GROWTH AND REPRODUCTION OF STENELLA CAERULEOALBA BASED ON THE AGE DETERMINATION BY MEANS OF DENTINAL GROWTH
LAYERSTOSHIO KASUYA*ABSTRACT

Growth and reproduction of Stene/la caeruleoalba caught on the Pacific coastof Japan in October to January were studied based on the annual growthlayers in dentine. The mating seasons are in May and June, and in November
and December. Calves are born after 12 months gestation at 100 cm in bodylength. They attain sexual maturity at 9 years at the lengths 212 cm infemales and 220 cm in males, and physical maturity at 14 to 15 years at thelengths 222 cm in females and 236 cm in males. Lactation lasts about 1.5
years. Mean reproductive cycle is about 3 years or slightly less. Oestrous
females assemble to form a school, which is retained at least for one reproductive cycle but can join with the schools in other growth and reproductive
stages of both sexes.

INTRODUCTION
The dolphin fishing was formerly operated in the several villages along the coast of
the Izu Peninsula, which situates in the Pacific coast of Ja pan (Japanese Fisheries
Bureau 1900). But in most of these places this fishing was abolished before the world
war II.
Though, until few years ago Arari on the west coast of the Izu Peninsula
operated this fishing for Stenella caeruleoalba (Meyen, 1833) and other delphinids inspring season (Nishiwaki and Yagi 1953, Nishiwaki et al. 1965), the operation haverecently ceased. At present, only Futo and Kawana on the east coast are cooperatively operating this fishing with several speed boats.
The fishing season for these two villages usually starts in the middle of Octoberand closes in December, but few catches have been made in January and early October. The catches are mostly Stenella caeruleoalba with occasional catch of Stenella
attenuata and other delphinids (Tobayama 1969). According to the private communication of Mr. N. Miyazaki, the mean annual catch of Stenella caeruleoalba in theboth villages is about 4980 individuals in recent 3 seasons from 1968-'69 to 1970-'71,
when the catch was regulated by the fishermen to control the market price.
The schools of Stenella caeruleoalba are found mostly in the waters off the south
east to north coast of Oshima Island, and then chased into the Kawana harbor or
the Futo harbor (Tobayama 1969). There are few catches in the west coast of the* Ocean Research Institute, University of Tokyo, Tokyo.
Sci. Rep. Whales Res. Inst.,
No. 24, 1972, 57-79.

58

KASUYAisland. As shown in Fig. 1, there is a strong inflow of warm water in the south east
entrance of the Sagami Bay. The migration of this species in the Sagami Bay and its
south east entrance seems to be related with this current.
This study was intended to obtain some informations on the growth andreproduction of Stenella caeruleoalba as a base of the population study.
141°E'--'~..J_...LJ....~.1..---'-----'=--=---'-~"'--'-----''---'---J.~"'--'-~'---'--'34°N

Fig. 1. Map showing the geography around Sagami Bay with the position of sightingof this species in Nov. 1971 (cross) and in Nov. 1970 (triangle). The surface
isothermal line is based on the observation in November 1971. Dotted line indicates 200 m line of the depth. A indicates Arari, F Futo, K Kawana, and otherwhite circles the position where dolphin fishing was formerly operated.

MATERIALS AND METHOD

Most of the materials used in this report were obtained from the 4 schools, schools
A, B, F, and Gin Table 1, processed at Kawana or Futo in 1967, '68, and '70.
School A, captured on 15 Nov. 1968 and processed at Futo, contained 1680 individuals, and 406 individuals were studied randomly. They were composed ofnewborn calves, juvenile, and mature individuals of both sexes, but lacking in thecalves from 110 cm to 140 cm in body length. Fetuses from 5 cm to 105 cm in bodylength were observed.
School B was caught on 16 Nov. 1968 and processed at Kawana. Among 319
dolphins constituted this school, 318 were studied. This school was composed ofjuvenile and mature individuals of both sexes ranging from 160 cm to 255 cm in bodylength, with an exception of a calf of 100 cm. No pregnant female was observed.
School F, caught on 26 Nov. 1967 and processed on 28 Nov. at Kawana, wascomposed of about 3,300 individuals, among which only 146 were studied randomly.
They were composed of newborn calves, juvenile and adult individuals ranging in
Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLATABLE I.

School
no.
A
B

c
D
E
F
G
H
ll

SOME INFORMATIONS ON THE SCHOOL COMPOSITION OF
STENELLA CAERULEOALBA

No.!) Sex ratio
No.
(male,
caught studied

%)

1,680
319
330
256
2.000
3.300
330
101

59

406
318
38
256
125
150
84
58
22

Ratio in mature female (%)

31.3
56.0

p
4 J.12)
0

34.0
41.7

2.0
23.3

p&l

r

Immaturity
in females
(%)

47.7
89.5

0.5
0

10.7
10.5

18.1
86.2

80.4
63.3

0
0

17.6
13.3

47.4
31.3

Fetus not included. 2 l 3 recently ovulated nonpregnant females included.
ing.
R resting. P&L pregnant and simultaneously lactating.

Date killed

16, 18, 22 Nov. '68
17 Nov. '68
21 Oct. '70
2 Dec. '70
22 Nov. '70
28 Nov. '67
14 Oct. '70
4 Oct. '71
19 Nov. '69
P pregnant.

L lactat--------·------··-----------------------80

n

70

'°
tll

:::J

60

"O

:~ 50

I

'O

11

s 40
0

... 30
QJ

-E 20
:::J

z 10
op...i.,.um..--~-...ai.mllm....-!J.,."-"lmlll.,.,L...Wlllmll...ll-

100

125

200
150
175
Body length in cm

225

250

Fig. 2. Body length frequency of the materials used in this study, schools A, B, F and
G are combined. Black indicates male and white female.

body length from 95 cm to 255 cm. But the dolphins of 150 cm to 195 cm were
scarce. Only three fetuses were observed.
School G was captured on 12 Oct. 1970, and processed and studied on 14 Oct.
Among 330 individuals constituted this school 84 individuals were studied randomly.
They were composed of juvenile calves ranging from 100 cm to 130 cm and from 165
cm to 200 cm, and adult individuals of both sexes ranging from 210 cm to 260 cm inbody length. Few fetuses were observed.
The body length was measured in each 1 cm interval in a straight line from thetip of upper jaw to the bottom of the notch of the tail flukes by two persons with atape measure. When the length was analized, each body lengths were grouped into
Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

60

KASUYAthe nearest 5 cm.
The mammary gland was observed in the field and classified into lactating andnon-lactating. Ovaries were collected from all the adult females and from most of
the immature females, and fixed with 10% formalin solution. The number of corpora albicantia and lutea was counted in the laboratory slicing the ovary into about 1
to 2 mm thickness. The both testis were collected from all the males and preservedwith same manner as used for ovaries. The weight was measured in the laboratory.
When a corpus luteum was observed in the ovary, the uterus was carefully searched
for small embryo.
Several teeth were collected with a hammer and a chisel usually from thecentral part of the upper tooth row, and fixed with 10% formalin solution.
For preparing the tooth for age determination, at first the connective tissue wasremoved with nife, then its half side was ground off perpendiculary with stones ofvarious granule. The polished surface was glued on a plastic board with synthetic
resin, then the other side was polished with the same manner to a thickness from 50
to 70 µ. The growth layers in dentine were observed under transmitted light withbinocular microscope ( X 20- X 50).
Other than the materials obtained from the schools A, B, F, and G, some tooth
samples and biological data collected at Kawana, Futo, and Taiji (33°35'N,
135°55'£) were presented by Dr. S. Ohsumi and Dr. K. Hirose. They are used inanalizing the growth curve. The informations on the body lengths of the fetuses
and juvenile calves of the schools C, D, E, and H were offered by Mr. N. Miyazaki,
and those of school I by Dr. K. Hirose. These body length frequencies are usedin analizing the reproduction of this species.
AGE DETERMINATION
The dentinal growth layers are used in this study as the age characteristics of Stenella
caeruleoalba. In the dentine of this species, as shown in Pls. I and II, there are observed the layers of opaque and translucent dentine arranged alternately. Though,the thickness of the layer varies at the positions on a tooth, the thickest is the layer
formed in the fetal stage. This fetal layer is usually composed of a opaque dentine.
But sometimes there are observed one or two faint laminated structure, on which no
study was conducted. Probably this will be a reflection of the physiological conditions of the mother which have affected the formation of the laminated dentine of themother. Though there is a neonatal line between the fetal dentine and the postnatal
dentine, this structure is inconspicuous in this species than in the sperm whale(Ohsumi et al 1963).
The structure, the thickness and translucency, of the postnatal dentine boundedon the neonatal line is variable between the animals. This will be related with thedate of birth and the season when the alternation of the opaque and translucent
layers occurs. There are usually observed one or two faint translucent layers in the
thick opaque layer accumulated just after the birth. But this fine structure was not
used for age determination, because it was not expected to show the annual accumulaSci. Rep. Whales Res. Inst.,
No. 21, 1972

GROWTH AND REPRODUCTION OF A STENELLA

61

tion cycle.
The thickness of the postnatal layers decreases with the age of accumulation,
especially the 7th or 8th and the latter layers are thinner. The dentine ofabout 15thor latter layers is composed of poorly calcified secondary dentine as in the case of
Globicephala melaena (Sergeant 1962). It is possible to read the laminations in thissecondary dentine with slight difficulty.
Though the boundary of opaque and translucent layers is usually not clear,
there is no indication to consider that there is significant difference of the thickness
between two layers.
Fig. 3 shows the seasonal change of the conditions of the newest dentinal growthlayer in the tooth with 13 or less layers, in which the layer is not too thin to makethe accurate observation. It is expected, from Fig. 3, that the alternation of dentinal
layer from the opaque to the translucent occurs in from November to December.

80

>.
u

60

c

QI

5- 40
QI

Lt

0 .
Oct.

.. ·.·.·.·.·.·.·.·.·. . . . . . . . ..

DecemberJan.
MonthFig. 3. The condition of the newest dentinal layers. The marks indicate thin opaquelayer, thick opaque layer, thin translucent layer, and thick translucent layer (fromtop to bottom). The numbers at the top indicate the sample size. The dates of thekill are grouped into the lst, 2nd, and 3rd decade of month.

NovemberIn this study it is presumed that the opaque layer is accumulated in summer
and the translucent in winter as in the case of Physeter catodon (Ohsumi et al 1963),
Tursiops truncatus (Sergeant 1959), and Globicephala melaena (Sergeant 1962).
For determining the age of the dolphins, it was tentatively assumed that theopaque and translucent layers indicate the growth in 6 months respectively. And
in the animals with the number of opaque layers 5 or less, the age was determined toeach nearest 1/4 years considering the conditions and thickness of the oldest (formed
just after birth) and the newest layer, in the animals with between 6 and 10 layers tonearest 1/2 years, and in the animals with more than 10 layers the number of the opaque layers was used as the approximate age of the animal.

Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

62

KASUYAGROWTH

Lengths and length frequencies
The length frequencies show characteristic features in each schools. This isconsidered to be resulted from the schooling behavior related with the maturity andreproductive cycle. Descriptions of the length frequencies of each schools areroughly made in the chapter of Materials and Method, and some discussions in thechapter of Reproduction.
When schools A, B, F, and G are combined, the smallest individual is found in
the length group of 95 cm, the largest female in 245, and the largest male in 260(Fig. 2). The highest frequency of the adult female is in the length group of 225 cm,
and that of male probably in 240 cm. The length of the adult mal~ seems to exceeds that of the female about 15 cm (see Growth curve).
No significant difference of length frequency between both sexes is observed in
the animal smaller than 175 cm.
250

.t:..

• •

'6i

I

=t •

c:

~

>-

-g 150

en

100
0

5

10

15

20

25

Age in year
Fig. 4. The mean growth curve of the female. One dentinal layer corresponds to one
year. Closed circle indicates immature female, open circle mature, and cross unknown. For explanation see text.

Growth curve
The relation between body length and the age in 218 females is shown in Fig. 4.
The growth is characteristic in the rapid increase of the length in the first 2 years.
The increase of the body length seems to stop at the age of 14 or 15 years. This age isconsidered to mean the time when most of the female individuals attain the physicalmaturity. As the mean body length of the 36 females at the age of 16 years or more isSci. Rep. Whales Res. Inst.,

No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLA

63

222.4 cm, it is considered that the mean maximum body length attained by the female
is approximately 222 cm. The age of oldest female was 26 years.
The mean growth curve shown by a solid line in Fig. 4 was drawn by eye, considering the mean body lengths in each ages. Based on this growth curve and themean body length at birth obtained separately, it was tried to adapt the Bertalanffy's
growth formula, L(t) =a(l-e.-fit), using Walford's finite differences diagram. But
it was impossible to adapt single growth formula for the mean growth curve of thefemale. For the growth of the first 2.5 years, the formula with the constances a=
192.2, a=-0. 7350, and /3= 1.1384 fits perfectly. The growth curve at the age of 2.5
years or more can be calculated from the above diagram, the mean final length 222.4
cm, and L(2.5) = 186.8 of the former formula. This gives the constances a=222.4,
a=-1.1813, and /3=0.2601, which is shown by the dotted line at the top in Fig. 4.
These two formulae can be considered to represent approximately the mean growth
curve of the female Stenella caeruleoalba. But the fittness of the latter formula is notgood for the ages between 6 and 12 years. Another formula which fits better only tothose ages is shown by the constances a=219.l, a= -1.3026, /3=0.2445, and shownby the dotted line at the bottom in Fig. 4.

..

250

E
u

•

I

.

•

•

I

. .
--- --- ----- -------- --- ------- -:

200

.s
.c
c

°'
~

>.

-g 150
CJ

0

5

15

10
Age

in

20

25

year

Fig. 5. The mean growth curve of the male. Solid line is drawn for male by eye.
Dotted line is the female growth curve shown by the solid line in Fig. 4.

The relation between body length and age in 191 males is shown in Fig. 5. Theline was drawn by eye. In the younger individuals the growth of the male is samewith that of the female. The difference of the body length between both sexes appears at about 6th year. After this age the growth is rapid and attaines the approximale length 230 cm at the age of 13 years. This rapid growth will probably
related with the attainment of the sexual maturity, and resembles with that of theSci. Rep. Whales Res. Inst.,
No. 24, 1972.

KASUYA

64

male Physeter catodon (Nishiwaki et al 1958). But the growth does not continue so long
period as the sperm whale. The physical maturity is considered from the growth
curve to be attained at 14 or 15th year as in the case of the female. The mean bodylength of the males of 16 or more years old is 235.9 cm or approximately 236 cm,
which is considered to be the mean maximum body length attained by the male.
This length is 14 cm larger than that of the female, which coincides with the value
expected from the body length frequency.
The age of the oldest male was 25 years.

Mean body length at birth
Because of the scarecity of the samples, the body length at birth was obtained
combining the both sexes. This will not give erroneous result because there is observed no sexual dimorphism in the body length of the younger individuals.
The mean body length at birth was calculated from the body length frequenciesof the large fetus and newborn calf. The data from the 4 schools, A, B, F and G wereused for this purpose. The body lengths were grouped into each nearest 5 cm units.
The largest fetus and the smallest calf were found in the length groups of 105 cm and95 cm respectively. So the length frequencies in the length groups between 90 cm
and 110 cm are considered here.
The numbers of calves and fetuses in the above range are 17 and 73 respectively,
and the frequencies of both categories are equal in the length group 105 cm (Table 2).
But there is a wide discrepancy between the total numbers of fetus and calf. As thecause of this phemomenon, it is possible to expect two reasons. One is related withthe method of fishing in which a school of dolphins is chased for several hours to be
driven into the harbor, during this chase some of newborn calves may be lost. The
other less probable cause is the difference of the schooling behavior or segregation of
the newborn calf and the mother.
TABLE 2.

BODY LENGTH FREQUENCIES OF FETUSES AND CALVES

Body length
(cm)

Futus
(no.)

Calf
(no.)

Fetus
(%)

Calf
(%)

% of calf(corrected)

90
95
100
105
110
Total

15
27
27
4
0
73

0

20.50
36.99
36.99
5.48
0
100

0
5.88
47.06
29.41
17.65
100

0
13.72
55.99
84.29
100
50.00

8
5
3
17

Though the correctness of these possible presumptions are not proved in thisstudy, the length frequencies were corrected to equalize the total numbers offetus andcalf. Then the ratios of the calf and the fetus were calculated in each length groups(Table 2). From this corrected percentage of the calf, the regression line y=
5.411 x-490.3 is obtained by the method of least squares, where y is the frequency ofthe calf in percentage and x is the body length in cm. The mean body length atbirth, 99.8 cm, is obtained from this formula as the body length corresponding to theSci. Rep. Whales Res. Inst.,
No. 24, 1972.

65

GROWTH AND REPRODUCTION OF A STENELLA

frequency of 50% (Fig. 6). But when the accuracy of the measurement is considered, it is better to say that the mean body length at birth is 100 cm.

•

iii 100
;:J

"C

:~

"C

80

.f
...c 60
0

.!l
~

Cl>
c 40

....0

Cl>
Ol

20

111
.....

c

Cl>

...

u

Ol
Ol

0
95
100
105
Body length in cm

90

Cl>

0.

Fig. 6.

110

Mean body length at birth, for explanation see text.

Age at the attainment of sexual maturity
In Table 3 the females with corpus luteum or albicans were identified as attained
sexual maturity, and the animals of the age of 6.5, 7.5, 8.5, and 9.5 years were
included into the age groups of 6, 7, 8, and 9 years respectively (see Materials andMethod). The oldest immature female and the youngest mature female were foundin the age groups of 10 years and 8 years respectively. As the materials are very few,
and the ratio of the mature individual fluctuates too wide to obtain the regressionline, another method was used to estimate the mean age at the attainment of sexual
maturity.
At first step, the percentages of mature and immature animals were calculated ineach age groups, then those of the immature animals were added from the older age
groups to the younger, and those of the mature from the younger to the older. These
accumulated percentages are shown in Fig. 7. The mean age of the female at theTABLE 3.

AGE FREQUENCIES OF IMMATURE AND MATUREANIMALS OF BOTH SEXES
No. of females

No. of males

Age
6
7
8
9
10
II
12
Total

Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

immature

matureimmature

mature

2
2
8

0
0

8
8
4

0

0
0
14

4
2
3
7
17

I
2
0
24

0
2
2
5
I

11

66

KASUYAattainment of the sexual maturity is obtained in the figure as the point where twolines cross. This gives the age 8.8 years.
-..!
0

Males

.!:: 140
>c:

u

~100

O"'

Cll
.....

-0
Cll

.....

60

..!!!

::J

N

§ 20
u

u
<t

0)

7

8

9

10
7
11 6
Age in year

8

Fig. 7. Mean age at the attainment of sexual maturity.
quency of mature individual, and cross the immature.

9

10

11

12

Circle indicates the fre-

300
200
~100
u

~ 80

0

:c: 60Cl"Qi

3: L.0 -------------------------------------20•i... ·i180•:•
-lo •!!i••.+•w•!•w•".,. •!- •

200

..
220

240

Body length in cm
Fig. 8. Relation between body length and the weight of both testis. Dotted line indicates the mean weight at the attainment of sexual maturity used in this study.

Another estimation can be obtained from the relation between age and numberof ovulations (Fig. 14), as the age when the number of ovulation is 1.0. The mean
age of the female at the attainment of sexual maturity obtained by this method is
8.9 years. These two figures coincide well.
Fig. 8 shows the relation between body length and the combined weight of bothtestes. Hirose and Nishiwaki (1971) showed with similar materials used in this study
Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLA

67

that the testes of more than 40 g are mature. Though they did not give the mean
weight of testis at the attainment of sexual maturity, it is considered to be between
30 g and 40 g. In the present study the testis with 35 g or more in combined weight
were considered to be sexually mature.
In the males, the oldest immature animal and the youngest mature were foundin the age of 11 and 7 years respectively. The mean age of the male at the attainment of sexual maturity, obtained by the same procedure used in the female, is 9.2
years (Fig. 7).
At present there is no reason to consider that the difference of the ages of theboth sexes at the attainment of sexual maturity is significant. And the best estimation for both sexes seems to be the age of 9 years.
The mean body lengths at the age of9 years obtained on the mean growth curveare 212 cm for fem ale, and 219 cm for male.
100.--~~~~~~~~~~~~~~--::::.--.~--wi

Fern ale

75
50 ----------------------E'-;;'!-

u'

25

N'

~·

c

~

N1

0

190

220

210

200

.......~-=---.i]ioo~~~~~~~~~~~~~~~~~~
m
Male
~

75
50 - - - - - - - - - - - - - - - - - -,

•
I

E:

25
200

210

220

230

Body length in cm
Fig. 9. The body lengths where 50% of individuals are sexually mature.
drawn by eye.

Lines areBody length at the attainment ef sexual maturity
The ranges between the largest immature individual and the smallest mature
are from 190 cm to 220 cm for females, and from 200 cm to 240 cm for males.
The mean body length at the attainment of the sexual maturity was obtained
directly from the ratio of the matured individuals in each length groups.
Fig. 9 is based on the 640 immature females and 604 mature females, and 53
immature males and 46 mature males. On the regression lines in Fig. 9, the length
where 50% of individuals are sexually mature is 212 cm for females, and 220 cm for
Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

68

KASUYA

males. These figures coincide well with those obtained from the mean age at theattainment of sexual maturity and the mean growth curve.
REPRODUCTION

Sex ratio
Table 4 shows the sex ratios in 4 schools caught at Kawana or Futo, and that of
TABLE 4. SEX RATIO IN PRENATAL AND POSTNATAL INDIVIDUALS,SHOWN BY THE NUMBER OF FEMALES PER 100 MALES
Prenatal
SchoolPostnatal~

male

femalemale

femaletotal

sex ratio

A
B
F

53
0

G

4
0
58
100

53
0
1
3
0
57
98.2

127
178
51
35
51
442
100

279
140
99
48
40
606
137.1

406
319
150
83
91
1,048

219.6
78.6
194.1
172.9
78.4
137.1

Taiji specimens
Total no.
Sex ratio

160 II
140

5 120
c

100 - - - - -

-c 80

L:.

OI

II

~

>-

-0
0

60

£I)

40
20
0

/

IV

v

VI VII VIII IX

x

XI

XII

III

II

Month
Fig. 10. Body length frequency of the fetus (solid line) and juvenile calf (dottedline). One month interval means 15 individuals. The growth curve of the fetus
was drawn presuming 12 month of gestation, and that of calf is based on the growthformula. I is started on the lst ofJune and II on the lst of December. The school
D, F, E, I, B (right side, no fetus) and A (left side), C, G, and Hare plotted against
the date of the catch (from right to left). A school in May is cited from Nishiwaki
and Yagi (1953).
Sci. Rep. Whales Res. Inst.,

No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLA

69

the catch off Taiji in January 1969. The ratio is shown by the number of females
per 100 males. The Taiji specimens were caught with hand harpoons from various
schools and studied by Dr. K. Hirose.
These data show that the sex ratio of the postnatal individuals varies in eachschools. This will probably be related with the characteristic schooling behavior
concerning the sexual conditions of the animal, and will not show the real sex ratio in
the population. The number of both sexes in fetuses is considered to be nearly equal,
and no uneven sex ratio is suggested.

40 A
2060 ········•···•·
B
40
20

20

11

Fig. 11. Frequency of the date of birth of the one year or younger individuals of both
sexes calculated with the growth formula. Square with smaller dots indicates theindividuals born 12 months prior to the date of catch. The numbers in the rightindicate the sample size, and the vertical rod the date of catch.

Parturition season
As there are observed many full termed fetuses and newborn calves in theschools caught in November and December (Fig. 10), this season seems to be at least apart of one of the parturition seasons of Stenella caeruleoalba.
Fig. 11 shows the frequency of the parturition infered with the growth formula,
the date of catch, and the body length of juvenile calf of 165 cm or less which isconsidered to contain one year old calves and all the younger individuals. It isseen in this figure that, though some parturitions may occur in any season of the year,
this species have two peaks of parturition one in May and June, and the other in
November and December. In this figure, the autumn peaks is smaller than thespring peak. But it will not be necessary to think that the autumn peak is smaller
than the other, because the frequency of the date of birth infered from the length
Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

KASUYA

70

and date of kill with fetal growth curve shows the reversal pattern (Fig. 12).

32
106

4
6

28
3

7

20
20

226

10
06;.;;;;:~~~~~iZ21:!:m:m~mlli~~~~~~

IV

V

VI

VII VIII JX

X

XI

XII

II

Ill

Month
Fig. 12. Frequency of the expected date of birth calculated with the growth curve of
fetus. Square with smaller dots indicates the fetuses expected to have been born 12month after the date of catch. The numbers at the right indicate the sample size,
and the vertical rod date of catch.

Mating season
Many small embryos are observed in the females caught in November andDecember (Figs. 10 and 13). In these figures the smallest length group includes theembryos smaller than 2.5 cm, which range is a half of the width of the length range of
5 cm applied to larger fetuses. Accordingly the height of the frequency of the smallest
group must be doubled to compare the relative abundance with that of the larger
length groups. Then it reveals that the relative abundance of the fetuses of2.5 cm orless in body length is higher than that of the next length group.
This will indicate that the beginning of the mating season is at slightly prior to
November, and the newly impregnated females are still increasing in November andDecember. Accordingly it is reasonable to consider that one of the mating seasons is
in November and December. As this season coincides with the autumn parturitionseason, another mating season in May and June is expected. This is supported by
the existance of small embryos, 10 fetuses under 10 cm, in a school caught in May at
Arari (Nishiwaki and Yagi 1953, reffered in Fig. 10 of this report).
Length of gestation
As the sampling period is restricted to October, November, and early December,
it is impossible to follow the seasonal increase of the body length of the fetuses to preSci. Rep. Whales Res. Inst.,

No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLA

71

sume the gestation period and the growth of fetus.
But it can be presumed from the seasons of mating and parturition, and bodylength frequency of the fetus. As mentioned above, both peaks of mating andparturition are in spring and autumn with the interval of6 months, then the approximate length of gestation must be a multiple of 6 months. On the other hand, there
are observed three peaks in the fetal length frequency in autumn season (Fig. 13).
80
C/I

<11
::J

-0

>

60 -1.0

0

-0.5

0.5

1.0

-0

.!:

...

QI

.0

E 20
::J

z

0

20

40

60

BO

100

120

140

160

180

200

Body length in cm
Fig. 13. Body length frequency of the fetus (dotted area) and juvenile calf (whitearea) in the catch of October to December. Figures at the top indicate approximate
age shown by the year from the birth. Schools A to I are combined.

The smallest peak is composed of the newly impregnated fetuses, the largest of the fulltermed fetuses, and the median of the fetuses impregnated in the preceding season.
If the length of gestation period is 18 months or more, there must be observed 4 ormore peaks, which is inconsistent with the above observation. Accordingly thegestation period of Stenella caeruleoalba is presumed to be about 12 months.
The mean fetal growth curve in Fig. 10 is drawn based on the above gestation
period using the method of Laws (1959). 0.13 X Gestation period is tentatively usedhere for Lt 0 of Laws (1959), which is the time when the extended straight line of the
fetal growth cuts the axis of time. The starts of gestation are put at the lst ofDecember (growth curve I) and at the lst ofJune (growth curve II). The growth curve of
the calf is based on the growth formula.
Fig. 12 shows the frequency of the date of birth infered with the fetal growth
curve, from the length frequency of the fetus and the date of kill. There are twoparturition seasons one in June and the other in November and December. This
result coincides well with the parturition frequency obtained independently (Fig. 11).
It is interesting that the shapes of the autumn parturition peak in Fig. 12 and that ofspring parturition peak in Fig. 11 are similar and the highest peaks are in December
and June respectively. This will indicate that the frequency of parturition is slightly
higher in December and June than in the preceding months, and will support thecorrectness of the presumed gestation period of 12 months.
For the comparison, the frequency of parturition was calculated on the asSci. Rep. Whales Res. Inst.,
No. 24, 1972.

72

KASUYA

sumption of the gestation of 11 months and that of 13 months. When 11 months
gestation and 0.18 X Gestation period for Lt 0 are assumed, the spring parturition peakis in April. And when gestation of 13 months and 0.13 X Gestation period for Lt 0
are assumed, the spring peak is in the range from April to June with the highest in
May. These results do not fit to that obtained from the length frequencies of thejuvenile calves.

Length of lactation
In this study the direct information indicating the length of suckling period was
not obtained, because no stomach content was studied. But it is expected that thecalf starts feeding on food before the age of0.5 year, because most of the teeth except
those in the anterior and posterior regions of the tooth row have erupted in all theindividuals of that age.
The length oflactation was estimated from the number oflactating females andthat of juvenile individuals caught together. There are observed several peaks inthe body length frequencies of each schools. The peaks of the younger individuals
are considered to represent the individuals born in some particular parturitionseasons, and their approximate age can be presumed from the body length at themode of frequency. The peak at 100 cm (schools A, B, and F) is considered to becomposed of individuals born in the autumn season when they were caught, and thatat 165 cm (schools A and B) is of the dolphins about 1 year old. The peaks at 120 cm(school F) and at 115 cm (school G) will have been born about 1/4 year before thecatch, and that at 140 cm about 1 /2 year before, and that at 170 cm about 1 1/4
years before the catch.
TABLE 5. LENGTH FREQUENCIES OF THE JUVENILE INDIVIDUALS,
THEIR APPROXIMATE AGE, AND NUMBER OF LACTATINGFEMALES.
Length frequency (cm)School
A

B

F

G

Range

Mode

Age

Number100-105
145-170
175-190200100160-170
175-21522095-1 JO
115-125130-145
160-175
185100-130
165---180
195-

JOO
165
185

0
I
2
7+
0

5
23
7
244

100
165
200
105
120
140
170
115
170

2, 2+
0
1/4
1/2
I 1/4
4+
1/4
I 1/4
4+

Number oflactating females
94

17
14
245
60
9
15
46
3
77
II
7
54

41

19

Sci. Rep. Whales Res. Inst.,

No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLA73

The number of the lactating females nearly coincides with that of calves
younger than I l /4 years old (schools B and G). The number of 2 years and older
calves caught together with the lactating females is very few comparing with thatof the younger individuals.
The school A was caught on 15 Nov. 1968, and the members were killed on 16,
18, and 22 Nov. when I had a chance to study some of them, but I had no chance tostudy many dead individuals picked up daily from the harbor and sold. The ratio of
the suckling calf may have been higher in these dead individuals which I missed to
study. This will be the reason of the low frequency of the juvenile calves in thesample compared with the lactating females. The quite reverse case is found in theschool F. This school was caught on 26 Nov. 1967, and I studied on 28 Nov. only thecarcases of the dolphins which had died in the water and picked up. In this school,
the number of juvenile calves is higher than that of the lactating females. But evenin these schools, A and F, it can be suggested that the 2 years old calves are rarely
accompanied by the lactating mother.
The lactating females are devided into two groups by the size of the largest
corpus albicans (Hirose et al 1970). The Futo school and Kawana school in Hirose
et al (1970) correspond to the school A and B of Table 1 respectively. Probably thelactating females with larger corpus albicans, which is few in number, will be themothers of newborn calves, and those with the smaller the mothers of the calves of theages from 1.0 to 1 1/4 years old.
As the conclusion of above discussions, it can be said that lactation lasts at least
for 1 1/4 years, and probably for about 1.5 years as in the case of Tursiops truncatus(Tabolga and Essapian 1957, Nakajima et al 1963), but rarely for 2.0years. The calves
which have started feeding on food by the age of about 0.5 year, will continue to take
both food and milk until the age of about 1.5 years.

Female reproductive cycle
As the females lactating and simultaneously pregnant are very rare in the catchand the recently ovulated non pregnant 3 females were not lactating (Table 1),
most of the fem ale seems not to ovulate in the period of lactation.
Between schools, there is wide variation of the ratio of the resting females, inwhich all the mature females other than the pregnant or lactating are included. But
it may be approximately between 10.5 and 17 .6 % of the total number of the mature
females (Table 1). If this ratio is compared with the total of the length of gestation(1 year) and length of lactation (about 1.5 years and surely more than 11/ 4 years), theaverage resting period of between 0.29 and 0.53 year (in case of 1.5 years lactation), or
between 0.26 and 0.48 years (in case ofless probable 11 / 4 years lactation) is obtained.
Accordingly the probable mean reproductive cycle of the female is presumed to
be about 3 years or slightly less (gestation 1, lactation 1.5, resting from 0.3 to 0.5year). This means that some females are impregnated in the same mating season
when they have weaned the calves, and most of the females in the next mating season.
In this species the corpus albicans probably remain in the ovary all through its
life (Fig. 14). The mean annual accumulation rate of the corpus albicans and luteum
Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

KASUYA

74

is 0.69, and the lowest and the highest accumulation rates are 0.25 and 1.6 respectively. To explain the mean accumulation rate and the mean reproductive cycle, there
must be expected about two ovulations in one reproductive cycle (Table 6). But as
indicated by the lowest accumulation rate, there must be few females which stayanoestrus for 3 years after giving birth to the calves and show the reproductive cycleof 4 years.

15

1.6 /year

Ill

c
0

~10
:J

V:0.69X-5.16

>
0

.....
0

5

0

z

/year

..

. ::::.
4

6

8 10 12 14 16 18 20 22 24 26
Age in year

Fig. 14. Relation between age and the number of corpora albicantia and luteum.
The regression line is calculated by the method of least squares, and the highest andlowest ranges are drawn by eye.
TABLE 6. MEAN ANNUAL OVULATION RATE CALCULATED FROM THE ASSUMEDREPRODUCTIVE CYCLE AND NUMBER OF OVULATIONS IN A CYCLE.
Number of ovulations

Pregnant(year)

Lactating(year)

Resting(year)

Repreductive
cycle(year)

2

3

4

5

6

1
I

1.25
1.25
1.25
1.25

0
0.26
0.48
1.75

2.25
2.51
2.73
4.00

0.44
0.40
0.37
0.25

0.89
0.80
0.73
0.50

1.33
1.20
1.10
0.75

1.59
1.47
1.00

1.25

1.50

1.50
1.50
1.50
1.50

0
0.29
0.53
1.50

2.50
2.79
3.03
4.00

0.40
0.36
0.33
0.25

0.80
0.72
0.66
0.50

1.20
1.08
0.99
0.75

1.60
1.43
1.32
1.00

1.25

1.50

Reproductive cycle and school
Fig. 15 shows the frequencies of the date of conception infered with the mean
growth curves of fetus and juvenile individual. The materials include all the fetus
and the calves smaller than 165 cm in body length. It is sure in the preceeding discussion that these calves are still accompanied by the mother. The frequencies ofconception calculated from the calves are shown by white area, and those calculated
Sci. Rep. Whales Res. Inst.,

No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLA

75

from the fetuses are shown by dotted area. The height of the highest peaks of theboth frequencies in each schools are modified to be equal.
When schools are seen from the date of the conception of the fet us, the schools
are grouped into two patterns. One includes the schools in which the fetuses aremostly composed of the autumn fetus (fertilized in autumn mating season), and theother the schools in which the fetuses are mostly composed of the spring fetus. The
schools A, C, D, E, G, and H belong to the former, and school I to the latter. Because of the scarecity of materials it is impossible to classify the school F. The school
B did not contain the pregnant female.

XII

Ill

V!

IX

XII

Ill

VJ

IX

XII

Month
Fig. 15. Frequency of the date of conception of the fetus and calf smaller than 165 cm
in body length, calculated by the growth curve. Figures at the top indicate theapproximate age of samples at the time of catch. Dotted area indicates the fetuses
and the white area calves. For further explanation see text.

Similar classification is possible by the date of conception of the juvenile calves.
The calves in schools A and B are mostly composed of the autumn calf (fertilized
and born in the autumn season), and those of the schools C, D, E, and F are mostly
composed of spring calf. The school G shows the intermediate pattern.
There is observed various combinations between the fertilized date of calves
and that offetuses in the same school. In the school B only calves are observed, in theschool A autumn calves and autumn fetuses are observed in one year interval, in theschool C mostly spring calves and autumn fetuses in half year interval, in the schools
D and E spring calves and autumn fetuses with about 1.5 year interval. In theschool F the sample of fetus is too few to conclude, but probably it belongs to thesame type with the school D. In the school G the larger fet uses and smaller calves
seems to belong to one continuous mating season which slightly deviates from theSci. Rep. Wahels Res. Inst.,

No. 24, 1972.

76

KASUYA

typical mating season, and there is also observed few fertilizations in half year intervals at both sides of the main peak. In the schools H and I, there is observed no
calves smaller than 190 cm and 205 cm respectively. These observation lead me tothe following hypothesis on the behavior of the mature female.
The oestrous females may assemble together in a mating season, and will be impregnated. This stage is observed in the schools Hand I. The school thus formed
is expected to be the base of other complicated schools, when seen only from the sideof mature females. This connection of the females will be retained through theperiod oflactation and at least untill the next oestrous cycle. This stage is seen in theschool B. Then the females will reconstruct the school in accordance with the stageof the reproductive cycle. This is the reason why there is observed a peculiarity ofthe season of the conception of calf or fetus in each schools. All the other schools
may have been formed as the result of sporadic annexation of the schools of pregnant
females and that of lactating females in various stage of reproductive cycle. No
special affinity between the original schools related with the size of calf and that offetus is expected. The annexation between the lactating schools, or between the
pregnant schools is not denied.
The schooling behaviors of the immature individuals of both sexes and of mature
males are not well studied. If the males of 220 cm or more in body length aretentatively classified into mature, the numbers of mature males in the schools A, B, F,
and G are 93, 41, 11, and 18, or 64.1 %, 242%, 21.6%, and 54.6% of mature females(212 cm or more in length) respectively. It is considered that numerous mature
males are found together with the mature females. There is an example of the school
composed of immature individuals. This school, composed of about 600 individuals,
was caught on 14 Dec. 1971, and random investigation was made on 303 animals byme and Mr. N. Miyazaki. Among these dolphins, all the 66 females were sexually
immature and only 6 males in 237 males had testes weighed more than mean testes
weight at the attainment of sexual maturity applied in this study.
Though there are observed many spring calves and autumn fetuses, the numberof spring fetus is scarce in the present materials (Fig. 13). This may suggest thepresence of the segregation of females related with the reproductive cycle. But
further confirmation is needed on this problem.
DISCUSSION
There have been published two contrary opinions on the accumulation rate of thedentinal growth layers. Sergeant (1959, 1962) showed on Tursiops truncatus andGlobicephala melaena that one opaque and one clear (translucent) layer are accumulated
annually, and that the former in summer and the latter in winter. Similar conclusion was obtained on Physeter catodon by Ohsumi et al (1963), But Gambell andGrzegorzewska (1967) on Physeter catodon, Brodie (1969, 1971) on Delphinapterus leucas,
and Kleinenberg and Klevezal (1962, cited in Brodie 1969) on Delphinus delphis considered that two layers are accumulated annually.
However, as indicated by Sergeant (1962), Ohsumi et al (1963), and Brodie
Sci. Rep. Whales Res. Inst.,

No. 24, 1972.

GROWTH AND REPRODUCTION OF A STENELLA

77(1969) there are observed several fine layers in one dentinal growth layer. Especiallyin case of the young sperm whale, one growth layer ofOhsumi et al (1963) is usuallycomposed of two parallel layers. Similar one or two vague layers are also observedin the lst layer of Stenella caeruleoalba.
I consider that the above mentioned contravention on the accumulation rateof dentinal layers may be mostly caused by the different interpretation of the finestructure of the layer. It will be most necessary, at present, to establish the standards
suited for each species, and then define the accumulation rates in each species.
According to the observation on the several western Pacific Tursiops truncatusborn in an aquarium, the body lengths at the ages of0.5, 1.0, and 2.0 years are 133 °/o,
155 %, and 185 % of the length at birth respectively (Nakajima et al 1963). In the
Atlantic T. truncatus, the similar figure at the age of 7 or 8 months is 160 % (Essapian1953). These figures coincide well with the growth of Stenella caeruleoalba presumed
in this study, and suggest the correctness of my interpretation of the dentinal growthlayers.
In Stenella caeruleoalba, the sexual maturity is attained in the 9th year in both
sexes as in the case of Physeter catodon (Nishiwaki et al 1958, Ohsumi et al 1963). But in
Globicephala melaena (Sergeant 1962) and Delphinapterus leucas (Brodie 1971), the mean
age of the female at the attainment of sexual maturity is from 4 to 6 years, which isabout half of that of the male. Further study is desired to have a conclusion if thisresult reflects the real growth of these species, or simply resulted from the different
interpretation of the dentinal layers.
The mating season of Stenella caeruleoalba is peculiar in the possession of bimodal
peaks. In the northern species, Physeter catodon has a mating season in April (Ohsumi1965), Delphinapterus leucas in April and May (Laws 1959, Brodie 1971), Globicephalamelaena in April and May (Sergeant 1962), Phocoena phocoena in from July to August(Laws 1959, Fisher 1970, M!llhl-Hansen 1954). The mating seasons of the former 3
species nearly coincides with the spring peak of Stenella caeruleoalba, but the mating
season of Phocoena phocoena situates in the summer, or at the intermediate season of thespring and autumn peaks of Stenella caeruleoalba.
With the relation of the various lengths of gestation period, the parturitionseasons are in August in northern Plryseter catodon (Ohsumi 1965), in June (Laws
1959) or in July and August (Brodie 1971) in Delphinapterus leucas, in August in
Globicephala melaena (Sergeant 1962), and in July (Laws 1959) or in June and July(Fisher 1970) in Phocoena phocoena.
These informations are obtained, with one exception of Plryseter catodon, from thespecies in colder waters. In these species the mating season and the length of gestation will have been adapted to put the parturition season in the northern summer orlate spring, when the environment is better for newborn calves. However, Stenella
caeruleoalba is considered to distribute, in the western Pacific, in warmer waters (Kasuya 1971, Kasuya and Oguro 1972), and not to migrate north of the northern boundary of the Japan current. Therefore it will not be an important factor for this species
to give birth to the calves in summer season. This will have a relation with theexistance of the two parturition season in Stenella caeruleoalba.
Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

78

KASUYA

Tursiops truncatus in captivity had the gestation period of about 12 months andnursing period of 18 months (Tavolga and Essapian 1957), or nursing period of 16months (Nakajima et al 1963). These figures show good coincidence with that obtained in Stenella caeruleoalba.
Ohsumi (1971) reported a hypothesis on the structure of the school of Physetercatodon. There are several structural differences between the school of Stenella
caeruleoalba and that of P. catodon. One is the continuity of the members in the schoolof mature female of Physeter catodon. If this exists in the school of Stenella caeruleoalba,
it is reasonable to expect the spring fetus and autumn fetus in same frequency in one
school. This was not observed in the present study and the continuity, in thestrict sense, of the members in nursing school can not be expected in Stenella caeruleoalba. Other characteristic features of the school of Stenella caeruleoalba are thelarger size of the school and the larger number of the mature males found together
with the mature females. The juvenile school is observed in Stenella caeruleoalba as in
the cases of Physeter catodon (Ohsumi 1971) and Globicephala melaena (Sergeant 1962).
Perrin (1969) considered the between-school differences in coloration detectedin Stenella grajfmani to suggest the generic entities of the school of this species. This isinconsistent with the informations on the schooling behavior of Stenella caeruleoalba
obtained in this study.
ACKNOWLEDGMENTS

Sincere thanks are due to Dr. S. Ohsumi of The Far Seas Fisheries Research Laboratory, Dr. K. Hirose and Mr. N. Miyazaki of The Ocean Research Institute, University of Tokyo, who offered the tooth samples and many biological informations.
Dr. T. Kajihara and Dr. M. Nishiwaki of The Ocean Research Institute areacknowledged for their cocperation and the valuable suggestions.
REFERENCES
BRODIE, P. F., 1969. Mandibular layering in Delphinapterus leucas and age determination. Nature, 221:956-958.
BRODIE, P. F., 1971. A reconsideration of aspect of growth, reproduction, and behavior of the white whale(Delphinapterus leucas), with reference to the Cumberland Sound, Baffin Island, population. J. Fish. Res.
Board of Canada, 28(9): 1309-1318.
EssAPIAN, F. S., 1953. The birth and growth of a porpoise. Natural History, 62 (9) : 392-399.
FISHER, H. D., 1970. Reproduction in the common porpoise (Phocoena phocoena) of the North Atlantic. J.
:(ool., 161 : 471-486.
GAMBELL, R., and C. GRZEGORZEWSKA, 1967. The rate oflamina formation in sperm whale teeth. Norsk
Hvalfangst-tid., 56(6): 117-121.
HIROSE, K., T. KAsuvA, T. KAZIHARA, and M. NISHIWAKI, 1970. Biological study of the corpus luteum andthe corpus albicans of blue white dolphin (Stenella caeruleoalba). J. Mamm. Soc. Japan. 5(1): 33-39.
HIROSE, K., and M. NrsHIWAKI, 1971. Biological study on the testis ofblue white dolphin, Stenella caeruleoalba.
J. Mamm. Soc. Japan, 5(3): 91-98.
JAPANESE FISHERIES BUREAU, 1900. Nikon suisan hosaishi (Fishing in Japan). Suisan Shoin, Tokyo. vol. 1, pp.
424. (injapanese).
KASUYA, T., and N. OauRo, 1972. A new tagging method of dolphins. Sci. Rep. Whales Res. Inst., 23: 81-85.
Sci. Rep. Whales Res. Inst.,
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GROWTH AND REPRODUCTION OF A STENELLA

79

LAws, R. M., 1959. The foetal growth of whales with special refference to the fin whale Balaenoptera physalus
L. Discovery Rep., 29: 281-308.
M0HL-HANSEN, U., 1954. Investigations on reproduction and growth of the porpoise (Phocoena phocoena L.)from the Baltic. Vidensk. Medd. fra Dansk naturh. Faren., 116: 369-396.
NAKAJIMA, M., K. TAKAHASHI, M. 0GURA, and K. SAWAURA, 1963. On the growth of infants of the small
size toothed whales. J. Jap. Ass. <:,ool. Gardens and Aquarium, 5(1): 16-22. (in Japanese).
NISHIWAKI, M., T. HIBIYA, and S. 0HSUMI (KIMURA), 1958. Age study of sperm whale based on reading of
tooth laminations. Sci. Rep. Whales Res. Inst., 13: 135-153.
NISHIWAKI, M., M. NAKAJIMA, and T. KAMIYA, 1965. A rare species of dolphin (Stenella attenuata) from Arari,Japan. Sci. Rep. Whales Res. Inst., 19: 53-64.
NISHIWAKI, M., and T. YAGI, 1953. On the growth of teeth in a dolphin (Prodelphinus caeruleoalba) (I). Sci.
Rep. Whales Res. Inst., 8: 133-146.
OHSUMI, S., 1965. Reproduction of the sperm whale in the northwest Pacific. Sci. Rep. Whales Res. Inst., 19:

1-35.
OHSUMI, S., 1971.

Some investigations on the school structure of sperm whale. Sci. Rep. Whales Res. Inst., 23:
1-25.
OHSUMI, S., T. KASUYA, and M. NrnHIWAKI, 1963. The accumulation rate of dentinal growth layers in the
maxillary tooth of the sperm whale. Sci. Rep. Whales Res. Inst., 17: 15-35.
PERRIN, W. F., 1969. Color pattern of the eastern Pacific spotted porpoise Stenella graffmani Lonnberg(Cetacea, Delphinidae). <:,oologica, 54(4): 135-151.
SERGEANT, D. E., 1959. Age determination in odontoceti whales from dentinal growth layers. Norsk
Hvaifangst-tid., 48(6) : 273-288.
SERGEANT, D. E., 1962. The biology of the pilot or pothead whale Globicephala melaena (Trail) in Newfoundland waters. Bull. Fisheries Res. Board of Canada, 132: 1-84.
TABOLGA, M. C., and F. S. EssAPIAN, 1957. The behavior of the bottle-nosed dolphin (Tursiops truncatus):
mating, pregnancy, parturition, and mother-infant behavior. <:,oologica, 42(1): 11-31.
ToBAYAMA, T., 1969. School size and its fluctuation in the catch of Stenella caeruleoalba in Sagami Bay.
Geiken Tsushin, 217: 109-119. (in Japanese).

EXPLANATION OF PLATESIn all the figures the contrast is reversed, or translucent layer is seen dark and opaque layer light.
PLATE I
Fig. 1.
Fig. 2.
Fig. 3.

Thin ground section of tooth of 92 cm fetus.
Thin ground section of tooth of a female, 116 cm, 1/4 year.
Thin ground section of tooth of a female, 173 cm, l year.
PLATE II

Fig. 1.
Fig. 2.
Fig. 3.

Thin ground section of tooth of a female, 193 cm, 4 1/4 year.
Thin ground section of tooth ofa male, 216 cm, 13 year.
Thin ground section of tooth of a male, 221 cm, 25 year.

Sci. Rep. Whales Res. Inst.,
No. 24, 1972.

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