GROWTH AND REPRODUCTION OF STENELLA
COERULEOALBA OFF THE PACIFIC COAST
OF JAPANNOBUYUKI MIYAZAKI
Department of Marine Sciences, University of the Ryukyus, Okinawa

ABSTRACTThis study is based on data from about five thousand specimens of S.
coeruleoalba. Mean length at birth is 100 cm. Mean lengths at the age of 1
year and 2 years are 166 cm and 180 cm, respectively. The species starts feeding on solid food at the age of 0.25 year (or 135 cm). Mean weaning age isabout 1.5 years (or 174 cm). Mean testis weights at the attainment of puberty
and sexual maturity of males are 6.8 g and 15.5 g, respectively. Mean ages at
the attainment of puberty and sexual maturity of males are 6. 7 years (or 210cm) and 8.7 years (or 219 cm), respectively. Females attain puberty andsexual maturity on the average at 7.1 years (or 209 cm) and 8.8 years (or216 cm), respectively. There are three mating seasons in a year, from February to May, from July to September, and in December. Mating season may
occur at an interval from 4 to 5 months. The overall sex ratio (male/female)is 1.14. Sex ratio changes with age, from near parity at birth, indicating
higher mortality rates for males.

INTRODUCTION

The striped dolphin, Stenella coeruleoalba are caught annually by the driving fisheryor hand harpoons in the Pacific coast ofJapan (Ohsumi 1972, Miyazaki et al. 1974).
According to Ohsumi (1972), Miyazaki et al. (1974), and Nishiwaki (1975), thestriped dolphins caught in the Pacific coast of Japan are suggested to belong to
one population.
The growth and reproduction of the species have been studied by some Japanesescientists (Nishiwaki and Yagi 1953, Hirose et al. 1970, Hirose and Nishiwaki 1971,
Kasuya 1972, Kasuya and Miyazaki 1976, and Kasuya 1976). Hirose et al. (1970),
and Hirose and Nishiwaki (1971) made histological examinations of the gonads,
but they did not make the age determination. Kasuya (1972) studied growth
and reproduction by the age determination by means of dentinal growth layers,and estimated that there are two mating seasons in a year. Kasuya and Miyazaki(1976) reported that the mean calving interval in the population in the coast ofJapan is decreasing in accord with a decrease in population level. Recently Kasuya(1976) reconsidered the life history parameter, based on cementa! layers, and indicated that the analysis of the life history by means of dentinal layers can not be
available in the older animals. However, Kasuya (1976) did not analize growth
Sci. Rep. Whales Res. Inst.,
No. 29 .1977, 21-48

MIYAZAKI

22

and reproduction by detailed examinations of the gonads.
The present study was undertaken to make histological examinations of thetestes and detailed observations of the ovaries, in order to reconsider the life history
up to the age of 13.5 years based on dentinal layers with abundant specimens, andto estimate the mating season of the species.
TABLE 1.

LIST OF MATERIALS USED IN THIS STUDY
Number of dolphins

Schoolno.

Date of kill

5
6
8
9
10
11
12
13
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
42
43
44

16, 18, 22, Nov. '68
17, Nov. '68
12, Oct. '70
13, Oct. '70
21, Oct. '70
24, Oct. '70
29, Oct. '70
22, Nov. '70
2, Dec. '70
10, Dec. '70
2, Oct. '71
4, Oct. '71
5, Nov. '71
17, 18, Nov. '71
20, Nov. '71
15, Dec. '71
3, Oct. '72
4, Oct. '72
7, Oct. '72
8, Oct. '72
13, 15, 16, Oct. '72
14, 18, 19, Oct. '72
17, 19, 20, Oct. '72
23, Oct. '72
26, Oct. '72
1, Nov. '72
5, Nov. '72
9, Nov. '72
10, Nov. '72
15, Nov. '72
16, Nov. '72
17, 20, Nov. '72
21, Nov. '72
23, 26, Nov. '72
7, 9, Dec. '72
19, Nov. '73
20, 22, Nov. '73
20 Jan. '73 - 15, Feb. '73

Total**

CaughtExamined

1,700
344
265
293
324
49
48
1,832
259
84
393
101
25
636
140
903
31
225
120
94
574
305
238
48
54
63
284
239
45
243
200
372
293
535
912
414
l, 724271 *
14,409

406
318
89
64
68
33
34
125
256
84
176
58
25
361
100
300
29
96
31
41
211
124
108
29
40
63
120
104
27
112
71
212
130
137
168
251
470
271*
5,071

Percentage
ofexamination
23.9
99.7
33.6
21.8
21.0
67 .4
70.8
6.8
98.8
100
44.8
57.4
100
56.8
71.4
33.2
93.5
42.7
25.8
43.6
36.8
40.7
45.4
60.4
74. l
1-00
42.3
43.5
60.0
46.l
35.5
57.0
44.4
25.6
18.4
60.6
27.3
100

Researcher
KasuyaKasuyaMiyazaki and Kasuya
MiyazakiMiyazakiMiyazakiMiyazaki
HiroseMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazaki and Kasuya
MiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazakiMiyazaki and Kasuya
Miyazaki and Kasuya
Miyazaki* animals caught by hand harpoon at Taiji.
* * exclude the animals caught at Taiji.

Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

23

MATERIALS AND METHODS

Biological informations were obtained from 5,071 striped dolphins from 37 schools
caught by the driving fisheries at Kawana and Futo on the east coast of Izu Peninsula
during the five years from 1968 to 1973, and from 271 dolphins caught by hand
harpoon at Taiji on the coast of Kii Peninsula in 1973 (Table 1). Of these specimens the data of the school nos. 5 and 6 were provided by Dr. T. Kasuya, and of theschool no. 13 by Dr. K. Hirose.
Body length was measured in I cm interval in a straight line from the tip of theupper jaw to the notch of the tail flukes.
Mammary gland was observed in the field and classified into lactating andnonlactating.
The ovaries were collected from most of females and fixed in 10% formalinsolution. In the laboratory the ovaries were weighed, the number of corpora albicantia and lutea was counted, and the three diameters of corpora were measured.
The mean diameter of the corpora was obtained as the cube root of the value multiplied their diameters.
Usually the testes were collected from most of males, fixed in 10% formalinsolution, and then transported to laboratory. Histological samples were takenfrom the center of cross section at the midlength of the testis, processed into microscopic slide, and stained with haematoxylin and eosin.
Several teeth were collected from the center of tooth row of each dolphin witha pair of gardening scissors and fixed in 10% formalin solution. The teeth wereprepared by the method of Kasuya et al. (1974). The opaque layers were counted
under a binocular microscope ( X 20- X 50) with transmitted light.
The ages of930 dolphins of the school nos. 5, 6, 22, 28, 43, and 44 were determined by Dr. T. Kasuya. As his age determination of the animals between theage of 1 and 12 years was biased by 0.27 year to the lower side ofMiyazaki's reading(Kasuya and Miyazaki 1976), there is no significant difference between their age
determinations. Therefore, in this study the age data determined by Dr. T. Kasuya
were used together with the same data prepared by Miyazaki.
AGE DETERMINATION

Growth layers in the dentine have been used for age determination of the animals
by various authors, for example in Tursiops truncatus (Sergeant 1959), Globicephalamelaena (Sergeant 1962), Physeter catodon (Ohsumi et al. 1963), Stenella coeruleoalba(Kasuya 1972), Stenella attenuata (Kasuya et al. 1974). Recently cementa! layers
were employed in the age determination of the animals in S. coeruleoalba and S. attenuata (Kasuya 1976).
Since the teeth on the anterior parts of the jaws are too small and those on theposterior are too strongly twisted, they were not used for the age determination.
The readability of the growth layers in teeth from different positions of the same
animal was checked on one individual (Table 2). The number of layers in theseSci. Rep. Whales Res. Inst.,
No. 29, 1977.

MIYAZAKI24

TABLE 2. AGE DETERMINATION BY TEETH COLLECTED FROMVARIOUS POSITIONS OF JAWS IN ONE DOLPHIN
Tooth No. from tip of jaw
Jaw
Left upper jaw
Left lower jaw
Right upper jaw
1 6 91

10

20

30

40

10.5
12.5
10.5

12.5
12.5
11.5

11.5
12.5
13.5

12.5
12.5
11.5

22 4 8 36 8 2

1330 1 9 1 0

10 0

i!

80

>u

c

60

Cl>
:J

.,er 40
u..
20
0
Oct.

Nov.

Dec.

Jan.

Feb.

s e a so n
Fig. 1. Seasonal change of the formation of opaque and translucent dentinal layers.
The marks indicate thin opaque layer, thick opaque layer, thin translucent layer,
and thick translucent layer (from top to bottom). The numbers at the top indicate the sample size. The dates of the kill are grouped into the lst, 2nd, and3rd decade of month.

teeth ranges from 10. 5 to 13. 5 years (average : 12. 0 years) indicating no significant
variations.
In order to find out the annual accumulation rate of dentinal growth layer,
the kind of the last layer and its thickness were examined on the teeth with threeor less opaque bands. Fig. 1 shows that the alternation from opaque to translucent
layer occurs in the period of December and January. This is slightly later thanthat estimated by Kasuya (1972). In the present study it is impossible to decide
when the alternation from translucent to opaque layer occurs because of the absenceof adequate samples. However, above result coincides with that of S. attenuata(Kasuya et al. 1974) in which the alternations occur in December or January andalso in April or May, and each opaque and translucent layer represents about 6
months. Accordingly it will be correct to presume that the alternation from opaque
to translucent layer of the dentine of S. coeruleoalba may occur in winter and thealternation -from translucent to opaque layer in summer, and that each opaque andtranslucent layer represents approximately 6 months, respectively.
In the present study an approximate age was calculated by the criterion of
Kasuya et al. (1974) taking into consideration of the kind of the first and the lastSci. Rep. Whales Res. Inst.,
No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

25

layers, and of their thickness in the dentine.
Since the dentine above 16 layers is composed of poorly calcified secondary dentine, it is impossible to read precisely the dentine with 16 layers or more. Kasuya(1976) showed that the accumulation of the dentine layers of S. coeruleoalba ceases ata large variety of age above 11 dentinal layers and that the accumulation of cemental
layers continues longer. The number of dolphins fallen in the range between x+ 1
and x-1 cemental layers was calculated at the x dentinal layer on the basis of Fig. 2of Kasuya (1976). The ratios of this number to the total at 12, 13, 14, and 15
dentinal layers were calculated at 80.9, 72.7, 54.8, and 13.8%, respectively. This
indicates that the ratio of the dolphins whose ages are underestimated becomes
gradually higher with the increase of the number of dentinal layers during 11 and 15
layers, and this ratio attains about 50% at 14 dentinal layers. Accordingly the age
determination of the animals seems to be reliable till 14 dentinal layers with precision of 50% or more. Therefore, this study intends to analize the animals at theage of 13.5 years or less. During 11 and 14 dentinal layers, however, the value ofthe mean growth curve or age-weight of testis relationship obtained in this study
appears to be higher than that expected from cemental layers by Kasuya (1976), andthe bias between two values is considered to become larger with increase of the number of dentinal layers.

GROWTH
Mean body length at birth
Since the body lengths of newborn calves do not show distinct sexual dimorphism and available data are scarce, the mean body length at birth was calculated
by combining both sexes. The body lengths were grouped at interval of 5 cm.
The largest fetus was 108 cm and the smallest calf 98 cm. Then the fetuses andcalves from 95 cm to 115 cm groups were used to calculate the mean body length
at birth (Table 3). The number of fetuses and calves was equal at 105 cm group.
However, the numbers of fetuses and calves between 95 and 115 cm groups were 68
and 20 animals, respectively, and there is a big difference between two numbers.
This seems to be caused by the loss of some newborn calves during the driving ofanimals as already suspected by Kasuya (1972). Then the frequency was corrected
to equalize the numbers offetuses and newborn calves (Table 3). The mean bodylength at birth was obtained at 100.5 cm as the body length corresponding to the
TABLE 3.

BODY LENGTH FREQUENCIES OF FETUSES AND CALVES

Length group
(cm)

Fetus
(no.)

Calf
(no.)

Fetus

Calf

(%)

(%)

Percent of calf
(corrected)

95
100
105
110
115
Total

33
30
4
1
0
68

0
8
4
5
3
20

48.53
44.12
5.88
1.47
0
100

0
40.00
20.00
25.00
15.00
100

0
45.39
77.28
94.45
100
50.00

Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

MIYAZAKI

26..----

100

a

u

0

50

______

/

c:
a.

0
0

u

...

1~

a.

a.

0

90

95

100

Body
Fig. 2.

/105

lengthMean body length at birth.

frequency of 50% (Fig. 2).

110

115

in

cm

120

For explanation see text.

This result coincides with that of Kasuya (1972).

Weaning
The weaning age of the animals was estimated by the following two methods.
First method: The food remains in the first stomach of 45 young animals were
examined (Table 4 ). The smallest animal containing solid food in the stomach was
133 cm and the largest containing milk was 176 cm in body length. The youngest
animal containing the solid food in the stomach was 0.25 year old and the oldest containing milk was 1.25 years old. Accordingly it is considered that some individuals
start feeding on solid food at 133 cm or 0.25 year, and some may continue to take
both milk and solid food until 176 cm in body length or by the age of 1.25 years.
However, this age seems to be underestimated, because when an animal contains
solid food, it is not easy to detect milk mixed in solid food.
The food items found in the stomachs of these young animals were fishes, squids,
and shrimps. This suggests that when the young animals start to feed on solid food,no preference was made among fishes, squids, and shrimps.
Second method: The next method is to compare the number of the young postnatal dolphins and that of the lactating females in the school. In the first place thecalves were counted in the order of their ages from the youngest one until the number
become equal to that of lactating females. Then the weaning age was determined
to be the age of the last calf in this counting in each school. Secondly the dolphinsof this age or younger were classified as suckling calves, and the dolphins older than
this age as juvenile calves in each school. Finally the ratio of juvenile calves to thetotal calves from 17 schools was calculated at each age as shown in Fig. 3. The oldest
suckling calf was at the age of2.5 years. As the relation between this ratio and age
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

27

GROWTH AND REPRODUCTION OF STRIPED DOLPHINTABLE 4.
Date of kill
3, Dec. '72
23, Nov. '72
17, Oct. '72
20, Nov. '71
20, Nov. '72
17, Oct. '72
20, Nov. '72
3, Dec. '72
20, Nov. '73
7, Oct. '72
20, Nov. '73
20, Nov. '73
20, Nov. '73
20, Nov. '73
17, Oct. '72
20, Nov. '73
20, Nov. '73
20, Nov. '73
26, Jan. '73
3, Dec. '72
20, Nov. '73
3, Dec. '72
20, Nov. '73
20, Nov. '73
20, Nov. '73
20, Nov. '73
3, Feb. '73
17, Oct. '72
13, Oct. '72
26, Jan. '73
3, Feb. '73
1, Nov. '72
1, Nov. '72
30, Jan. '73
1, Nov. '72
31, Jan. '73
31, Jan. '73
26, Jan. '73
26, Jan. '73
31, Jan. '73
21, Jan. '73
21, Jan. '73
26, Jan. '73
3, Feb. '73
26, Jan. '73

WEANING AND TOOTH ERUPTION

Field
no.

Sex

Body
length

Age

155
2
40
41
112

M
M
M
M
F
M
F
M
M
F
M
M
M
M
M
F
M
M
M
F
F
M
F
F
M
F

102
103
110
112
122
130
132
132
133
135
142
145
149
149
150
151
152
153
154
154
154
157
157
158
159
163

0.25
0.25
0.25

F
F
F
F
M
M
F
F
F
M
M
F
F
F
F
M
M
F
M

163
170
172
174
176
182
184
184
184
184
184
185
185
186
187
187
188
189
196

114
133
92
7
126
182
27
28
3
12
13
11
20
144
136
143
167
214
127
49
23
4
11
30
22
2
10
9
3
30
3
7
5
3
4
5
29
4

0.25
0.25
0.25

Milk

Food

L:+Jo,R:+Jo
r

none

r

none

L : 30Jo, R: 33Jo
L: 35Jo, R: 35Jo

milk only
milk only

L: 37J28, R: 37J28
L: +Jo, R: +Jo

0.5

r
r
none
r
none
r
r
r
none
none
r

0.5

r

r
r

0.5
r
r
r

beaks of squids
fish lensmilk only
beaks of squids
milk only
beaks of squids
beaks of squids
milk only
milk only
fish lens
L: +J+, R: +J+
beaks of squids
L: +J+, R: +J+

0.5
0.5
1.25

1.25
2.50
2.25
0. 75
1. 75
1.50
1. 75
2.50
2.75
3.00
1. 75
3.00
2.75
1. 75
I. 75
2.75
3.00
2.25

r
r

r
r
r
r
r
r
r
rr

r
r

r
r

r
rr

L, left; R, right; M, male ; F, female ; r, few; rr, abundant; +, erupted

Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

Comments

L: oJo, R: oJo
L : oJo, R: oJo

r
0.25
0.5
0.25

No. of erupted
teeth

beaks of squids
beaks of squids
beaks of squids
beaks of squids
and shrimps

fish

28

MIYAZAKI

can be shown in a straight line during the ages from 0.25 to 2. 75 years, this line wascalculated by the least squares during this range (Fig. 3). The age at which theratio shows 50% is calculated to be ·1.5 years. This indicates that the mean weaning
21

48

9

26

40

43

·13

12

39

100

•

50

•

16

185

•

~Y=46.7164X-----------•

2• • • •

100•

520.8200( r= 0.9747)

.,,1
·I
~,

0 ._......

~____..__,__.___,~~~~~~~~~~-'-~

0.5

0

1 .5

1.0

Age

3.0

2.5

2.0
in

3.5

years

Fig. 3. Age of the weaning in S. coeruleoalba. Closed circles indicate the frequencyof the juvenile calves. The numbers at the top show the sample size. For explanation see text.

!!Eu85 106

99

82

64

53

74 72

69

96 106

13

10

14

B

6

3

4

1

6 71 60 93 122 135 139 114 139 133 105 98 59 86 71 72 42 41 48 39· 30 9

17

9

10

5

1

1

1

54

66

85

81

78

69

57

40

40

1

1

250

c

.r:.

200

Cl

c
<I>

150

>·u
0

JJ'.l

1 00
10

0

A g

e

io

15

n

y

e

a

25

30

s

Fig. 4. The mean growth curves of the male and the female. Closed circles indicate the male and open circles the female. The numbers at the top indicate thesample size. For samples greater than 30, ± standard deviations indicated asvertical line. Solid line is drawn for the animals younger than 13.5 years old andthe dotted line for the animals older than 13.5 years old.

Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

29

age is 1.5 years and the body length at this age is 174 cm as indicated by the mean
growth curve (Fig. 4). This method is not adequate to estimate the minimum
weaning age. However, it is more reliable to estimate the maximum weaning age.
Accordingly the mean weaning age is 1.5 years, and some dolphins may continue tosuck milk until they become 2 to 3 years old. However, this mean weaning age
seems to be overestimated, because the possible loss of newborn calf is expected(see page 25 ).
From these informations it is concluded that S. coeruleoalba starts feeding on
solid food at the age of about 0.25 year (or 133 cm), the mean weaning age of thespecies is approximately 1.5 years (or 174 cm), and some dolphins may continue totake milk till the age of 2 to 3 years.

Tooth eruption
At birth the teeth of young animals have not yet pierced the gums. Teeth
begin to erupt at about 110 cm and are almost erupted at approximately 130 to
150 cm in body length, as may be seen from the data shown in Table 4. The orderof tooth eruption is: first, the 10-40th tooth of the upper jaw, and then the same
numbers of the lower jaw, and finally those close to anterior and posterior ends.
The body length in which teeth of both upper and lower jaws almost erupt wellagrees with the body length of the animals which start feeding on solid food.
Postnatal growth
As the age determination of S. coeruleoalba is reliable till 14 dentinal layers (seepage 25 ), in this section the postnatal growth till 14 dentinal layers was analized
in detail with abundant data.
Based on the data of 1, 752 males and 1,535 females, the mean body length wasplotted against each dentinal layer, and the mean growth curve of S. coeruleoalba
was drawn by assuming body length at birth is 100 cm (Fig. 4). Fig. 4 shows that
S. coeruleoalba attains the body length of 166 cm and 180 cm at the age of 1 and 2
years in average, respectively. The sexual dimorphism of the body length appears
at the age of about 3 years. The mean body length of the males is larger than thatof the females at the ages from 3 to 10 years. The mean growth rate of the females
increases at the age from 7 to 8 years. However, the mean body length of the females in this period does not exceed that of the males at the same age. Afterthe age of
9 years, that is close to the mean age of attaining sexual maturity in the females, themean growth rate of the females becomes slower, and the growth in the mean bodylength stops at the age of 12 years. On the other hand, the mean growth rate of themales is almost constant during the ages from 5 to 10 years. The mean growth rateof the males increases at the age of 11 years. Then the increase of the mean bodylength stops at the age of 13 years. The mean body lengths of 284 males older than
14 years, and of 584 females older than 12 years were calculated at 238 cm and 225
cm, respectively. According to Kasuya (1976) the asymptotic ages of S. coeruleoalba
in the female and the male are 17 and 21 years, respectively. However, the mean
body length offemales older than 12 dentinal layers is close to the asymptotic length
Sci. Rep. Whales Res. Inst.,
No 29, 1977.

MIYAZAKI

30

of the female obtained by Kasuya (1976), and that of males older than 14 dentinal
layers is larger than the asymptotic length of the male obtained from Kasuya (1976).
This indicates that if more abundant data are available, the asymptotic lengths of
S. coeruleoalba in both sexes would be slightly larger than those expected by Kasuya(1976).
REPRODUCTION IN THE MALE

Testis weight and the maturity
The bilateral difference of testis weight was examined for 30 individuals. The
ratio of the heavier testis to the lighter one came between 1.00 and 1.32, and theratio less than 1.20 was observed in 93% of all the pairs. The ranges of the testis
TABLE 5.
Weight(left, g.)

T-TEST OF THE SYMMETRY OF THE TESTIS WEIGHT

Range(L. - R.)

<6.8
6.8-15.4
15.5;;;;
Total-1.2 -1.0 -1.5 -1.5 -Meandifference

SamplesizeStandard
error

Probability-0.2100-0.1700
3.29000.9700

10
10
10
30

0.14870.19322.30910.86660.80<p>0.90
0.50<p<0. 70
0.80<p<0.90
o. 70<p<0.80

0.3
0.9
22.0
22.0

500
300
OI

r:::

l'

l/)

I

:;: 100.:·. • ..
p!:
: r.
....
•.

l/)

i

GI

....... 50
~

GI
.i::.

:

30

OI

:

:

.•

:

i

I :

!. - •- . t<
:.

0

.i::.

i.: ! !
i.;:.
.. i:

11 •

~

;.

10

D

~:~

I~:.

.~

~

·~r~~ ~

GI~.
: i~;~t'~.

;;:: 5
3

.: .
0

..
i

'

5

10

15

20

25

30

Age in years

Fig. 5. Relation between age and the weight of the left testis. The weight of theleft testis is plotted on logarithmic scale. Closed circles indicate the mature animals, open circles the immature, and open circles with rod the pubertal.
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

31

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

weight of immature, pubertal, and mature males on the average were obtained atless than 6.8 g, from 6.8 to 15.4 g, and at 15.5 g or more, respectively, as indicatedin the latter sections of this study. The mean differences of the testis weights inthese ranges are small enough to consider that there is no significant asymmetryof the testis weight (Table 5 ). Therefore, the weight of the left testis was used in
the present study.
In order to check the maturity tissues were taken from the center of the crosssection at the midlength of the testis and the tissues collected from testes of 350
males were histologically examined. Immature, pubertal, and mature males were
defined as the animals having testes containing only spermatogonia, both spermatogonia and spermatocyte, and spermatozoa, respectively.
Fig. 5 shows the relation between age and testis weight. The testis weightrapidly increases at the ages of 2.5 years when pubertal male first appears and of9years which is about the mean age attaining sexual maturity. The increase of
testis weight stops at about the age of 13.5 years. However, Kasuya (1976) reported that there was no increase related with the age of the animal after 15 years
TABLE 6.
Testisweight(g)

MATURITY OF THE MALE AND ACTIVITY OF THE MATURE
MALE BY GROUP OF THE LEFT TESTIS WEIGHT
MatureImmature

12345678910152025303540455060708090100150200Total

Total

PubertyMI3(100)
12 (100)
13 (100)
13(65.0)14(73. 7)
3 (20.0)5(35. 7)4(57.1)I (20.0)

7 (35.0)5(26.3)12 (80. 0)8(57.1)3(42.9)4(80.0)8(61.5)7 (43.8)
1 ( 8.3)
1 ( 5.3)
1(10.0)1 ( 6. 7)5(38.5)7 (43.8)
7(58.3)15(78.9)4(40.0)6(40.0)5(38.5)3(27 .3)
5(31.3)2 (22.2)
1(10.0)2(12.5)4 (33. 3)
3(15.8)5(50.0)8(53 .3)
6(46.2)6(54.5)8(66. 7)
6(37 .5)
3 (33 .3)
I (10.0)5( 9.1)2(13.3)

68

58

Sci. Rep. Whales Res. Inst.,

Miil

Total

I ( 7. 1)

61

Figures in parentheses indicate percentage.

No. 29, 1977.

MII

59

2(15.4)2(18.2)4(33.3)5(31.3)4(44.4)8(80.0)17 (100)
50(90. 9)13(86. 7)3(100)
108

5
9
11
18
9
14
13
11
12
16
9
10
17
55
15
3
228

3
12
13
20
19
15
14
7
5
13
16
12
19
10
15
13
11
12
16
9
10
17
55
15
3
354

Maturity
(%)
0
0
0
0
0
0
7.1
0
0
38.5
56.3
91. 7
94.7
90.0
93.3
100
100
100
100
100
100
100
100
100
100

32

MIYAZAKI

old. This age difference appears to be related with differences in the methods ofage determination.
Table 6 shows the ratio of immature males to the total males for each testis
weight. The testis weight of the pubertal males ranges from 4.0 to 37.0 g. The
relation between this ratio and testis weight in the range from 3.5 to 10.5 g can beshown by a straight line. The following formula was obtained by the least squares
method, y= l l.332lx-26.6626 r=0.8407; x: testis weighting, y: the ratio, r:
correlation coefficient. The testis weight that 50% of the animals attain the
puberty is 6.8 g.
Table 7 shows the ratio of immature males to the total males at each age. The
youngest pubertal male was 2.5 years and the oldest immature was 11.5 years. The
relation between this ratio and age in the range from 3.5 to 10.5 years can be shownby a straight line. Calculation by the least squares method gives the formula,y=l5.2060x-51.5292 r=0.9652; x: age in year, y: the ratio, r: correlation
coefficient. The age when 50% of the animals attain puberty is 6. 7 years
and the body length at this age is 210 cm as indicated by the mean growth curve(Fig. 4).
The ratio of mature males to the total males for each testis weight is shown in
TABLE 7.

MATURITY OF S. COERULEOALBA BY AGE GROUPS
Male*

Female**Age
immat.
0.5
1. 5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5
17.5
18.5
19.5
20.5~

Total

pub.

mat.

total

1(100)
4(80.0)11 (100)
17 (85.0)
15(78.9)9 (50. 0)5(17.9)3 (14.3)
4 (14. 8)
0 (0)I ( 4. 8)
0(0)
0(0)
0(0)

0(0)
1 (20.0)0(0)3(15.0)4(21.1)7 (38. 9)15(53.6)9(42.9)6(22.2)2(10.0)0(0)5 (15. 6)
3 ( 9. 4)
I ( 4.2)

0(0)
0(0)
0(0)
0(0)
0(0)
2(11.1)8(28.6)9(42.9)17 (63.0)
18 (90. 0)20(95.2)
27(84.4)29(90.6)23(95.8)

5
11
20
19
18
28
21
27
20
21
32
32
24

0(0)
0(0)
0(0)
0(0)
0(0)
70

0(0)
0(0)
0(0)
0(0)
0(0)
56

21 (100)
11 (100)
12(100)10(100)15(100)
222

21
11
12
10
15
348

immat.

pub.

mat.

total

4(100)
5 (100)22 (95. 7)
42 (97. 7)32(86.5)
20 (74.1)
17(63.0)9(36.0)6(16.7)6(14.3)3 (8.3)0(0)I (1. 3)
0(0)
0(0)
0(0)
0(0)
0(0)
0 (0)
0(0)
0(0)
167

0(0)
0(0)
1(4.3)
I (2.3)3 (8. I)
6 (22 .2)
8 (29. 6)
8 (32.0)16(44.4)
7(16.7)5(13.9)2 (3.4)1(1.3)
0(0)
0(0)
0(0)
0(0)
0 (0)
0(0)
0(0)
0(0)
58

0(0)
0(0)
0(0)
. 0(0)
2(5.4)
I (3. 7)2 (7. 4)8(32.0)14(38.9)29(69.0)28(77 .8)
56(96.6)
73 (97 .3)
54(100)57 (100)
46(100)44 (100)
40(100)27 (100)
21 (100)
42 (100)
544

4
5
23
43
37
27
27
25
36
42
36
58
75
54
57
46
44
40
27
21
42
769

* The testis was histologically examined.
* * The ovaries were weighed and the number of corpora albicantia and Iutea was counted.
in parentheses indicate percentage.

FiguresSci. Rep. Whales Res. Inst.,
No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

33

Table 6. The testis weight of the mature males varied from 7.7 to 225.2 g. The
relation between this ratio and testis weight can be shown by a straight line in therange from 2.5 to 27.5 g. The following formula was obtained by the least squares
method, y=4.2577x-15.8157 r=0.9819; x: testis weight in g, y: the ratio, r:
correlation coefficient. The testis weight that 50% of the animals attain sexual
maturity is 15.5 g.
Table 7 shows the ratio of mature males to the total males at each age. The
youngest mature male was 6.5 years and the oldest pubertal was 14.5 years. The
relation between this ratio and age can be shown by a straight line in the range from
5.5 to 11.5 years. Calculation by the least squares method gives the formula, y=
17.0643x-97.7893 r=0.9923; x: age in year, y: the ratio, r: correlation coefficient. The age when 50% of the animals attain the sexual maturity is 8. 7 years
and the body length at this age is 219 cm as indicated by the mean growth curve(Fig. 4).
Fig. 6 shows the length frequencies of mature animals and pubertal or immatureones at each age from 6.5 to 10.5 years. This figure clearly shows that the body
Female

60
40

Male

6.5

years

7.5

years

20

"'c:

0

.s:

40

CL

-

20

..,

0
40

0

20

.

0

.0

E
20

z
0

'"''~

20

180
B

o

200

220

y

I

d

240

260

180

200

e

g

h

i

n

t

n

· 2W
c

240

260

m

Fig. 6. The body length frequencies of both sexes in each age. White squares indicate immature or pubertal animals, and black mature animals.

Sci. Rep. Whales Res. Inst.,.No. 29, 1977.

MIYAZAKI

34

length of the mature animals is larger than that of the pubertal or immature onesat the same age, and maturity rate of the older animals is higher than the younger
ones at the same body length group. These suggest that the larger animals attainsexual maturity earlier than the smaller in the same age, and the sexual maturity
rate of the animals increases with the age in the same body length.
In order to estimate the spermatogenic activity of the testis, 20 seminiferoust<iro1415141s 25 24 13 1s

s

1

16

s

16

s

1

3

3

a

2

1

I

200

c
·- 150

I

"'N
"iii
~100

"
"

.0

I-

0

150

100

50

0

Weightof left

I est is200
in g

Fig. 7. Relation between the weight of the left testis and the diameter of seminiferous tubules. Closed circles indicate mean and vertical lines range. Solid line
is drawn by eyes. The numbers at the top indicate the sample size.
TABLE 8.

SEASONAL VARIATIONS OF TESTIS WEIGHT AND SEMINIFEROUS TUBULE

SeasonOct.

Immature

Pubertal

No.

T.W.

S.T.

No.

T.W.

S.T.

No.

T.W.

S.T.

E.

6

4.4

40

14

7.5

48

66

127

M.

9

5.1

40

4

15

59

2
(6.3)
4
(8.0)

50

94

4

8.5

61

I

35

121

47

109

29

110

29

84

II

59

33

98

L.

Nov.

Dec.

MI

E.

2

4.4

46

5

13

63

M.

5

6.9

48

5

9.5

57

L.

6

9.6

50

II

15

63

E.
M.

42

3.9

43

15

7.0

47

(25. 0)8(15.4)
22(56.4)
20(55 .6)3(42.9)

L.

Total
Mean

70

58
4.8

44

60
(27. 3)
10

55

T.W. and S.T. indicate mean left testis weight (g) and mean seminiferous tubule size (µ),respectively.
Sci. Rep. Whales Res. Inst.,

No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

35

tubules selected at random from each testis of the sexually mature males were histologically examined. The mature males of type MI and type Miii were definedas the animals having testes containing spermatozoa in only one tubule and in all
20 tubules, respectively. The mature males of type MII was defined as the animals at the intermediate stage of type MI and type Miii. The testis weights of themature males in type MI, MII, and Miii range from 7.7 to 80.0 g, from 17.2 to
159.7 g, and from 42.2 to 225.2 g, respectively. The ratios of the mature males
in type MI, MII, and Miii to the total mature males for each testis weight are
shown in Table 6. The relation between the ratio of type Miii and testis weight
can be shown by a straight line in the range from 40 to 100 g. Calculation by theleast squares method gives the formula, y=l.5680x-58.1632 r=0.9639; x:
testis weight in g, y: the ratio, r: correlation coefficient. The testis weight thatthe ratio of type Miii shows 50% is 68.9 g. This indicates that the mean testis
weight at the attainment of the full sexual maturity is considered to be approximately 70 g. This value well coincides with the weight at the age of 13.5 years(Fig. 5).

Relation between the testis weight and the diameter ef seminiferous tubule
The testes of 348 males were histologically examined. The mean of the measurements of 20 seminiferous tubules selected randomly from a testis was used as thediameter of the tubules of the specimen. The diameters of the tubules in 70 immature, 58 pubertal, and 220 mature males range from 33.0 to 65.0 µ (mean: 43.5 µ),from 34.0 to 104.0 µ (mean: 54.6 µ), and from 45.0 to 214.0 µ (mean: 130.8 µ),
IN THE IMMATURE, THE PUBERTAL, AND THREE TYPES OF THE MATURE MALES
Mature
MIII

MII

Total

No.

T.W.

S.T.

No.

T.W.

S.T.

No.

7
(21. 9)7(14.0)
3(75. 0)16(30.8)
16(41. 0)12(33. 3)

97

170

119

168

121

158

23
(71. 9)39(78. 0)

119

155

47

156

49

122

45

119

35

112

32
(100)
50
(100)
4
(100)
52
(100)
39
(100)
36
(100)

17

73

1
(14. 3)62(28.2)

28(53.8)
1(2.6)
4(12 .1)3(42.9)

98

72

152
169

49

103

55

92

130

108

Figures in parentheses indicate parcentage in the mature.
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

S.T.

111

166

113

150

44

148

78

136

37

115

33

95

31

75

73

131

220
(100)

98
(44.5)
55

7
(100)

T.W.

153

MIYAZAKI

36

respectively.
Fig. 7 shows the relation between the seminiferous tubule size and testis weight.
The increase of the diameter of the tubule is rapid when the testis weight becomesfrom 15 to 20 g, that coincides with the mean testis weight at the attainment of sexual
maturity. This increase becomes slower at testis weight of 70 g or more where themales attain full maturity. These suggest that the testis grows with the increase ofdiameter of seminiferous tubule till about 70 g, and then the testis weight and thediameter of seminiferous tubule show no significant correlation.

Seasonal change of reproductive activity in the testis
In order to find out the seasonal change of reproductive activity in the testis,studies were made on the variations of testis weight, of the diameter of seminiferous
tubule, and of the ratio of mature males in type Miii to the total mature males. Table 8 shows the variations by ten days period. This Table indicates that there is noseasonal change in both the testis weight and seminiferous tubule size for immatureand pubertal males during early October and middle December. As for the mature• -Octobero --- November• -·- December
Cl

200

c:
Ill

-.
I ll

-

150

-..

0

..c:

~

t\

100

: \ It

Cl

.
. 50

I

?

/

;:

~I\ R
I~
ii\~/\\!\,''it\•If H I I
\

,,

/'
p

c:

0

II
/\

t/

0

\

\

I

:::;:

.l

\

b

&j1.,,,\. !·"'·\..i''V/

I

f

_,l"

J\

p'

1

....

J"-1.

.6 •.

0 LI:!:~:i::l.....L..L..W....W....L-J'"-L-L..l-._,_.L..L.L...L...W...J....J.-l....1-L-JL.-L-'--L..J
25
30
15
20
10
5
0
Age

in

years

Fig. 8. Relation between age and the mean weight of the left testis. Closed circles(solid line) indicate the males caught in October, open circles (dotted line) themales in November, and closed squares (chain line) the males in December.
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

37

males, the seasonal variations of the testis weight, of the seminiferous tubule size, andof the ratio of the mature males in type MIU are also shown in Table 8. This indicates that the testis weight, the tubule size, and the ratio of mature males in type
MIU gradually decrease from early October till middle December. As it is expected that such a seasonal change of the reproductive activity in the testis might
be affected by the difference of the age composition of the animals, the testis weightof the males at each age was analized by each month from October to December(Fig. 8). This shows that after the mean age at the attainment of sexual maturity(8. 7 years), the mean testis weight is heaviest in October and lightest in December.
From these informations, it is suggested that the reproductive activity of thetestis is highest in October and then it gradually decreases till December between
October and December. This does not well coincide with the main mating seasons estimated from the latter part of this study. Such a seasonal change of reproductive activity of the mature males seems to be affected by the difference of thetime in the migration period of mature males into the fishing area, because the sampling period is short.
REPRODUCTION IN THE FEMALE

The maturiry
In order to determine the sexual conditions of the females, the ovaries, the mammary gland, and the fetuses were examined in details. Immature animals were identified as those that had the ovaries with no corpus luteum or albicans, pregnant ones
as those that had the ovaries with corpus luteum, the lactating ones as those had the

01

c
.>-

....

Cl

>

0

--

5

<I>

0

.c

01
<I>

3::
0

5
Age

10
ii')

yea.rs.

Fig. 9. Relation between age and the left ovary weight.' Double crosses indicate
the mean weight of the left ovary.
Sci. Rep. Whal.es Res. Inst.,

No. 29, 1977.

MIYAZAKI

38

mammary gland secreting milk, and the resting females as those that were matures
but neither pregnant nor lactating.
The pubertal females were identified by the following method. As the leftovary of S. coeruleoalba becomes mature earlier than the right (Ohsumi 1964), therelation between the weight of the left ovary of the immature and age is shown in
Fig. 9. This shows that the weight of the ovary was about 2.0 g or less during 2
and 6 years, and it rapidly increases after the age of 6 years, when the mature female
first appears. The weight frequencies of the left ovary with one corpus albicans(251 individuals) and of the left ovary with no corpus albicans or luteum (191 individuals) are shown in Fig. 10. The former had a mode in 3.5 to 4.0 g and the

Ill

100

c
.i::

n.
0
-0
~

0

50

"'

..CJ

E
:J

z
0
0

'·

2
Weight

0 f

left

8

6
ovary

in

10

g.

Fig. 10., Frequency of the left ovary weight. White squares indicate the left ovary
with no corpus albicans and black the left ovary with one corpus albicans.

latter also one in 1.0 to 1.5 g. This indicates that the weight frequency of the leftovary can be divided into two groups at the ovary weight of approximately 2.0 g.
From these informations the pubertal females were tentatively defined as the immature females who had the left ovary weighing 2.0 g or more. The youngest pubertal
female was 2.5 years and the oldest immature female was 12.5 years. The ratio of
the pubertal females to the total females at each age is shown in Table 7. The
relation between that ratio and age in the range from 3.5 to 11.5 years can be shownby a straight line. Calculation by the least squares method gives the formula,y=l3.1883x-42.9792 r=0.9803; x: age in year, y: the ratio, r: correlation
coefficient. The age when 50% of the animals attain the puberty is 7.1 years, andthe body length at this age is 209 cm as indicated by the mean growth curve (Fig.
4).
Table 7 shows the ratio of mature females to the total females at each age. The
youngest mature female was 4.5 years and the oldest pubertal was 12.5 years. The
relation between this ratio and age in the range from 5.5 to 12.5 years can be shownby a straight line. The following formula was obtained by the least squares method,
Sci. Rep. Whales Res. Inst.,
No. 29, 197.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

39

y= 15.1036x-83.0946 r=0.9846; x: age in year, y: the ratio, r: correlation coefficient. The age when 50% of the animals attain sexual maturity is 8.8 years,
and the body length at this age is 216 cm as indicated by the mean growth curve(Fig. 4).

Diameter of corpus luteum
Fig. 11 shows the relation between the mean diameters of corpus luteum of 206
pregnant females and the time in months after the start of the last gestation. The
date of conception of pregnant female was estimated from the date of kill and thetime after conception given by the fetus size on the basis of the mean growth curve(Fig. 15). The pregnant female confirmed no fetus was tentatively considered as

30

E

25

E

0

c:

·-

20
c:
0

.. 1 5
E
Cl

0

10

5

-

I
I

Cl

, .

a..

:!
1 .•

-1~2~~~~~~0~~-'-----'--"--"-,2-L--l.--l.--1--1--12_4_i__,__,__,__J

MonthsFig. 11. Change of the diameter of corpora lutea during the period of pregnancy
and regression of the diameter of corpora albicantia after parturition. Closed
circles indicate the reliable combinations obtained from Fig. 13 and open circles
the mean diameter of corpora lutea in the interval of 0.25 month.

the earliest pregnant female. The mean diameter of corpus luteum of these females
was 20. 7 mm. It may continue the growth until 3 months after the conception andattain the diameter of 29.3 mm. Then the mean diameter of corpus luteum slightly
decreases with the lapse of time. If the regression line is calculated by the leastsquares, it gives the relation: y=-0.447lx+23.5098, where y is the mean diameterin mm and x is the proceeding time after conception. The mean diameter at thetime of parturition was calculated at 23.5 mm from this formula.

Regression of corpus albicans
Fig. 12 shows frequency of diameters of corpora albicantia at each age. The
modal diameter gradually decreases with the increase of the age of the animal until
Sci. Rep. Whales Res. Inst.,

No. 29, 1977.

40

MIYAZAKI

14.5 years. Then the highest peak of the mode stays between 2 and 5 mm in diameter. The minimum size of the corpora albicantia was 2.3 mm. These indicatethat the corpora albicantia reaches to final size within several years after the startof regression and may stay in the ovary all the life time.
The regression curve of the diameter of corpora albicantia was estimated by themethod of Kasuya et al. (1974). As the size frequency of the largest corpora albicantia of lactating females in the same school may represent the frequency of parturition seasons, the length frequencies of calves and size frequencies of the largest
corpora albicantia oflactating females in corresponding schools are shown in Fig. 13.
The time after parturition was estimated from the mean modal length of calves on
the basis of the mean growth curve (Fig. 4).
No
5

2

11

2 3 24ofcorpora2 0 1 9 24 2 3 1t

10

4

2

2

6

E 10
E
c

·....

...

<I>

<I>

E
a 5

''

--

0

10

15
Age

20
in

25

years

Fig. 12. Distribution of the diameter of corpora albicantia. The dotted line indicates the position of the mode and the numbers at the top show the sample size.

The mean modal diameters of the largest corpora albicantia oflactating females
were plotted against the time after parturition in Fig. 11. It shows that the mean
diameter of corpora lutea at parturition (23.5 mm) rapidly decreases to 7.0 mm
within 4 months. Then the speed of regression gradually slows down and after 18
months the diameter decreases in an almost straight line. However, it is difficult todistinguish between the corpora albicantia of ovulation and those of the last parturition. If a few probable corpora albicantia of ovulation are omitted as in thecase of S. attenuata (Kasuya et al. 1974), the regression curve of the diameter of corpora albicantia seems to become slightly earlier than that expected here.

Ovulation diagram
Of 296 pregnant females, the 26 females on which all the ovulations occured
within 12 months before the last conception were picked up in order to make ovulaSci. Rep. Whales Res. Inst.,

No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN
8 0 d y120

100

length

140

41

cm

160

180

200

I

29

33
I

34
36
37
39
40
42

---43

44

20

15
D

10

5

ameter(mm)

0Fig. 13. Comparison of the frequency of the diameter of the largest corpora albicantia of lactating females (black square) and the length frequency of juvenile
calves (white square). The numbers at the left indicate the school no.

tion diagram. All corpora albicantia of these pregnant females may be probably corpora albicantia of ovulation, because these pregnant females were not synchronously
lactating. This diagram was made by finding out the conception date of the pregnant females from the kill date and fetus size, and the ovulation date of them fromthe size of corpora albicans and the regression curve of corpora albicans. Fig. 14
shows ovulation diagrams of each pregnant female. This figure shows that all theanimals have not regular ovulation rhythm. In the animals of field no. 29 fromschool no. 15, of no. 13 from school no. 34, and of no. E6 from school no. 42, theovulation occurs at about 4 months interval and sometimes does at approximately
one month or less interval. It also occurs at about 8 months interval in the animals
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

42

MIYAZAKIof field no. El from school no. 36, of no. 44 from school no. 38, and of no. 344 fromschool no. 44. These suggests that the ovulation may usually occur at about 4
months or 4 x N months interval (N: integer), and sometimes at approximately
one month or less interval. Accordingly the ovulation of the species may occur by
combination of several ovulation intervals.

Schoo I Fie I d
n o

no

27

14

29

2

29

12

29

15

29

22

34

13

34

15

34

42

34

E5

36

E1

36

E6

38

18

38

3"

38

43

38

44

39

10

40

I8

40

35

40

107

42

E5

42

ES

43

121

44

135

44

232

44

29 0

44

34 4

...
.
..
..

c 1

•

2

M

0

3

4

6

5

h

s

8

9 10 11 12

7

No.of
No.of
corporc corpora

0

0

0

0

0

0

0

0

0

0
0

«>

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

I

n

0
0
0

0

0

0
0

•o
«>

•

0

•

0

0

0

00

0
0
0
0

0

0
0

0 0
0
0

0

0

0

0

0
0

0

0

0
0

0

0
0

tut ea albicontia
1

3

1

2

1

3

1

2

1

1

1

2

1

1

1

3

1

1

1

1

1

1

1

2

1

4

1

1

1

1

1

3

1

3

1

1

1

2

1

I

1

3

1

2

1

4

1

2

1

1

1

1

Fig. 14. Ovulation diagrams of 26 pregnant females. Closed circles indicate thecorpora albicantia of pregnancy and open circles the corpora albicantia of ovulation. C means conception date. For explanation see text.

Mating season
Since the main sampling period was limited in October to December, it is difficult to define the mating season of S. coeruleoalba with precision. In this study themating season of the species was estimated from the following two methods.
First method: Length frequencies of 603 fetuses and 73 newborn calves bymonth are shown in Fig. 15. Records of 21 individuals caught in May 1951 werecited from Nishiwaki and Yagi (1953). As they did not report the body length offetuses smaller than 15 cm but gave the range and the number of fetuses, the middle
value of the range was used in this study. Fig. 15 shows that there are three peaks inthe body length frequency in October, 10.0, 68.3, and 110 cm and also in November,
5.0, 45.0, and 78.3 cm. Laws (1959) showed that fetal growth in odontoceti can. be described by a linear plot oflength except for the very early stage of growth. As
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

43

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

the gestation length of S. coeru:eoalba was estimated at 12 months (Kasuya 1972)
and the mean body length at birth of the species was about 100 cm (see page 25), if
Lto of Laws (1959) is tentatively .used as 0.15 x gestation length, the fetal growthrate in the linear plot of length is calculated at 9.8 cm per one month. Taking
account of this fetal growth rate and also by using the method of Laws (1959), themean fetal growth curve can ?e drawn from the following four points, 8.1 cm in May,.o cm in October, 78.3 cm m November, and 94.0 cm inJ anuary (Fig. 15). When
68
this growth curve is moved for 4 months, any mode observed in Fig. 15 well fits to
it. On the other hand, from ~he differences of the mean body lengths between
these peaks at each month, the mtervals of the. mating seasons are estimated as 3.4
months (mean: 4.5 months) on the basis of this fetal growth rate. These60:~g~est that the mating season may occur in the interval of approximately 4 months.

E

100

u

--~Y : /Ieng! h at birth---

.-c

-"'

.c

c

-"' 5.0
,,>0

'1l

0
J

F

M

A

M

A

S

0

N

D

F

MonthsFig. 1 . Body length frequencies of .fetuses and newborn calves by month. The
5
feta! growth curve is dra_w~ pre~ummg 12 ~onths of gestation. The specimens in
May are cited from N1sh1wak1 and Yagi (1953). White squares indicate thefetuses, black squares the newborn calves, and closed circles the mean body lengthin each mode.

Second method: Since the gest~tion length of S. coeruleoalba was 12 months (Kasuya) the mating season was estimated from the parturition season on the basis of1972
h p~rturition dates of 567 fetuses, 238 newborn calves younger than 1.0 year (ort e II r than 166 cm), and 144 juvenile calves from 1 to 2 years of the age (or from
sma e
..
d
cm to 180 cm). The partunt10n ate of the animal was predicted from the166
kill date and from body lengt~ as indicated by the mean fetal and postnatal growth
curve. The seasonal fluctuation of the parturition is shown in Fig. 16, with the
49 specimens caught from 1968 to 1972. Fig. 16 shows that there are year-to-year
9variations in the timing and the number of mating peaks, and when the data for all
years are combined it can be said th.at the mating season is prolonged, with threehigher points in Feburary to May, ~~July to September, and in December. To
estimate the interval of the partunt1on date, the frequency of parturition dateSci. &p. Whales Res• .lnst.,

No. 29, 1977.

44

MIYAZAKI
1966

60

"'c:

40

.c:
a.

20

196 7

·1 9 6 91 9 6B

1sio

0

0
"O

150

Tola I

6

60
40

.a

E
~

z

50

20
0
M

0

n

h

s

Fig. 16. Frequencies of parturition dates estimated from the body length of fetuses
and postnatal calves on the basis of the mean growth curve. Black squares indicate the parturition dates estimated from fetus size, striped squares from thebody length of newborns younger than 1 year old, and white from the body lengthof the calves at the age between 1 and 2 years. Double crosses indicate peak.
TABLE 9.
Stage or age
fetus

0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5
17.5
18.5
19,5
20.5Total*

SEX RATIO OF S. COERULEOALBA BY AGE GROUPS
No. of males

No. of females

269

247
54
66
85
106
99
82
64
53
74
72
69
96
106
85
81
78
69
57
40
40
59
1,535

71

60
93
122
135
139
114
139
133
105
98
59
86
71
72
42
41
48
39
30
55
1,752

} 131
875

}

419

327

} 120
563

}

428

424

Sex ratio

1.09
1.31
0.91
1.09
1.15
1.36
I. 70
I. 78
2.62
1.80
1.46
1.42
0.61
0.81
0.84
0.89
0.54
0.59
0.84
0.98
0.75
0.93
1.14

} 1.09
1.55

}

0.98

0.77

* exclude the fetuses.
Sci. Rep. Whales Res. Inst.,
No. 29, 197..;.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

45

between 1969 and 1972 was available, because the frequency in these years was composed of the data of three stages, which were fetus, newborn calf, and juvenile calf
stages. The interval between the nearest peaks ranges from 3 to 7 months (mean 4. 7
months). This indicates that the mating season of the species may occur in the interval from 4 to 5 months.
From above informations it is concluded that there are three mating seasons ina year, namely, from Feburary to May, from July to September, and in December,and the mating season may occur in the interval from 4 to 5 months.

Sex ratio
The sex ratio in this study is shown by the number of males per one female.
The overall sex ratio was 1.14. Table 9 shows the change of the sex ratio in relation to the age of dolphins. In both the fetuses and newborn animals of the age
younger than 2.0 years, the number of males slightly exceeds that of females. The
ratio of the males at the age between 2 and 9 years is extraordinary high. Thissuggests that there is sexual segregation in this period. The number of males isalmost equal to that of females in the young mature animals at the age between 9
and 14 years. However, the ratio of the males at the age older than 14 years is toolow. This suggests that the mortality of the older mature males higher than thatof females at the same age group.
DISCUSSION

It was reported that a pair of opaque (in summer) and translucent (in winter)layer is annually accumulated in the case of Tursiops truncatus (Sergeant 1959),
Globicephala melaena (Sergeant 1962), Physeter catodon (Ohsumi et al. 1963), andStenella attenuata (Kasuya et al. 1974). The seasonal change of the dentinal layers
in the present results reveals good agreement with those of the above four species.
On the basis of the comparison of the dentinal and cemental layers of the same
tooth on S. coeruleoalba, Kasuya (1976) indicated that the accumulation of dentinal
growth layers ceases at a large variety of the age above 11 dentinal layers and thatthe accumulation of cemental layers continues longer. However, the dentinal
growth layer is more useful than the cemental layer to determine the age in theyounger animals in which the former is more readable than the latter. Thus thepresent study analized the growth and reproduction of the animals younger than
13.5 years old by means ofdentinal growth layers.
Ohsumi (1966) showed an allometric relationship, B=0.532 M 0 • 916 , between
mean body length at birth (B in m) and that of female at the attainment of sexual
maturity (M in m) for Physeter, Berardius and several species of Delphinidae. If
this formula is applied for S. coeruleoalba assuming M=2.16 (m), 1.08 m is obtained
as the mean body length at birth. This figure is too larger than the length obtainedin this study. Same tendency is observed in S. longirostris (Perrin et al. 1975),
S. attenuata in the Pacific coast of Japan (Kasuya et al. 1974), and S. attenuata in theoffshore eastern tropical Pacific (Perrin et al. 1976). On the other hand the ratios
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

46

MIYAZAKIof the mean length at birth to the mean length of the females at attainment of sexual
maturity were calculated as 0.45, 0.49, 0.46, and 0.46 in S. longirostris (Perrin et al.
1975), S. attenuata in the Pacific coast of Japan (Kasuya et al. 1974), S. attenuata inthe offshore eastern tropical Pacific (Perrin et al. 1976), and S. coeruleoalba (this study),
respectively. The constancy of these values clearly indicates that the mean length
at birth well correlates with that of the females at the attainment of sexual maturity
in the genus of Stenella. Then the relation between the mean length of the females
at the attainment of sexual maturity and the mean length at birth in Stenella wascalculated by the least squares method. This gives the relation y=0.4758x- l.9152(r=0.9584), where y is the mean length at birth in cm, x is the mean length of thefemales at attainment of sexual maturity in cm, and r is correlation coefficient.
According to the observations on T. truncatus born and kept in aquallia (Tabolga
and Essapian 1957, Nakajima et al. 1963) and to the examination of stomach contents on S. attenuata (Kasuya et al. 1974), these species start feeding on solid food fromabout 0.5 year old. This coincides with the result obtained on S. coeruleoalba in thisstudy. T. truncatus continue to take both solid and milk till about 1.5 years. Thisis similar to the present result on S. coeruleoalba. However, Kasuya and Miyazaki(1976) pointed out that the length oflactation in S. coeruleoalba became shorter withthe decrease of population size. On this problem further study should be done in
the future.
According to Kasuya et al. (1974) and Perrin et al. (1976) the age of S. attenuata
at attainment of sexual maturity in the female is slightly younger than that in the
male. However, the present study indicates that the sexual maturity in S. coeruleoalba
is attained at about 9 years of the age in both sexes. This result well coincides withthat in S. coeruleoalba obtained by Kasuya (1972, 1976).
Kasuya (1972) showed that the mating seasons of S. coeruleoalba were expected
in November and December, and in May and June. The present study, however,shows that there are three mating seasons in a year, namely, from February to May,
from July to September, and in December. This is rather similar to result of S. attenuata (Kasuya et al. 1974). According to Kasuya et al. (1974), the species in thecolder waters have one mating season in spring or in summer and those in the warmer
waters have two or three in a year. The present result seems to support this idea.
The present work shows that the sex ratio at birth is almost parity, indicating
coincidence with the case of S. attenuata in the offshore eastern tropical Pacific (Perrin
et al. 1975). However, in the cases of S. attenuata in the Pacific coast of Japan(Kasuya et al. 1974) and S. longirostris (Perrin et al. 1975) the number of the males
born slightly exceeds that of the females suggesting that the sex ratio at birth appears to be dependant on the population, but not on the species.
For S. attenuata (Kasuya et al. 1974, Perrin et al. 1976) and S. longirostris (Perrin
et al. 1975) it was reported that the mortality rate for the adult male was higher thanthat for the adult female. The present study that confirms these observations
strongly suggests that mortality rate of the adult male is higher than that of thefemale in the genus Stenella.
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

GROWTH AND REPRODUCTION OF STRIPED DOLPHIN

47

ACKNOWLEDGMENTS

The collections of specimens were made under the kind help of the fishermen's cooperative unions at Kawana, Futo, and Taiji. I am indebted to Dr. T. Kasuya of
the Ocean Research Institute of Tokyo University, Dr. K. Hirose ofTokai Regional
Fisheries Research Laboratory, and Mr. N. Oguro of the Oyster Research Institutefor collecting the data and samples. The greatest thanks are due to Dr. T. Kasuya,
and Professor M. Nishiwaki and Dr. T. Higa of Department of Marine Sciences,
University of the Ryukyus, who kindly read the manuscript and provided useful
criticism. I also thank to the following persons for their kind help and valuableadvices: Drs. T. Kajihara and K. Numachi of the Ocean Research Institute ofTokyo University, and Professor T. Fujiyama of Department of Marine Sciences,
University of the Ryukyus.
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and the corpus albicans of blue white dolphin (Stenetla caeruleoalba). J. Mamm. Soc. Japan, 5 (1): 3339.
HIROSE, K., and M. NISHIWAKI, 1971. Biological study on the testis of blue white dolphin, Stenella caeruleoalba. J. Mamm. Soc. Japan, 5 (3): 91-98.
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.
KASUYA, T., 1976. Reconsideration of life history parameters of the spotted and striped dolppins based on
cementa! layers. Sci. Rep. Whales Res. Inst., 28 : 73-106.
KAsUYA, T. and N. MIYAZAKI, 1976. The stock of Stenella coeruleoalba off the Pacific coast of Japan.
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OHSUMI, 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.
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cetacea. J. Mamm. Soc. Japan, 3 (I): 3-7.
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0HSUMI, S., T. KASUYA, and M. N1sHIWAKI, 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., D. B. HOLTS, and R. B. MILLER, 1975. Preliminary estimates of some parameters of growth
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48

MIYAZAKI

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Sci. Rep. Whales Res. Inst.,
No. 29, 1977.