1
6

68~

5
3

4.8

10

2.2

23.4

281.9

8
7

2
9
17
195.6

17.3
17

18.1

7
6
2
158,6
Sci. Rep. Whales Res. Inst.,.No. 26, 1971

STENELLA ATTENUATA

217

conclusive at present as to whether there is a school of mature females which does
not contain puberal animals. However it is highly probable that the connection
between puberal animals and mature females is less strong and the former form
smaller groups when the school is dispersed or leave the latter school to join with aschool of immature animals.
If the sexual activity of the puberal males, or males from 6 g to 67.9 gin testis
weight, is shown by the density of spermatozoa in the testis smear, the activity is highin schools 2, 3 and 4 (Table 26). And a parallel relationship is observed between
the sexual activity of puberal males and that of mature males indicated by smears orby the mean weight of testes. However it was not conclusive as to whether it is related with the reproductive cycle of the male or with the schooling behavior.

Mature female
The age frequency of mature females in each school does not show the predominance of a particular age class, but seems to be evenly distributed and affected onlyby the mortality (Fig. 47).
As shown in Table 27, the breeding school of S. attenuata always contains bothpregnant females and lactating females. But the ratio of the latter to the former
varies widely between schools. The lowest ratio about 1.0 is found in the schools
I and 5, and the highest ratio 6.4 in the school 4. Most of the mature females inschool 4 were in lactating or resting stage. The abundance of resting females ismore variable from 0 in schools I and 5 to 3.5 times of the pregnant females in school

7.
The history of the reproductive activity of the females in each school is known
from the length frequency of the juvenile calves and of fetuses (Fig. 48). The simplest example is in school 6, where the mating peaks in the past are in March, January, and August (from left to right in Fig. 48). The intervals between the peaks
are 17 months and 12 months respectively. The feature of school 1 are very similar
to that of school 6, except for a small peak in October. The intervals between
the peaks in the school I are 16, 4.0, and 10.5 months.
School 2 shows complicated features. The peaks of conception are in February, late May, October, January, May, August, July, and October. The intervals
are 3.5, 4.0, 3.5, 4.0, 3.0, 11.0, and 3.0 months. Possibly the schools 4 and 5 belong
to a similar pattern with the approximate interval of the peaks of 4.0, 4.5, 4.0, and
9.0 months for school 4, and 4.5, 3.5, and 11.0 months for school 5. School 7 has
no small fetus, but the arrangement of mating peaks in schools 3 and 7 is notlargely different from that of school 2.
It is already suggested that S. attenuata in the Japanese coastal waters has 3mating seasons. If all the mature females stay in the same school for their life time,
there should be observed the peaks of 3 mating seasons in one school because thelength of reproductive cycle is highly variable between animals and it is difficult
for all the females to conceive synchronously in only one of the 3 mating seasons.
No such case was observed in any of 8 schools studied by us or by Nishiwaki et al,(1965). This strongly suggests that the member of mature females in any shcool is
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KASUYA, MIYAZAKI AND DAWBIN

218

IV

VII

X

SJ,

z
3
4

=
n

Q--,
6

0

VII

X

IV

VII

X

I

IV

Vll

X

IV

x

VII

Month
Fig. 48. Frequency of date of conception (scale at the top) and of parturition(scale at the bottom). Dotted areas indicate the frequency obtained from the bodylength of postnatal animals, white from that of fetuses, and dotted lines cited from
Nishiwaki et al (1965). Numbers indicate the school.

TABLE 27.

RATIO OF REPRODUCTIVE CONDITIONS OF FEMALE S. ATTENUATA.
MatureDatekilledImmature* *25, Oct.
2
3SchoolTotal***

unknown

pregnant

p. & I.

lac tating

resting

total*

24
33.3%

12

15
41. 7

3
8.3

18
50.0

0
0

36
100

72

15, Nov.

67
42.7%

4

19
22.1

2
2.3

47
54.7

18
20.9

86
100

157

4, Nov.

5
20.0%

0

4
20.0

0
0

15
75.0

20
100

25

5.0

4

13, Nov.

15
26.3%

3

3
7.7

1
2.6

28
71.8

7
17.9

39
100

57

5

18, Nov.

5
33.3%

2

4
50.0

0
0

4
50.0

0
0

8
100

15

6

12, Feb.

13
18.8

8

14
29.2

0
0

21
43.8

13
27.1

48
100.1

69

7

2; Jui.

3
16.7%

2

2
15.4

0
0

4
30.8

7
53.8

13
100

18

132
32.0%

31

61
24.4

6
2.4

137
54.8

46
18.4

250
100

413

Total

* : Excluding the unknown.
* * : Percentage of total.

%***: Excluding the maturity unknown.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

219

STENELLA ATTENUATA

not fixed but some of them can be exchanged between schools.
The interval between two recent mating peaks is usually long, but the intervals
obtained from length frequencies of postnatal individual is often appears short.
This seems to indicate that the oestrous females assemble together to conceive as suggested in S. coeruleoalba (Kasuya, 1972), and this connection is retained during theperiod of gestation. However after parturition the connection becomes weakerwith the elapse of the time, and some of the lactating females may move to anotherschool accompanying the calf in some case. Possibly the shift of the lactating females between schools is related to the cycle of postpartum or postlactum ovulations.
The schooling behavior of resting females is probably similar to the late lactating
females, but the ratio of the between school movement will be much higher.
The frequency of interval between the two mating peaks in the present 7 schools
has 4 modes with the modal means of 3.8 month (13 examples), 8.1 months (4),11.2 months (4), and 16.5 months (2). These modal means are a multiple of 4months which is the interval of mating season of this species. If the combination of
the peaks of conception observed in the present schools are formed through therandom combination of each peaks, the frequency of each mode are expected tolie on the straight line shown in Fig. 49. However, the above data shows a higher
frequency of the 4 months interval. This means that the neighbouring two matingpeaks are not completely independent. Probably the females which failed to conceive in a mating season may often stay in the same school to conceive in the next
mating season. This case is represented by a smaller peak next to the main peak
TABLE 28.

MATURITY FREQUENCIES AND SEX RATIO OF MALE S. ATTENUATA.
Testis weight in g

Schoolno.

Datekilled25, Oct.
2

15, Nov.

34, Nov.

4

13, Nov.

518, Nov.

6

12, Feb.

7
Total

2, Jui.

unknown

Total

Sex ratioof themature

imm.*

mat.*

5.9;?;

6.067.9

16
34.0%
49
48.5%
1
4.3%
11
19.0%
8
100 %
2
4.0o/o
7
41.2%
94
30.9%

21
44. 7

0
0
4
4.0

2
4.3
20
19.8

8
17.0

3

50

0.605

11

7

108

0.311

0

23

0.450

58

0.762

0

8

0

12
24.0

30
51. 7
0
0
15
30.0

0

0

50

0.286

2.0
0
0
41
13.5

0

17

0.067

5.9
21
6.9

9
39.1
15
25.9
0
0
20
40.0
8
47.1
74
24.3

9
39.1

0
0

4
17 .4
0
0
0
0

10

314

0.409

17
16.8
0
0
2
3.4

* : Maturity of testis was identified by eye on the beach.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

68;£

10.9

5.9
74
24.3

220

KASUYA, MIYAZAKI AND DAWBIN

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8

10

12

14

16

Interval in months
Fig. 49. Frequency of the interval of mating peaks in the 7 schools. Open circles
and dotted line indicate the total frequency in each mode plotted against themodal mean of the interval. Straight solid line shows the frequency expected whenmating peaks are independent.

Sum

50
40
30
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Diameter in mm

10

Fig. 50. Diameter of the largest Graafian follicle. White spots indicate pregnant
females, black spots the lactating, and lines the resting. Schools are shown by
the number at right.

observed in schools 2 and I. In some cases, oestrous females from other schools
exceeding in number the oestrous females in the original school, may join to thisschool. This is suggested by the 6th peak (from the left) of school 2, where thepreceeding 5th peak is smaller. Of course there can be the case where all females
which should have constituted the subsidiary peak shift to another school as sugSci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

221

gested by the long intervals of peaks.
The origin of other peaks of short interval or of peaks of long interval might be
the result of the annexation of schools or random exchange of members between two
or more schools. The relative abundance of the 8, 12, and 16 months interval ofpeaks of conception decreases at the rate expected in random combination.

A1ature males
As the diameter of seminiferous tubules and the weight of testis show a linear
relationship, the weight of testis of mature male can be used as an indicator of re-

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Weight of testis (g)Fig. 51. Relation between the mean weight of testis of mature animal and mean
diameter of Graafian follicle of the female in a same school. Large closed circle
indicates the follicle diameter of resting female, open circle that oflactating female,and small closed circle that of pregnant female. Schools are shown by the numbers .

.

a~-~-~4-~-~-~-~-~~,,.....

,
.6

o·6
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----.'~
0'---~--'----'---~-~-~-~~

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1

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5

6

Diameter of follicle in mm
Fig. 52. Relation between the mean diameter of Graafian follicles of restingfemales and the sex ratio of matureindividuals in the same school. Schools
are shown by the number. The dottedline indicates the highest and lowest
range.

0 '--2~00-'~30~0_...~40~0__,_~50~0--L.-6~0-o~-7~0~0~

Weight of testis(g)Fig. 53. Relation between sex ratio of
mature animals and mean weight oftestis of mature males in the corresponding school. Schools are shown by
the number.

productive activity. The mean weight of testis of mature male was compared withthe mean diameter of the largest follicle of the mature females caught in the same
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

222

KASUYA, MIYAZAKI AND DAWEIN

school (Fig. 51 ). The mean weight of testis shows a positive correlation with thediameter of the follicle of resting females. As resting females with large follicles
are ready to ovulate, it is reasonable to expect males of high activity in the sameschool.
The sex ratio of mature individuals is small in schools containing resting femaleswith large follicles (Fig. 52). A similar but clearer tendency is observed when it
is compared with the mean weight of testis of mature males (Fig. 53). Though it
is not shown in Figs. 51, 52 and 53, school 5, which does not contain resting females nor lactating females with follicles larger than 1.0 mm in diameter and is considered to be in low mating activity, it has no sexually mature male.
These results suggest that in schools at the beginning of the approach of heat
many mature males may assemble in the same school, but at the peak of the heatmost of them except a few males of high activity leave the school. It is highly possible that there is intrasexual aggression between the males of S. attenuata as in thecase of another delphinidae species Globicephala melaena suggested by Sergeant ( 1962).

Fig. 54. Schematic figure showing one of the possible between school exchange ofmature females. Arrows indicate the annexation or the split of the school. White
spots indicate pregnant females, black spots the lactating, lines the resting. Mating
seasons in each 4 months inteval are shown by vertical rods.

Discussion
It is well reported in several species of delphinidae that their school is compact
in some cases, but in other cases it is composed of many smaller schools dispersed ina wide area. Sergeant (1962) and Saayman and Tayler (1973) suggested that it
will be related to feeding or sexual behavior of the animal. Our observation on alarge school of S. coeruleoalba, found in the evening of 24, Oct. 1972 at 34°52' N,
141°05' E, showed that it covered a wide area of about 21 km in one diameter andcomposed of numerous small groups of dolphins. This smaller groups had a distanceof about 3 km to 15 km from each other, and were composed of 10 to 200 individuals.
Some of them contained calves accompanied by the mother and several were composed mostly of smaller animals. This suggests that the larger schools of S. coeruleoalba contains smaller units.
It is probable that several small units leave the main larger school after whicha larger dispersed school again forms a compact school, or that the smaller units
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STE.NELLA ATTE.NUATA

223

move to an other school when the two groups come to a close distance. Fig. 54
was drawn on the assumption that the exchange or shift of the members between
schools can occur through the procedure mentioned in the above. In this figure,though the composition of the reproductive condition at both ends of the diagram
is similar, the members are largely different. The immature animals are not shown
because their behavior is simple, the mature male was also excluded as its behavior
is not well understood.
ACKNOWLEDGMENTS

The collection of data and samples were made through the cooperation of thefishermen's cooperative unions at Kawana, Futo, and Taiji. We had the help of
Dr. T. Kajihara of the Ocean Research Institute and of Mr.]. H. Perano in collecting smaples. Dr. W. F. Perrin of the National Marine Fisheries Service kindly readthe draft. Dr. K. Tatsukawa of the Ocean Research Institute, and Dr. A. Ohsumi
and Dr. A. Suda of the Far Seas Fisheries Research Laboratory gave us valuablesuggestions. This study was technically assisted by Miss. S. Wada and Miss. K.
Nagayama and Mr. K. Kureha. These persons are deeply acknowledged.
REFERENCESAsDEL, S. L., 1964. Patterns of mammalian reproduction (2nd ed.), 670 pp. Constable & Co., London.
BEST, P. B., 1969. The sperm whale (Pftvseter catodon) off the west coast of South Africa 3. Reproductionin the male. Invest[. Rep. Div. Sea Fish. S. Afr., 72: 1-20.
BRODIE, P. F., 1969. Duration of lactation in cetacea: an indicator of required learning? Amer. Midland.Naturalist, 82(1): 312-314.
CALDWELL, D. K. and M. C. CALDWELL, 1966. Observation on the distribution, coloration, behavior andaudible sound production of the spotted dolphin, Stenella plagiodon (Cope). Contribution in Science, 104:
1-28, Los Angeles County Museum.
DAWBIN, W. H., 1966. Porpoise and porpoise hunting in Malaita. Australian .Natural History, 15(7):207-211.
EVANS, W. E. and J. BASTION, 1969. Marine mammal communication: social and ecological factors. The
Biology of Marine Mammals (ed. Andersen, H. T.), p. 425-475. Academic Press, New York and London.
FRASER, J. F. D. and A. ST. HUGGETT, 1973. Specific foetal growth rates of cetaceans. J. :(,ool., Landon.
169: 111-126.
GRAY, J. E., 1846. Zoology of the voyage qf H.M.S. Erebus and Terror, I (Mammalia): 44, pl. 28.
HARRISON, R.J. and S. H. RIDGWAY, 197 I. Gonadal activity in some bottlenose dolphins (Tursiops truncatus).
J. :(,ool., Land. 165: 355-366.
HIROSE, K. and M. NISHIWAKI, 1971. Biological study on the testis of the Blue white dolphin, Stenella
caeruleoalba. ]. Mam. Soc. Jap., 5(3) :91-98.
KAsUYA, T., 197 I. Consideration of distribution and migration of toothed whales off the Pacific coast ofJapan based upon aerial sighting record. Sci. Rep. Whales Res. Inst., 23: 37-60.
KASUYA T., 1972. Growth and reproduction of Stenella caeruleoalba based on the age determination bymeans of dentinal growth layers. Sci. Rep. Whales Res. Inst., 24: 57-79.
LAws, R. M., 1959. The foetal growth of whales with special reference to the fin whale Balaenoptera physalus
L. Discovery Rep., 29: 281-308.
MIYAZAKI, N., T. KAsUYA, AND M. NrnmwAKI, 1974. Distribution and migration of two kinds of Stenellain the Pacific coast of Japan. Sci. Rep. Whales Res. Inst. 26: 227-243.
Sci. Rep. Whales Res. Inst.,.No. 26, 1974

224

KASUYA, MIYAZAKI AND DAWBIN

MizuE, K. and K. YOSHIDA, 1962. Studies on the little toothed whales in the west sea area ofKyusyu-lX,
about Prodelphinus sp., so called "madara iruka" in Japan caught at Arikawa in G:lto Is., Nagasaki
Pref. Bull. Faculty of Fish. Nagasaki Univ., 13: 1-8.
NAKAJIMA, M., K. TAKAHASHI, M. 0GURA, AND K. SAWAURA, 1963. On the growth of infants of the small
size toothed whales. ]. Jap. Ass. :(,ool. Gardens and Aquarium, 5(1): 16-22. (in Japanese).
NISHIWAKI, M., M. NAKAJIMA AND T. KAMIYA, 1965. A rare species of dolphin (Stene/la attenuata) from
Arari, Japan. Sci. Rep. Whales Res. Inst., 19: 53-64.
0HsUMI, S., 1964. Comparison of maturity and accumulation rate of corpora albicantia between the left
and right ovaries in cetacea. Sci. Rep. Whales Res. Inst., 18: 123-148.
0HsuMI, 1972. Catch of marine mammals, mainly of small cetaceans, by local fisheries along the coast ofJapan. But. Far Seas Fish. Res. Lab. (Shimizu, Jap.), 7: 137-166.
PERRIN, W. F., 1969. Color pattern of the eastern Pacific spotted porpoise Stene/la graffmani (Cetacea, Delphinidae). :(,oologica, 54(4): 135-151.
PERRIN, W. F., 1973. Variation and taxonomy of spotted and spinner porpoises (genus Stenella) of the eastern
tropical Pacific and Hawaii. Dissertation Abstracts International, 33(10).
PERRIN, W. F. AND E. L. ROBERTS, 1972. Organ weights of non-captive porpoise (Stenella spp.) Bull. Southern
Calif. Acad. Sci., 71 (1): 19-32.
SAAYMAN, G. S. AND C. K. TAYLER, 1973. Social organization of inshore dolphins (Tursiops aduncus andSousa) in the Indian Ocean. ]. Mamm., 54(4): 993-996.
SERGEANT, D. E., 1962. The biology of the pilot or pothead whale Gtobicephala melaena (Trail) in Newfoundland waters. Bull. Fish. Res. Board of Canada, 132: 1-84.
SERGEANT, D. E., D. K. CALDWELL, AND M. C. CALDWELL, 1973. Age growth, and maturity of bottlenosed dolphin (Tursiops truncatus) from northeast Florida. ]. Fish. Res. Board Qf Canada, 30(7): 1009-1011.
TAVOLGA, M. C. AND F. S. EssAPIAN, 1957. The behavior of the bottlenosed dolphin (Tursiops truncatus):
mating, pregnancy, parturition, and mother-infant behavior. :(,oologica, 42(1): 11-31.
TosAYAMA, T., 1969. School size and its fluctuation in the catch of Stenella coeruleoalba in Sagami Bay.
Geiken Tsushin, 217: 109-119. (in Japanese).
YABLOKOV, A. V., 1963. On the type of color of cetacea. Bull. Moscow Sci. Experimentation in Nature, Biol.
Sec., 68(6): 27-41, (in Russian with English summary).

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

STENELLA ATTENUATA

EXPLANATION OF PLATESPLATE I
Fig. I. S. attenuata, female, 118 cm in body length. Pigmentation of newborn stage.
Fig. 2. S. attenuata, female, 178 cm in body length. Dark spots have started toappear only on the !ewer jaw, and the white area on the tip of jaws is also started.
The both characters are classified into stage I.
Fig. 3. S. attenuata, data unknown. The white tips of jaws are classified into stage
2, and the dark and white spots into stage 3.
Fig. 4. S. attenuata, male, 209 cm in body length, same animal with Fig. 4 in Plate
II. The white tip of jaws are classified into stage 2, white dorsal spots into stage 2,
and dark ventral spots into stage 3.
Fig. 5. S. attenuata, adult male. The white tip of upper jaw is classified into stage 3,that of lower jaw into stage 2, white dorsal spots into stage 2, and ventral dark
spots into stage 3.
Fig. 6. S. attenuata, dorsal view of left flipper of juvenile animal, showing the dark
spots on the surface.
PLATE II
Fig. I. S. attenuata, female, 117 cm in body length. Showing the pigmentation of
newborn animal. An animal in the other side is also newborn calf.
Fig. 2. S. attenuata, female, 189 cm in body length (center). The conditions of two
kinds of spots are classified into stage 3, and the white areas at the tip of jaws
into stage 1. An animal at this side is a juvenile male.
Fig. 3. S. attenuata, male, 181 cm in body length. Ventral white spots are classified
into stage 3, and other characters into stage I.
Fig. 4. S. attenuata, male, 209 cm in body length, ventral view. Same animal with
Fig. 4 in Plate I.
Fig. 5. S. attenuata, ventral wiew of an adult male.
PLATE III
Fig. I (top). Underwater photograph of a group of adult or subadult S. attenuata inthe school 5 enclosed in the harbor, showing the conspicuous white tip of beak.
(Photographed by N. Miyazaki)
Fig. 2(bottom). Mother and suckling calf in the same school, showing the differenceof the pigmentaticn of ventral region. (Photographed by N. Miyazaki)
PLATE IV
Tooth section of S. attenuata. photographed under transmitted light.
Fig. I. 104 cm, male, killed on 12th, Feb. A thin translucent layer is accumulated.
Less than 1/4 year.
Fig. 2. 112 cm, male, killed on 18th, Nov. A thick opaque layer is accumulated.
About 1/4 year old.
Fig. 3. 135 cm, male, killed on 12th, Feb. A thin opaque layer and a thick tramlucent layer of full thickness are accumulated. About 3/4 year old.
Fig. 4. 175 cm, female, killed on 13th, Nov. A thin opaque layer and 2 opaquelayers of full thickness are accumulated. About 2 1/4 years old.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

225

226

KASUYA, MIYAZAKI AND DAWEIN
PLATE V
Fig. !. 166 cm, male, killed on 2nd July. 3 opaque layers are accumulated. The
both first and last layers are thin translucent layer. About 2 3/4 years old.
Fig. 2. 177 cm, female, immature, killed on 2nd, July. 6 opaque layers are accumulated. The first and the last layers are the thick opaque layer. About
5.5 years old.
Fig. 3. 205 cm, female, two corpora in the ovaries, killed on 2nd, July. 11 opaquelayers are accumulated and the pulp cavity is still wide. About 10.5 years old.
Fig. 4. 195 cm, female, 11 corpora in the ovaries, killed on 13th Nov. 24 opaquelayers are observed, and pulp cavity is filled with secondary dentine. Real age
will be more than the number of the opaque layers.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

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