RECONSIDERATION OF LIFE HISTORY PARAMETERS
OF THE SPOTTED AND STRIPED DOLPHINSBASED ON CEMENTAL LAYERSTOSHIO KASUY A
Ocean Research Institute, University of Tokyo, Tokyo

ABSTRACT

Growth layers are visible in both cementum and dentine of thedecalcified and stained teeth of Stenella attenuata and S. coeruleoalba. Theiraccumulation is annual in young dolphins, but in older animals only theaccumulation of cementa! layers are detected. Growth of the dolphin
lasts longer than expected from previous studies. The maximum age
is more than 45 years and sexual maturity is attained at 9 years infemales of both species. Annual natural mortality rate of 7.5% in S.
attenuata and the total mortality of 11.8% in S. coeruleoalba are estimatedfor adult females. In immature animals the mean mortality is lower,
and higher in adult males. Annual ovulation rate is about 0.41 inboth species. Reproductive parameters are analysed in relation to theage of animals and the present population level.
INTRODUCTION

The age determination of delphinids has been made by counting the growthlayers in dentine or cementum. The accumulation of layers was shown to beannual in case of Tursiops truncatus based on the animals born and died in an
aquarium (Sergeant 1959, Sergeant et al. 1973). Sergeant (1962), Kasuya (1972),
and Kasuya et al. (1974) also indicated the possibility of the same accumulation
rate for Globicephala melaena, Stenella coeruleoalba, and S. attenuata
respectively based on the materials covering limited seasons, and analysed thelife history of those dolphins.
However, through the analysis of life history parameters of an unexploited
population of S. attenuata, Kasuya et al. (1974) concluded that the accumulationof dentinal growth layers ceases in older animals as already indicated bySergeant (1962) for G. melaena. This made it difficult to study the life historyof the species, and even if the life history parameters are estimated by otherindirect method the accuracy was unsatisfactory.
The present study was undertaken to reconsider the life history parametersof the spotted dolphin, S. attenuata, and of the striped dolphin, S. coeruleoalba,
which were fomerly estimated through the analysis of dentinal growth layers
or by other indirect method (Kasuya 1972, Kasuya et al. 1974, Kasuya andMiyazaki 1976). Since the cemental layers abandoned in the study by Kasuya
Sci. Rep. Whales Res. Inst.,
No. 28, 1976, 73-106.

KASUYA

74

et al. (1974) were found in this study to be readable if the teeth were stained
after the decalcification of a thinner ground section.
MATERIALS AND METHOD

All the materials used in this study were collected from the catch of the drive
fishery off the Pacific coast of Japan. Though the samples were collected
randomly from the catch, as the species have the tendency to form a separate
schools in relation to the sex, growth and reproductive stages (Kasuya 1972,
Kasuya et al. 1974, and Miyazaki 1975) and also some members of a school
will be lost during the drive, the samples are not necessarily an unbiased
representative of the population. This bias was considered in the analyses.
As shown in Table 1 all the samples of S. attenuata are the same as thoseused by Kasuya et al. (1974) except for 2 schools (nos. 8 and 9), which were
examined by Mr. S. Matsui of The Taiji Whale Museum and by myself
respectively. The data from these two schools were used only for the estimationof the sex ratio at birth.
TABLE I.
Schoolno.

SpeciesS. attenuata

SOURCE OF MATERIALS USED IN THIS STUDY
Date ofdriving

23, Oct., '70
10, Nov., '70
4, Nov., '72
3
4
13, Nov., '72
516, Nov., '72
12, Feb., '73
6
72, Ju!., '73
824, Jan., '75
916, Jan., '76
S. coeruleoalba 14, Dec., '71
22
38
17, Nov., '72
15, Nov., '73
43
20, Nov., '73
44
23,24, Nov., '75
45,46

Place ofdriving
Kawana

2

Fu to
Taiji

KawanaNo. dolphinscaught examined
264
1381

131
273

189
192
67
146
60
102
91
903
411
414
1724
1000

48
117
23
122
36
102
44
3061 )
203
250
470
399

No. dolphins aged
males females
17
9
17
54
7
45
9

27
15

217 2l

62 2l

37
105
78

68
65
81

13
54
15
67
9

1) All immature except 7 sexually mature males.
2) Based on dentine layers read by ordinary method.

The materials of S. coeruleoalba were collected from the catch of six drives
off the lzu coast. The school numbers in Table 1 are the same as those usedby Miyazaki (1975). Though the five schools from nos. 38 to 46 in Table 1 werecomposed of mature and immature animals of both sexes, the school no. 22 waspeculiar in the extreme scarcity of mature animals and the higher ratio of themales as indicated in Table I and Figs. 18 and 19 showing the age composition.
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

75

From school nos. 45 and 46, only the informations on body length, sex, and ratioof reproductive stages of adult females are available.
The growth layers were counted independently by three methods on alongitudinally sectioned tooth. The first is the ordinaly one (Kasuya et al.
1974), or to count the opaque dentinal layers under transmitted light with astereoscopic microscope ( x 50 to x 80). The thickness of these sections was 50
to 70µ. For the second and the third methods, the tooth slides used for the
first method were ground thinner to a thickness between 10 to 20µ, anddecalcified in the 5% water solution of formic acid for several hours. Then they
were stained with Mayer's haematoxylin solution and mounted with glycerin.
By this preparation the dentinal and cementa! growth layers are observed clearlyas the well stained and poorly stained layers arranged alternatively. For theage determination by the second method, the number of stainable dentinal layers
was counted with a microscope ( x 100). The f et al dentinal layer is not includedin the count. The disagreement of repeated count by this method was less than
5%. The counting of cementa! layers, the third method of age determination,
was made with a microscope ( x 150) on a same slide used for the second, andat a part of the thickest cemental layer, which usually situates at a position
between cervix and the midlength of the root. Though the arrangement of
cementoblasts seems to be correlated with the stainable cementa! groundsubstance, the counting of the layer was made on the condition of the latter. Three
counts were usually made on the best spot of a slide and the middle figure wasused as the number of cementa! layers. The disagreement of the repeated counts
was usually from 5 to 15%. The discrepancy of the repeated counts is larger
in older animals. The fetal cementa! layer was not usually detected by theabove method, and possibly not included in the count.
As one accumulation cycle of the layers consists of a stainable and an
unstainable layer, the real number of the cycles of an animal with n dark layers
is expected to be between n-1.5 and n+0.5. Then the average number of thecycles of the animal is considered to be n-0.5, if the samples are obtained
evenly from all seasons. Though, it is not actually proved if each kind of layer
represent the period of 6 months, the figure n - 0.5 was used tentatively as theapproximate age of the animal.
The presence of corpora in the ovaries was used as the indication of thesexual maturity of the female, and their number was counted by the ordinary
method (Kasuya et al. 1974). Usually the weight of left testis was measured,and the right testis only when the left is not available. The maturity of themale was determined by the testis weight based on the mean weight of single
testis at the attainment of sexual maturity 68 g for S. attenuata (Kasuya et al.
1974) or 16.5 g for S. coeruleoalba (Miyazaki 1975, Kasuya and Miyazaki 1976).

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

76

KASUYARESULTS

Comparison of the methods
The relationship between the number of dentinal growth lyers counted byordinary method and that counted on decalcified and stained dentine is shownin Fig. 1. As the stainable dentinal layer seems to correspond approximately
with the translucent layer or with the part transitional from opaque totranslucent layer of undecalcified dentine, the two counts compared are notindicating the number of same kind of layers. So, the disagreement of only one

25

0

./

S. attenuata

510

S. coeruleoalba

20
25 0
5
10
No. layers (ordinary method)

15

15

20

25

Fig. 1. Comparison of count of dentinal layers made by ordinary method (undecalcified)
and that made after decalcification and staining. Open circle indicates female, closed circle
male, and solid line the equal accumulation rate. Larger circle indicates five animals, andthe smaller one.

layer is not significant, and it is suggested that the two methods can give almost
same result for the dolphins younger than 8 layers and that after this age thecorrelation becomes less. The count on undecalcified tooth with more than
12 layers gives, in general, smaller figure than that on decalcified and stained
dentine. There is detected no sexual difference in this relationship. The
disagreement of the counts of dentinal layers by the two methods will be causedby the difficulty of reading the layers in secondary dentine, or by the difficultyof distinguishing the thin layers of older animals. If so, the count made on
decalcified and stained preparation of thinner tooth section can be a better
indication of the age, since the layers are more clearly observed by thispreparation.
However, when the number of dentinal layers in decalcified and stained
tooth is compared with that of cemental layers of the same tooth slide, the twocoincide extremely well only up to the age of 9 layers in S. atteniiata or theSci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

S. attenuata

77lS. coeruleoalba

25
<II

.£
..... 20
c
<II
-0

.£ 15
(/)

......

<II

fi 10
0

z

5

5

10

15

22 32 42 05No. layers in cementum ·15

22 32 42

Fig. 2. Comparison of the accumulation rate of dentinal and cementa! layers.
counted after decalcification and staining. For other marks see Fig. I.

Layers wereage of 11 layers in S. coeruleoalba (Fig. 2). After these ages the latter usually
gives the higher number, and there is observed almost no correlation between
the number of layers in dentine and cementum in teeth with more than 20
cemental layers. This must indicate that the acumulation of the dentinal layers
ceases at an large variety of age above 9 layers (S. attenuata) or 11 layers (S.
coeruleoalba), and that the accumulation of cemental layers continues longer asin the case of G. meiaena (Sergeant 1962). The ages of the oldest animal were46 cemental layers in S. attenuata and 50 in S. coeruleoalba. They are about
twice as high as those estimated from the dentinal growth layers.

Acciimiilation rate of layers
As the sample of S. attenuata covers wider range of seasons, the seasonalchange of the dentinal growth layer was analysed for this species, Table 2
shows the condition of the dentinal growth layers being calcified at the time
TABLE 2. CONDITION OF DENTINE BEING CALCIFIED
AT THE TIME OF DEATH OF S. ATTENUATA~ntine*Date~

OctoberNovemberFebruary
July

Stainable layer
no. dolphins %

16
62
40

76.2
96.9
95.2
7. 7

* Stained dark or pale by haematoxylin.
Sci, Rep. Whales Res. Inst.,
No. 28, 1976.

Unstainable layer
no. dolphins
%
5

23.8

2

3. 1

2

4. 8
92.3

12

Total
no.

/o

21
64
42

100

13

100

0/

100
100

78

KASUYAof death. Only the animals younger than 9 growth layers were selected here,
because the accumulation of the layer ceases in some animals after this age.
The frequency of the dolphins with the stainable layer at the pulp wall is about
75% in the catch of late October, and it increases to about 95% in November
and February. However the ratio is reversed in early July. This is anindication of that the formation of stainable layer starts, on the average, inearly October and that of the unstainable layer in between March and June.
Kasuya et al. (1974) suspected from the observation of undecalcified tooth
sections of the same animals that the translucent dentine is formed in the seasonfrom December or January to April or May and opaque dentine in the otherseason. As it is easily expected that Kasuya et al. (1974) might have overlookedsome of the translucent layers or opaque layers which were too thin to be
detected on the thicker and undecalcified section, the seasons when the alternationof the layer occurs can be shown slightly delayed in their study than the
result of this study. So the slight difference between the two conclusions
is not significant. This .is another indication of the fact that the dentinal layers
stainable with haematoxylin correspond, as mentioned before, with translucent
layers or with the part transitional from the opaque layer to the translucent
layer of undecalcified section (Plate III, Fig. 2). As the seasonal change of thecondition of the incomplete latest dentinal layers of S. coeruleoalba studied by
Kasuya (1972) on undecalcified tooth section is same with that of S. attenuata
reported by Kasuya et al. (1974), the seasonal alternation of the dentinal layersof the two species seems to follow the same pattern. The present results on theseason of formation of dentinal layers is consistant even with the results on G.
melaena (Sergeant 1962), Hyperoodon ampullatus (Christensen 1973, Mitchelland Kozicki 1975), and Physeter catodon (Ohsumi et al. 1963).
In order to know the growth rate of dentinal layers, the seasonal changeof the thickness of the most recent incomplete layer must be detected. As a
preparatory procedure of this, the change of the thickness of the layers
accompanied with the age of the animal was checked on the enlarged photographsof the decalcified and stained tooth sections of two S. attenuata and two S.
coeruleoalba, some of which are shown in Plates. In these measurements thefirst incomplete layer which start from the neonatal line was not included.
The measurements of the subsequent layers are shown in Fig. 3. The thicknessof the layer of S. coeruleoalba decreases rapidly untill 3rd or 4th layers. After
this age the decrease is almost constant, and the thickness of a layer is about
95% of that of the preceeding layer. This feature seems to be same even in
the case of S. atteniwta, though the thickness of each layer is larger during
the first few years.
The following relationship is lead from this result.

when tn=the thickness of the most recent incomplete layer
T n=the expected thickness of the same layer at the time of completion
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA~---------------~

!''~

.30

'
R, ,o,,.ft ,<i

.25

E

E .zo

,R,

/\

;

I

'

\

P... ..

:~:::~\ \ / \/ \ \~! \~·! 1
:

\

if

..c .15: :\:,:·,:· ·:',:

3 .10

1.0
.8

\,f
ll

::'•

-0

1.2

337

!..•..,',,\.',,,

''

.~

79

0

0

.6 0:

\!
~

.05

.4

161

o~~~5~-'--'~,~o-'-'-~~,L5-'--'-'-L2Lo-'--J-'--L2~5_,_L-..J.2

No. of layers
Fig. 3. Thickness of dentinal growth layers shown in relation to the age of theanimals. Solid line indicate the thickness of each layer, and circle and dottedline ratio of one layer to the former layer. 27: S. attenuata, 183 cm, male, school
no. 7. 30: S. attenuata, 202 cm, female, school no. 7. 161: S. coeruleoalba, 221 cm,
female, school no. 38. 337: S. coeruleoalba, 232 cm, male, school no. 44.

T n-l =the thickness of the one cycle earlier complete layer
T n- 2 =the thickness of the two cycles earlier complete layer
Then, the rate of completion of the most recent incomplete layer, tn/T n' isshown as follows.
_t'!....=t ._Tn-2
Tn Tn-1 2
In the present study the thickness of one layer was measured from the start ofa stainable layer to that of next stainable layer, on the selected teeth of S.
atteniiata at the age betwen 4 to 12 layers. These teeth were selected considering
the preparation, clearness of layers, the regularity of layers, and in older animals
the coincidence of the numbers of layers in dentine and in cementum.
The result is shown in Fig. 4, where 4 schools caught in November aregrouped and plotted on the mean date of catch. Though there is observed asmaller peak in October, November and February, they will have been causedby the measurements of young tooth where the thickness changes rapidly. If thehighest modes were connected there is suspected a slower growth in the period
from November to June. The extraporation of the line connecting the modesin October and November suggests that the starting date of the accumulationof stainable layer is in early October, which coincides well with the estimation
has been made from the seasonal change of the frequency of the two kindsof layers being calcified. As the completion rate at the beginning of Julyis only 45%, the rate of 16.5% per month, which is close to the rate in October
15.0%, must be achieved in the accumulation of unstainable layer formed in
the summer months in order to expect the annual formation of the layer. Thisrate will not be improbable in view of the accumulation rate of opaque layers
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

KASUYA

80

100 ,
0

0-

80

.\;;

c 60

.Q

Q;

a.

E 40
0
u
20
0

0

N

D

F

M

A

M

A

s

Month
Fig. 4. Seasonal change of the completion rate of dentinal growth layers.
The numerals at the top indicate the sample size. The interval of one
month corresponds to 5 animals. For further explanation see text.

of Hyperoodon ampullatus, about 30% in the fishing season of May and June,suspected by Mitchell and Kozicki (1975, see their discussion) based on the datareported by Christensen (1973). Possibly the growth of dentine in S. attenuata
is rapid in the summer season when the unstainable layer ( ·. opaque layer onground section) is accumulated, and slower in winter when the stainable layer( ·. translucent on ground section) is accumulated. Though Ohsumi et al. (1963)
showed a different feature on the growth rate of dentine of Physeter catodon,
as they measured the layer not on the thin ground section but on the half-cut
longitudinal section, the exact comparison is difficult at present.
As the conclusion it will be safe to say that the accumulation rate of thedentinal growth layers is annual in case of S. attenuata, and that the similarrate will be expected on the closely related species S. coeruleoalba. Since thenumbers of layers in cementum and dentine are equal in young animal andthe alternation of the layers will be affected by the same physiological change,
the accumulation of cemental layer is also considered to be annual.

Mean growth curve
The relationship between body length and age in S. attenuata and in S.
coeruleoalba are shown in Figs. 5 and 6. As the ages are grouped into eachn-0.5 years, where n indicate the number of stainable cemental layers, and thestainable layer start to be accumulated in the average in early October, theanimals of almost similar age can be grouped into separate age groups affectedby the slight difference of the dates of birth and death, or of the formation ofstainable layer, even if there is no misreading of the layer. In other words,
the difference of one year in the age of each individual is not significant.
The mean growth curves of S. attenuata in Fig. 5 were drawn by eye
referring the curves of S. coeruleoalba where the samples are more abundant,
Sci. Rep, Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

81

200
E 180
u

.!;

..c. 160f>- 140 . "

"C

S

H
120

,,

1001:

0

5

10

15

20

25

35

30

40

45

Age in years
Fig. 5. S. attenuata, scatter diagram of the relationship between body length and age estimatedfrom the cementa! layers (200 females and 158 males). Open circle and dotted line indicate
female, closed circle and solid line male, smaller circle one animal, and larger circle mean
body length. Data in parenthesis are excluded from the calculation.

---.o.;-~---------·-------- ...----------;.-----------------------

140
120
5

10

15

20

25
30
Age in years

35

40

45

50

Fig. 6. S. coeruleoalba, scatter diagram of the relationship between body length and age (340females and 488 males). The age data of the school no. 22 estimated by ordinaly method
were included. The age of other animal was based on cementa! layers. For other marks
see Fig. 5.
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

KASUYA

82

and are liable to the alteration when more data is accumulated. The bodylength at birth is about 89 cm (Kasuya et al. 1974). The animals of both sexes
attain the mean length of 142.5 cm in the first year. Then the male length
seems to exceed that of female at the age of two years. Though the data isscanty and the minor change of the growth rate is not clear, there might beagain a period, approximately at the age between 9 and 11 years, when themean length of the female may exceed or equal with that of the male. Apartfrom these minor and uncertain features, the general trend of the mean growth
curve of the immature stage does not differ from that shown by Kasuya et al.
(1974). But, as reasonably expected from the possible underestimation of theage in their study, the growth in immature period shown in the present study
appears slightly slower (Table 7). The mean asymptotic length of the female
S. atteniwta is attained at 11 or 12 years (Fig. 7), and the length is estimatedto be 193.9 cm from the mean length of 135 animals at the age more than 12
years. The mean growth curve of the male indicate slower growth after 12
years of age and seems to reach the asymptonic length of 207.1 cm at the age
100 [S. coeruleoa!ba

5. attenuata

E so

u

c

-~

60

11
..c 40
~

e 20

~

0

5

10

15

20

0

5

10

20

15

Age in years
Fig. 7. Annual growth rate of the two kinds of dolphins. The increase in mean
body length between n-0.5 and n+0.5 year groups is plotted at n years of age.
Closed circle and solid line indicate male, and open circle and dotted line female.
The feta! length was cited from Fig. 7 of Kasuya et al. (1974).
TABLE 3.

ESTIMATION OF THE. MEAN ASYl\fPTOTIC LENGTH

SpeciesS. coeruleoalba

S. attenuataSex).1ales

Females

Males

Females

Age range (years)~17. 5~22. 5~12. 5~21. 5

No. of animals

21

135

41

89

Smallest (cm)

195

177

220

200

Largest (cm)

216

211

248

250

Mean (cm)207. I

193.9

236.0

225.3

Standard dev. (cm)

8. 63

4.04

6. 14

15.29

Standard error (cm)

1. 88

0.35

0. 96

1. 62

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

83

of about 22 years.
The mean growth curves of S. coeruleoalba was drawn by eye assuming
99.8 cm for the body length at birth (Kasuya 1972). The age of the school no.
22 was estimated from the number of dentinal growth layers counted by theordinary method, because this does not seem to give bias since the animals
constituting this school are young ranging from 1.5 to 10.5 years of age (Figs.
18 and 19). As the dolphins of larger body length tend to attain the sexual
maturity at an age younger than the shorter animals do (see Kasuya 1972 Fig.
4), and other schools are mainly composed of adult animals, it is necessary to
add the age-length data of the school no. 22 to eliminate this bias.
S. coeruleoalba attains the mean length of 164.0 cm at the age of I year.
The sexual dimorphism of the body length appears at the age of about 2 years
as in the case of S. attenuata, and then the lengths of both sexes increase atan almost constant rate retaining the difference of about 4 cm until the age of
6 years. As shown in Table 7, the growth rate of this period is slightly smaller
than that estimated by Kasuya et al. (1974). The mean growth rate of thefemales increases at the age from 6 to 7 years, and attains at age from 7 to 8
years a length slightly exceeding that of the male. After the age of 8 years,
which is about one year before the onset of sexual maturity, the mean growthof the female becomes slower, and mean growth curve reaches the asymptotic
length of 225.3 cm at about 17 years. Though the growth of the male is slightly
faster at the age between 7 and I 0 years than the preceeding period, theaccelerated growth is not conspicuous. After the age of 10 years the growth ofthe male becomes slower and attains the asymptotic length of 236.0 cm at theage of about 21 years (Table 3).
The mean growth curves of the two species of Stenella discussed here show
similarpattern. The ages when the growth rate changes correspond to theacompletion of weaning at 1.5 to 2.4 years indicated by Kasuya (1972) and in
the latter section of this study, attainment of sexual maturity discussed below,
and the completion of the attainment of physical maturity. Though theaccelerated growth of the male near the attainment of sexual maturity is notconspicuous, the growth rate of the fem ale seems to become higher just before
the attainment of sexual maturity. The fact that the body length of the female
near the onset of sexual maturity exceed that of male at the same age is at
close resemblance to G. melaena (see Sergeant 1962, Fig. 11). The low growthrate preceding the age at asymptotic length might by a reflection of thedecrease of the growth rate of each animal and of the increasing ratio of theanimals which have attained the physical maturity. The age when mean growth
curve attains the asymptotic length will correspond to the age when all theindividuals attain the physical maturity.

Attainment of sexual maturity
When the presence of the scar in the ovary or the testis weighing morethan 68 g (Kasuya et al. 1974) is used as the criterion of the sexual maturity in
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

KASUYA

84

S. atteniiata, the youngest sexually mature animals are found in the age groupof 7.5 years (both sexes) and the oldest immature animals in that of 11.5 years(female) or 15.5 years (male). The following formulae are obtained by the least
aquares for the relationship between the ratio of mature animals (y) and theage of the animals (x).
Female: y=l7.43x-106.93

6.5~x~l2.5

Male

6.5~x~ 16.5

: y= 10.59x-74.84

The age where 50% of the animals are sexually mature is estimated from theabove formulae to be 9.0 (female) or 11.8 (male) years (Fig. 8). These values
are slightly higher than those obtained by Kasuya et al. (1974), as expected fromthe limit of ageing by dentinal layers.

.8

S. attenuata
36 females
73 males.~.... .6

138 males

,,

;;J

0

,'1

~ .4

'

'

,' 0

o'

.2

'

I
I

:

I
I

,/

:

/

0 /
6

S. coeruleoalba
110 females

f

~

J

8

10

12

14

16

4

6

8

10

12

Age in years
Fig. 8. Relationships between age and the ratio of sexually mature animals. Opencircle and dotted line indicate female, and closed circle and solid line male. In
S. caeruleoalba smaller circles include school no. 22, and the larger exclude it.

In case of male S. coerideoalba the mean weight of testis at the attainmentof sexual maturity, 16.5 g, was determined Miyazaki (1975), where the presenceof spermatozoa or spermatid in the tissue taken from the center at the midlengthof testis was considered to be mature. In spite of this criterion, there werealways observed both spermatozoa and spermatid in the mature testis (Miyazaki1975) as in the case of S. atteniiata (Kasuya et al. 1974). As shown in Table 4,
this criterion seems to give a same age at the onset of sexual maturity withthe criterion based on the presence of spermatozoa in the epididymal smear.
However other criterion may give different results. For the female S. coeruleoalba, the maturity was determined by the presence of the corpus in the ovaries.
If the age-maturity data of 152 (146 immature and 6 mature) males and
25 immature females of the school no. 22 are dealt together with the data ofschool nos. 38, 43 and 44, the ratios of immature males are unreasonably low
in the age groups below 9 years and the age of 9.7 years is obtained as themean age of the male at the attainment of sexual maturity (Fig. 8). However,Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

85

TABLE 4. COMPARISON OF DIFFERENT CRITERIONSFOR THE MATURITY OF MALES. COERULEOALBA
No. oflayersAge in
years

2

I. 5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
I I. 5
12.5
13.5
14.5
15.5

3
4
5
6
7
8
9
10
II
12
13
14
15
16

Testicular smear
imm. mat.
mat.%
2
8
0
2
3
4
0
2
0
0
0
0
0
0

0
2
I
3
6
I
7
6
8
17
29
2
7
4

0
20.0
100
33.3
50.0
85. 7
20.0
100
75.0
100
100
100
100
100
100

Epididymal smear
imm. mat.
mat.%
2
9
I
3
4
2
5
2

0
1

0
10.0

0
0
2

3
0

4
8
16
27

0
0
33.3
71. 4
0
71. 4
57. I
100
94. I
96.4
100
85. 7
100

2
0
I
0

5
0
5

3
6
5

Testicular weight 1l
imm. mat.
mat.%
9
11
8
7
7
7
7
6
7
4
4
1
0
0
0

0
0
0
0

0
0

5
6
12
22

0
0
12.5
22.2
12.5
45.5
46.2
75.0
84.6

32
6
11
8

97.0
100
100
100

2

I) Less than 16.5 g was considered to be immature (Miyazaki, 1975).

as mentioned before, this school is mostly composed of the immature males
affected by the special schooling behavior, and the data from this school may
not be suited for the present purpose. On the other hand, as the other 3 schools
show less specialized age distribution even if the frequency below 9 years israther low, they will give better estimation of the mean age at onset of sexual
maturity. The least squares regressions of the age-maturity relationship calculated for the data of these 3 schools are as follows.
Female: y=l2.60x-56.51

4.5~x~ 13.5Male

4.5~x~ 13.5

y= 12.05x-58.94

They give 8.5 years for female and 9.0 years for male as the age where 50%of the animals are sexually mature. Though the figure of the female is slightly
lower compared with the corresponding age of female 8.8 years (Kasuya 1972)
or 9.1 years (Miyazaki 1975), these difference may not be significant because aslight underestimation is expected from the scarecity of the immature animals
below 9 years of age in the 3 schools discussed here. The mean age of the maleat the attainment of sexual maturity estimated above shows good coincidencewith 9.2 years and 9.0 years estimated by Kasuya (1972) and Miyazaki (1975)
respectively.
Fig. 9 shows the relationship between age and the weight of a testis. The
weight shows a rapid increase at the age between 9 and 13 years in S. atteniiata
or between 7 and 11 years in S. coeruleoalba, which correspond to the age at
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

86

KASUYA

600 s attenuata
400
300

S. coeruleoalba

:

200

: .
.: .

..

..

. ...: .

100
Ol 80.......,
60--.:.t~iti~:·::,;.

-~

lll
Cl.I

._
~

. i .:. : ~· .
.,;.. .. :

40
30

:

0

Ol

·a;

S:

. '..

20

._

.c

.

' . ...

:

10

~·r

. .:....

. '. : ..

8
6

:

...
•I

.l~ i

l1;

:

4
3

·::

•

2 -·

I

5 9 13 17 21 25 29

5 9 13 17 21 25 29 33 37 41 45

Age in years
Fig. 9. Scatter diagrams of the relationship between age and the weight of testis.
Two age groups are combined. In S. coeruleoalba data from school no. 22 areincluded. Weight of right testis is plotted if the left is not available. Larger
circle indicates 5 animals and the smaller one.

onset of sexual maturity. However after the age of 15 years, the weights are
distributed between 120 g and 600 g (S. attenuata) or between 40 g and 150 g(S. coeruleoalba) and there is observed no increase related with the age of theanimal. This stage will correspond to the full sexual maturity and probably
to the stage of social maturity as suspected by Miyazaki (1975). This feature
is completely different from that observed in Physeter catodon where the weightof testis increases after the attainment of sexual maturity at or little above theprevious rate (Gambell 1972).

Accumuration rate of corpora in the ovaries
The persistency of the corpora for entire lifetime of the female wassuggested in S. attenuata by Kasuya et al. (1974). Possibly this is also true forS. coeruleoalba (Hirose et al. 1970, Miyazaki 1975).
In S. attenuata, though the individual variation is large, there is expected
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

87

a linear relationship between the number of corpora and the age (Fig. 10). The
following regression is obtained by the least squares for the animals older than
7 years of age,
y=0.412x-1.97
where y indicates the number of corpora, and x the age in years. This indicatesthe mean annual ovulation rate of 0.412, or 1.62 ovulations in one reproductive
cycle of 3.94 years (see page 88). Kasuya et al (1974) estimated, through theanalyses of corpora diameter, the annual ovulation rates of between 0.407 and0.595 for 5 age groups classified by the corpora number. And they concluded
from these estimates that the annual ovulation rate changes from 0.432 of young
animal to 0.487 of the animals with highest corpora number. Though they
considered that this indicates the change of the ovulation rate related with theage of the animal, it is theoretically incorrect because the number of corpora
in the ovaries is the function of both the age of the animal and of the individual
specific ovulation rate, and these two factors could not be separated in theirstudy. It should have been interpreted that the mean annual ovulation rate ofthe dolphins which may attain higher corpora number is higher than the mean
rate of the other dolphins. Furthermore, the diameter of the corpora is not amuch reliable indicator of the time elapsed after the ovulation, and the ovulation
rate estimated from its analysis is less accurate. Accordingly the ovulation
rate of S. attenuata estimated in this study will be the present best estimate.
As shown in Fig. 11, the relationship between the number of ovulations
and the age of S. coeruleoalba may not come on a single linear relation. If
the regression lines are calculated separatly for the age classes from 5 to 25
years and for those from 26 to 43 years. The following relations are obtainedby the least squares.
y=0.414x-2.40
5~x~25y=0.115x+6.55 26~x~43.
However, the first regression line seems to fit to the animals up to 33 years ofage. There are expected three reasons for this. The first is the simple sampling
error caused by the scarcity of the samples of higher age, the second is thedecline of the ovulation rate or the resorption of corpora at higher age, andthe third is the recent increase of ovulation rate might have happened as the
response of the population to heavy exploitation started since the last war.
The possibility of miscounting of the corpora will be neglected, because thecounting was made by myself with the same method for both species of Stendla.
The third possibility is also improbable because the ovulation rate of 0.115 perannum is too low even as the rate at initial population level, and because theovulation rate of S. attenuata possibly at the initial level (Kasuya et al 1974) isalmost identical with the first rate 0.414 per annum. Accordingly, the possible
explanations can be only the first or the second. And, though the first might
be more probable, the conclusion on this problem will have to wait until moredata are accumulated.
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

KASUYA

88

ig 15
0

0

:i

~10

0

6

z

5

8 i:

0

20
25
30
Age in years

15

10

5

35

.40

.45

Fig. 10. S. attenuata, scatter diagram showing the relationship between age and numberof corpora in the ovaries (124 females). Thin straight line and thick straightline indicate mean corpora number and regression line obtained by least squares
respectively, for both only the data reoresented by closed circles are used.

20

C/l

c 15

.Q

0

:i
>

0

:

10

.

0

.

d

z

5

:
:

.

0
5

10

15

20
25
30
Age in years

35

.40

Fig. 11. S. coeruleoalba, scatter diagram showing the relationship between age andnumber of corpora in the ovaries ( 170 females). The data from school no. 22 are
not included. Larger circle indicates five animals and smaller one. For other marks
see Fig. 10.

Reproductive cycle
In this section the reproductive parameters of the adult females are analysed
in relation to the age of the animals. When the ratio of pregnant females inthe total adult females is shown by P, that of pregnant and simultaneously
lactating by PL, that of lactating by L, that of resting by R, and the mean
length of gestation by G years, the annual pregnancy rate and the mean calving
interval in years are estimated as follows.
Sci. Rep, Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

89
('/,)

tt

10

10.0
11.1
12.5

8

14.3
16.7

in 6
.....

Cl

20.0

QJ

>

25.0

4

33.3

1. \

''
\\
/'r:\
\.... _~~..~ _/ !

\

35

40

50.0
100

45

Age in years
Fig. 12. S. attenuata, relationship between age and the mean reproductive parameters
estimated from the ratio of reproductive stages. C: calving interval (left scale)and annual pregnancy rate calculated from it (right scale), L: length of lactation,
R: length of resting period, PL: length of the period lactating and simultaneously
pregnant. Each value is calculated combining the data of nearest 5 age classes.
The age classes indicated by arrow containe no pregnant female.

TABLE 5. DIFFERENCE OF REPRODUCTIVE PARAMETERS
OF ADULT FEMALES BETWEEN AGE CLASSES
S. attenuata

Species
Age class (year)

7-15

15-25

25-35

35-46

No. animals

31

64

22

13

130

Pregnancy (years)
PL (years)

0.83
0.10

0. 72
0.22

1. 46
0. 83
3.23
31. 0

2.04
0.55
3.53
21. 7

0.94
0
1. 40
o. 70
3.04
28.8

0. 79 (0. 82)
0. 14 (0. 12)

Lactation (years)
Resting (years)
Calving interval (years)
Annual pregnancy rate (%)

0.94
0
4.06
1. 87
6.87
14.6

0. 77 (0. 77)
3.69 (3. 94)
27. 1 (25.4)

Species
Age class (year)

5-15

15-25

S. coeruleoalba
25-35

35-49

Total

72
0.95

63
0. 73

No. animals
Pregnancy (years)
PL (years)
Lactation (years)
Resting (years)
Calving interval (years)
Annual pregnancy rate (%)

Total')

1. 99 (2. 23)

22

8

165

0.94

0. 75
0.25

0. 86
0.14

0.05

0.27

0.06

0.24
0.07

0.46
0.07

0.24
0.06

1. 31
76.4

1. 54
65. 1

(164)

0. 75

0.33

1. 29

0.25
2.00

0.08
1. 41

77. 3

50.0

70.9

I) In parenthesis includes age unknown animals cited from Kasu ya et al. 1974, Table 14.

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

90

KASUYA

Annual pregnancy rate=(Calving interval)- 1 = P+cJL
Then mean length of each reproductive stage is estimated by the formulax

P+PL·G
where X indicates the ratio of females at one of the above 4 stages. This method
gives correct estimates only when the samples are not seasonally biased. As thepresent data, especially those of S. coeruleoalba, were collected in the limited
season, there might occur some bias of this kind. However, since the matingof both species of Stenella occurs in wide range of the seasons and there exist
probable three mating peaks in S. attenuata (Kasuya et al. 1974) and two(Kasuya 1972) or three (Miyazaki 197 5) peaks in S. coeriileoalba, such kind ofbias can be small enough compared with the bias caused by the inclination of
the dolphins to form separate schools by the sex, growth and reproductive stages
suggested by Kasuya (1972), Kasuya et al. (1974), and Miyazaki (1975).
The length of gestation of S. attenuata was estimated by Kasuya et al (1974)
to be 11.24 months or 0.94 years. If this gestation length is used for the above
calculation, the mean length of calving intervals is estimated to be 3.94 years,
mean length of lactation (L + PL) 2.35 years, and mean length of resting period
0. 77 years (Table 5). Though the data used here are same with those in Kasuya
et al (1974), as the treatment of the females pregnant and simultaneously lactating is different the figures are slightly changed. The analyses of the parameters in relation to the age of the female reveals that the mean calving interval
is shorter in the females below the age of 23 years and only in those age classes
the pregnant and simultaneously lactating females are observed. After this age
the length of calving interval increases, which at first step accompanies the increase of the length of lactation and then the increase of resting period. Thisphenomenon can be interpreted that, in the younger females, the lactation ceases
at about 2 years after the former parturition almost synchronizing with the startof the next conception, and in the older females as the start of the next conception delays the period of lactation is extended up to about 4 years in the
average when the calf stops suckling affected by the change of the behavior.
This age of the calf is the time when the calves are expected to leave the schoolof their mothers (Kasuya et al 1974). If, then, the next gestation does not start
for more than 4 years, there can occur the increase of resting females. The
extremely rapid increase of the mean calving interval after the age of 30 years
seems to suggest the presence of the females that ceases the ovulatory activity.
The decline of the length of calving interval or the increase of annual pregnancy
rate after the age of 37 years is the reverse of the former trend, but the similar
phenomenon is found also in S. coeruleoalba. One possible explanation for it
might be that the old females which ceases the ovulatory activity die, by theage between 35 and 40 years, earlier than the still active females which is fewer
in number, and even the latter females they often die at the last parturition
without followed by the lactation.
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

91

25.0

•

28.6

33.3

•

40.0
0-------

50.0----- ---- -----

66.7------0--0__ _
-----

100
(°lo)

0'--~"---'--'--'--'---'-1.---'--'----'----'---'---'---'---'---'---'--'--'--'--'--L-'

'55

'60

'65

'70

'75

Vear
Fig. 13. S. coeruleoalba, annual fluctuation of the mean reproductive parameters estimated from the ratio of the reproductive stages. Closed circle and solid line indicate
the length of calving interval (left scale) and the annual pregnancy rate (rightscale), open circle and dotted line length of lactation (L+PL, left scale), andsquare and thin solid line length of resting period (left scale). For further explanation see text.
('/,)

33.3

3

40.0
50.0
66.7

100

5

10

30

35

40

Age in years
Fig. 14. S. coeruleoalba, relationship between age and the mean reproductive parameters.
For further explanation see Fig. 12.

For S. coeriileoalba, there is no reliable estimation of the gestation length,
however it was roughly suspected by Kasuya (1972) to be about 12 months.
In the present study this estimation is used without further confirmation. Kasuya
and Miyazaki (1976) indicated that the mean calving interval in the populationof S. coeruleoalba off the Pacific coast of Japan is decreasing accompaniedwith a decrease in the population level. If the data of 33 pregnant, 44 lactating,and 18 resting females obtained from the two schools caught in 1975 (see Table
1) are added to the data reported by Kasuya and Miyazaki (1976), the following
trend is estimated by the least squares (see also Fig. 13).
Calving interval= -0.040838x+82.944
Length of lactation= -0.044982x + 89.811
Resting period= 0.001951 x-3.485
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

92

KASUYA

where x indicates the calender year, and length of each stage is indicated by
year. The length of lactation includes both the period of lactation and thatof lactating and simultaneously pregnant. The above formula gives the estimation of the mean calving interval of 2.39 years in between 1972 and 1973
when the present materials were collected. This is slightly longer than thecalving interval estimated from the regression in Kasuya and Miyazaki (1976).
As shown in Fig. 13, the catch in these two seasons is strongly biased by thehigh ratio of pregnant females, and the mean calving interval estimated fromthe materials used in this study is only 1.41 years (see the short bar in Fig. 13),which is only 59.0.% of the unbiased estimate calculated from the above
regression. Accordingly only a general trend of the change of reproductive
parameters related with the age of the female can be discussed in this study,
and if necessary the above bias must be corrected.
In S. coeruleoalba, the period of shorter calving interval lasts about 10 years
longer than S. attenuata until about 32 years of age, and the period when thelactating and simultaneously pregnant females occur also last longer. After
this period, the pregnancy rate declines rapidly and then increases as observedin S. attenuata. Though the reason of the difference of the period of thehigh reproductive activity between the two species of Stenella is not conslusive, it seems to be possible to attribute it to the difference of the populationlevel because the Japanese coastal population of S. attenuata is close to theunexploited level (Kasuya et al. 1974) and that of S. coeruleoalba has been
exploited since before 1897 and the decline of the population is indicated (Kasuya and Miyazaki 1976).

Sex ratio
The sex ratio of S. attenuata was already analysed by Kasuya et al. (1974)
and the predominance of the male in fetal and juvenile stages was indicated.
Table 6 shows the sex ratio of fetus and of the juvenile animals not exceeding
142 cm in body length which corresponds to the age of one year on the mean
growth curve. As the sex ratio of the species at the time of birth will be between those of the above two growth stages, the ratio of females at the instanceof birth, 41.6.%, was obtained by combining the data of the two categories.
The ratio of females at the instance of birth in the population of S. coeruleoalba was estimated by Kasuya and Miyazaki (1976) to be 47.6.% based onlarge number of samples (Table 6). Though they included the animals corresponding, on the mean growth curve, to the age up to 1.25 years, it will notgive significant bias.
Fig. 15 shows the change of the sex ratio related with the age of the dolphins. The extraordinaly high ratio of the males at the age between 5 and11 years in S. attenuata or between 2 and 15 years in S. coeruleoalba is considered to be the result of the sexual segregation in the period between weaning
and the attainment of full sexual maturity. Possibly the ages when the young
dolphins move from the nursery school to the school of immature animals andSci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLATABLE 6.

SEX RA TIO OF THE DOLPHINS AT BIR TH

fetus

S. attenuata
juvenile 2 J

No. females

39
31

79
53

Female, %Male/Female

44.3
1. 26

41. 4
1. 49

SpeciesStage
No. males

93

S. coeruleoalba 1J

Total

fetus

juvenile 3l

Total

118

351
310

385
359

736
669

46.9
1. 13

48.3

47.6

1. 07

1. 10

84
41. 6
1. 40

1) Data cited from Kasuya and Miyazaki 1976.
2) Not exceeding 142 cm.
3) Not exceeding 172cm.

3.0 .,-----.---:0..-----r-----,-----.-----.----~-~
0

20

0

0
0

.2
.1'--"-'--'--'--'---'--'--'----'---'-.1........L-'--L-l...-'-.--'--'-'----'---'-.l........L-'---'-'-'--'--'--'--'---'--'--'

0

5

10

15

20

25

30

Age in years
Fig. 15. Relationship between sex ratio (male/female) and the age of S. attenuata(closed circle and solid line) and of S. coeruleoalba (open circle and dotted line).
The ratio for the age more than one year is calculated combining the data of thenearest 5 age classes. For other explanation see text.

then to the school of adult dolphins will be different between the sexes, andthis in combination with the kinds of schools studied by me will give sucha bias in the sex ratio. If these age classes are excluded there is observed adecline of the ratio with the progress of the age of the animals, which will
indicate the sexual difference of the mortality rate. When µ and µ' indicate
the annual natural mortality coefficient of male and female respectively, andRx the sex ratio (male/female) at the age of x years, there is expected thefollowing relationship.
The regression lines in Fig. 15 and the following figures are calculated by theleast squares based on the sex ratio at birth obtained above and the points in
Fig. 15 corresponding to the ages above 11 years (S. attenuata) or 15 years (S.
coeruleoalba). In case of S. coeruleoalba the mortality is affected by natural
mortality and fishing mortality. However, as the fishing mortality rate of theSci. Rep. Whales Res. Inst.,
No. 28, 1976.

94

KASUYA

species is not expected to be different between sexes, it can be neglected in thecalculation.
S. attenuata
S. coeruleoalba

µ-µ'

Ro

0.04120.0171

1. 32
1. 06

These figures indicate that the difference of the natural mortality coefficient
between the sexes is higher in S. attenuata, and that it is partially compensatedby the higher ratio of the male at birth.

Age composition and mortality
The age composition of S. attenuata obtained from 158 males and 200
females are shown in Figs. 16 and 17. The age composition suggests the presence of segregation related with the age and the sexual maturity of the animals
below the 13 years of age as already mentioned in the former sections. The
slope of the age frequency at the right side of this point indicates the natural
mortality, as the fishing mortality is negligible in this population. Though themortality seems to be slightly higher in the females above 31 years, the leastsquares calculated ignorering this minor change gives the following annual
mortality rate for S. attenuata.
Sex
FemaleMale

Coefficient
0.0779Rate in%
7.49

Age in years

0.1135

10. 7314~

14~

The difference of the annual mortality coefficient between sexes is 0.356 andcoincides well with the corresponding figure estimated from the sex ratio.
Since the above rates can be considered to represent the mean annual
mortality rate of the adult animals, the mean mortality of the immature stage
is estimated as follows. When the following abreviations are used
A : Number of mature females
lx : Number of females at the age of x years
C : Calving interval in years
L : Litter size
m: Mean age at the onset of sexual maturitys: Ratio of females at birthµ 1 : Mean annual mortality coefficient of immature femalesµ 2 : Mean annual mortality coefficient of mature females
there must be the next relations in an unexploited stationary population.

lo=

A·L·s
C

l_n = 10 e-µim
A= 1-n ~:e-µ2cx-mldx
As C=3.94, s=0.416, m=9.0, and µ 2 =0.0779 are obtained in this study, and theSci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

95

40
30
20

c 15 •

Ill

:.c0.
0

"'O

....0
d

z

10
8
6

.

.

''

4
3
2

6

9

13 17 21 25 29 33 37 41 45 49

Age in years
Fig. 16. S. attenu:ita, age composition of the 200 females based on cementa! layers.
4 indicate School no. 4, 6 school no. 6, S sum of the school nos. I, 2, 3, 5 and 7,
and the open circle and dotted line the total of the seven schools. Each two age
groups are combined.

40
30
20
Ill 15c::c0. 10
"O

8
6

0

4
3

0

....0
z

2

5

9

13 17 21 25 29 33 37 41 45 49

Age in years
Fig. 17. S. attenuata, age composition of the 158 males. Closed circle and thick solidline indicate the total of seven schools. For other explanation see Fig. 16.

litter size is 1.0 (Kasuya et al. 1974), the following estimate of the mean annual
natural mortality rate of immature female of S. attenuata is obtained from theabove formulae.
Sex
FemaleSci. Rep. Whales Res. Inst.,
No. 28, 1976.

Coefficient
0.0338Rate in%
3.32

Age in years;;;;9.0

96

KASUYAAs the difference of the mortality coefficient between the sexes is between 0.0412
and 0.0356 or approximately 0.038, the rough estimate of the mean annual
natural mortality of the immature male is as follows.
Sex
Male

Coefficient
0.072

Rate in%

6.9

Age in years;£9. 0The natural mortality rates of the immature dolphins estimated here are themean of the mortality of all the age classes from 0 year to the age at the attainment of sexual maturity, and the possible higher mortality expected for the
suckling calves is not separated. It was already indicated by Kasuya et al. (1974)
on S. attenuata that the mean natural mortality rate in the immature period
must be lower than that in the adult stage. The present study gives results
similar to those obtained in the previous study.
The age compositions of 341 females and of 488 males of S. coeruleoalba
are shown in Figs. 18 and 19. In these age data, the age of 62 females and
217 males of the school no. 22 was determined by counting the dentinal layers
by ordinary method. Because of the segregation of the immature individuals(Kasuya 1972, Miyazaki 1975) as in the case of S. attenuata, only the slope
above the age of 12 years in females or that above 14 years in males can indicate the mortality under certain assumption mentioned below. They are as
follows.
Sex
FemaleMale

Coefficient
0.09190.1143Rate in%

8. 78
10.80

Age in years
12 ;£
14;£

Above values can indicate the mean annual total mortality during theperiod from 1920's to 1959's only when the population is assumed to bestationary. However, as the exploitation of this population became higher
near the end of the last war as in the case of Phocoenoides (Kasuya 1976 a and b)
and the population has been decreasing (Kasuya and Miyazaki 1976), theestimation is not fully accurate. The total mortality rate in the postwar period
must be estimated separately. This can be made using the frequency of corpora
number, because the method can afford more abundant data. Fig. 20 shows
the age composition of sexually mature females calculated from the frequency
of corpora number cited from Kasuya and Miyazaki (1976) and the accumulation
rate of corpora (0.414/year) estimated in this study. There is observed two slopesin the age frequency. The first is from 15.9 (4 corpora) to 28.4 years (9 corpora)of age and the second slope is from 30.9 years (10 corpora) and above. If themortality is calculated separately, the following figures are obtained as theestimates of the total annual mortality rate of mature females.
Sex
FemaleFemaleFemaleCoefficient
0.12580.09510.0896Rate in%
11. 829.07

8.57

Age in years
15.9-28.4
30. 9-51. I
15.9-51.l

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENE,LLA..." .
' .

60

f,

40
30

,'

'

.

~

! \ :·~·"~·
20
&

C/l

c

22 ~11,'

'\

~-<t"

I•

d

:.c0. 15
0

-

R

,'1

I

\

10

" 86
0

d

z

4
3
2

5 9

13 17 21 25 29 33 37 41
Age in years

Fig. 18. S. coeruleoalba, age composition of the 341 females. Numerals indicate theschool number. The age composition of school no. 22 was based on the ordinary
method of reading of dentinal layers. For other marks see Fig. 16.

100

BO
60
40
30
C/l

c 20
.c 15
0.
0 10

-"
0

6

z

8

6
4
3
2

5

9

13 17 21 25 29 33 37 41 45 49
Age in years

Fig. 19. S. coeruleoalba, age composition of the 488 males.
For marks see Figs. 17 and 18.
S1i. Rep. Whales Res. Inst.,
No. 28, 1976.

97

98

KASUYA
100

80
60
40
30
VI

c

:c0.

0

"- 108
0

d

z

6

4
3
2

10

15

20

25

30

35

40

45

50

55

Age in years
Fig. 20. S. coeruleoalba, age composition of the 772 adult females based on the numberof corpora in the ovaries. Number indicate the sampling year, and closed circle andthick line the total. For further explanations see text.

The mortality rate calculated from all the age classes above 15.9 years coincides
well with the figure obtained above based on the cemental layers. This may
indicate the correctness of the above calculation. Furthermore, since the dateof birth of dolphins at the boundary of the two slopes, 28 to 30 years before
1972 when most of the samples were collected, approximately coincides withthe end of the last war in 1945, the annual mortality rate of 11.8% seems toindicate the rough estimation of the mean total mortality rate of the females
in the years near the end of the war to late 1950's. As the difference of theannual mortality coefficient between sexes is in between 0.0171 and 0.0224 orabout 0.020, the total mortality coefficient of the adult male in the corresponding years might be 0.126+0.020=0.146 or 13.6% per year. Though the meanmortality rate in the immature stage is expected to be lower than that of adultdolphins as in the case of S. attenuata, it is not possible to estimate it atpresent. The natural mortality rate is also unknown.
DISCUSSION

Age determination method
When the layers in dentine and cementum are compared, the higher number is almost always found in the latter in case of the dolphins older than 9(S. attenuata) or 11 (S. coeruleoalba) years of age. If, after these ages, thenumber of dentinal layers plotted against the number of cemental layers showed
an increase slower than that in younger stage, it would be possible to interpret
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

99

that the accumulation rate of cementa! layers increases after these ages and thatof dentinal layers stays constant. However, as the number of dentinal layersof older age groups is constant without showing any correlation with that of
cemental layers, it is concluded that the accumulation of the dentinal layers
stops at various ages between 9 or 11 years and about 25 years, or that thedentinal layers become too thin to be distinguished by the present method.
Recently same phenomenon was confirmed by Dr. P. B. Best (person. comm.)on Lagenorhynchiis obscurus through tetracyclin marking. He considers that"dentinal layers accumulate annually in young dolphins, but deposition ofdentine slows down in older animals and may cease altogether". As the toothat this stage is hardly distinguishable from that at preceeding stage throughthe observation of the pulp cavity, it seems to be safe to use the cemental
layers for the age determination of the dolphins with more than 9 (S. atteniiata)
or 11 (S. coeruleoalba) dentinal layers. However it must be noted for thepractical use of the cemental layer that the counting of the layers is moredifficult and unreliable than that of dentinal layers, and that it is necessary
to confirm the coincidence of the counts of the two kinds of layers on the
decalcified and stained preparation of the tooth of young dolphin.
Furthermore, it should be remembered that the suitable way of preparing
the tooth for age determination can be different between species of dolphins.
If the present method of preparation is applied on Phocoenoides or Lagenorhynchiis tooth, it gives good result for reading the cementa! layers. However,in cases of Globicephala macrorhynchus and Pontoporia blainvillei the decalcifi.cation and staining are not necessary if the tooth is ground thinly enough,and the thick and· irregular cemental layers in Platanista gangetica seems
difficult to use for age determination.

Life history parameters
From the above reasons, the life history parameters estimated previously
by reading dentinal layers and concerning the sexually mature dolphins are
now unreliable. They are the accumulation rate of the corpora, mortality rate,
and the age at the attainment of asymptotic length. In case of S. atteniiata,
the mean age at the onset of sexual maturity must also be corrected. However,as the accumulation rate of the corpora of this species was estimated from thecomparison of corpora diameter and the growth of suckling calves, and themortality rate from the corpora number and the accumulation rate (Kasuya
et al. 1974), they are not much affected by the present study (Table 7). It isalso confirmed that the ovulation rate and the calving interval estimated in thepresent study give the mean number of ovulations in one reproductive cycle
1.62 which is close to the estimation of 1.7 to 1.8 made by Kasuya et al. (1974).
Miyazaki (1675) estimated, for S. coeruleoalba, the annual ovulation rateof 0.59 based on the age-number of corpora relationship of the animals between
the age of 11 and 15 dentinal layers, and this result was used by Kasuya andMiyazaki (1976) for the estimation of the mortality rate and the population.
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

100

KASUYATABLE 7.

COMPARISON OF LIFE HISTORY PARAMETERS OF THE TWO SPECIESOF STENELLA OFF THE PACIFIC COAST OF JAPANSpeciesS. attenuata
Present
study

S. coeruleoalba

Parameters

Previous
study 1 J

Previous
studies

Present
study

I. Gestation (months)

11. 2489142. 7142.5

1299. 81)
164.5

164.0

} 166.0

163.0
161. 5194.0
191. 0

193.0
189.0

191. 5
190.0

186.0
187.5

220.5
217.0

222.0
218.0

2. Length at birth (cm)
3. Length at 1 year of age (cm)4. Length at 3 years of age (cm)malefemale5. Length at 9 years of age (cm)malefemale6. Length at 50% maturity (cm)malefemale218. 33 )211. 5194.3
181. 97. Asymptotic length (em)234. 41)
223.5

236.0
225.3

22

12

12

JI

21
17

10.3
8.2

11. 8
9.0

9. 2 2)

8.8

9.0
(8.5)

24
24
4. 19
2.44
23.8
0.43-0.49

40
46
3.94
2.35
25.4
0.412

25 1)
26
2. 20 ('73) 3)
0. 89('73) 3)
45. 5 ('73) 3)
0. 590 4 )5049
2. 37 ('73)
I. 06('73)
42. 2 ('73)
0.414

12. 8 3)

I I. 8

203.3
194.9

207. I
193.9

male

12

femaleJI

malefemale8. Age at asymptotic length (years)

9. Age at 50% maturity
male
female10. Maximum age (years)
male
female11. Calving interval (years)
12. Length of lactation (years)
13. Annual pregnancy rate (%)
14. Annual ovulation rate
15. Annual mortality rate (%)
adult female

7. 4

7.5

immature female

3.2
14.8
11. 8
38.9

3.3

6.9
41. 6

8.3

9

16

2. 40 5 )0. 965,6)

adult male
immature male

16. Ratio of female at birth (%)
17. Maximum no. of calves produced
by one female
18. Mean no. of calves produced
by one female

JO. 7

13.6
47. 6 3)

1) Kasuya et al. 1974, 2) Kasuya 1972, 3) Kasuya and Miyazaki 1976, 4) Miyazaki 1975,
5) e- 9µ1J~ e-µ2-"dx/calving interval, 6) µ 2 =0.07, fishing mortality coefficient 0.0558, andcalving interval 2.90 years in 1960 are assumed.

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

IOI

However, in view of the result of the present study, it seems to be improbalethat the age-number of corpora relationship can fit such a linear line asindicated by Miyazaki (1975, and see Kasuya and Miyazaki 1976). Actually,
any data possessed by myself or by Dr. Miyazaki, both of which were used in
Miyazaki (1975), failed to reproduce the graph cited in Kasuya and Miyazaki(1976). Possibly many individuals younger than 16 dentinal layers and withhigh corpora count might have been arbitrarily excluded from his calculation.
Accordingly, the accumulation rate of corpora estimated in this study, 0.414 perannum, is considered to be more reasonable.
However, as this ovulation rate is estimated from the gradient of
age-number of corpora relationship, it gives only the ovulation rate in theyear when the dolphin attained the sexual maturity. Even if the ovulation
rate is changing in recent years, it will not be detected by this method until
the recently matured females attain a certain age and the change of the gradientis detected. As the ovulation rate of S. coeruleoalba estimated in this study
is not different from that of S. attenuata at initial population level, and isclose even with 0.38 to 0.45 the rate of the P. catodon estimated by dentinal
layers (Bannister 1969 cited in Gambell 1972, Best 1970, Gambell 1972), 0.5 of
H. ampullatus (Christensen 1973), and 0.39 to 0.51 of G. melaena estimated byindirect method (Sergeant 1962), it is suspected that the annual ovulation rateof toothed whales at their initial population level will not be much different
between the species, and that the annual ovulation rate of two kinds of Stenella
estimated in this study must be that at initial population level.
The population of S. coeruleoalba off the Pacific coast of Japan has been
depleted in recent years to about half of the initial level, and the mean calving
interval is changing shorter (Kasuya and Miyazaki 1976, and see Table 8 andFig. 14 of this study). If the mean calving interval of this population in
between 1972 and 1973 is 2.39 years as calculated from the regression line (seepage 89) and the mean annual ovulation rate is 0.414, the mean number ofovulations in one reproductive cycle is only 0.989. This is not significantly
different from one ovulation per cycle, or the rate only theoretically possible
when all the ovulations are followed by the conception and seems to be actually
impossible. This problem can be explained by one or two of the following
assumptions.
I. Annual ovulation rate has started to increase in recent years, when thecalving interval is so shortened that the further shortening can not be
achieved by the improvement of the ratio of successfull ovulation.
2. Length of calving interval is underestimated.
Though it is sure that the estimation of the mean calving interval is liable toalteration when better estimation of gestation length is obtained or when thedata of the ratio of females at various reproductive stages covering wider seasons
are obtained, the bias of the present estimation was expected not to be large(page 88). And I suspect that the first assumption is more probable.
It has been generally supposed that when the population is depleted by
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

102

KASUYAthe exploitation, the pregnancy rate or the calving interval of the cetaceans
will change in order to result in the increase of recruitment (Laws 1961). Thisphenomenon seems to have been detected with insufficient accuracy in the
population of S. coeruleoalba off the Pacific coast of Japan (Kasuya andMiyazaki 1976). The difference of pregnancy rate between S. coeruleoalba andS. attenuata indicated in the present study can be the reflection of different
strength of exploitation. If the relationships between pregnancy rate and ageof the animals of the two species of Stenella are compared (Figs. 12 and 14) inview of the above hypothesis, there are expected several factors influencing thechange of mean pregnancy rate in a population. The first and the most direct
is the shortening of calving interval in all the age classes. This is expectedfrom the fact that the mean calving interval of the unexploited S. atteniiata
is about 3.4 years in the age classes of high reproductive activity, and thecorresponding figures of exploited S. coeriileoalba is about 1.4/0.590=2.4 years
when the bias of the data is corrected (see page 90). The second factor is theextension of the period of high reproductive activity from up to 22 years of age
as observed in S. attenuata to up to 32 years of age in S. coeruleoalba. These
two factors will result in the decrease of the mean length of lactation, which
is suggested from the facts that the shortening of mean calving interval of S.
coeruleoalba in recent years is achieved by the shortening of the period of
lactation (Kasuya and Miyazaki 1976, page 89 of this study) and that the lengthof lactation in the ages of high reproductive activity is generally shorter thanthat in older age classes (page 88). The third and the most indirect factor willbe the increase of the ratio of young and reproductively active females, whichis the result of the high recruitment rate brought by the former two factors
and of higher fishing mortality rate. Though the attainment of sexual maturity
at younger age is expected when the population decreases as already observedin fin whale (Lockyer 1972), it ts not clear if the difference of the mean age
at the onset of sexual maturity between the two species of Stenella observedin this study is due to it. It seems to be probable in the delphinids that thelength of nursing period is more variable than the age at the onset of sexual
maturity. So, when the population is depleted by the exploitation, the shortening of the calving interval directly related with that of nursing period as indicated in this study will occur before the change in the age at the attainmentof sexual maturity does.
Kasuya et al. (1974) estimated from the analysis of the size distribution of
the corpora in the ovaries that the maximum number of calves produced by
one female is 8.3 in S. attenuata. It was shown in the present study that theage of the oldest female of the species is about 46 years, mean calving interval
3.94 years and the mean age at the start of the sexual maturity 9 years. Thesefigures give the estimation of the maximum number of calves that can be produced by a female, or (46-9)/3.94=9 calves. This figure coincides extremely
well with the estimate made by Kasuya et al. (1974) through the analysis ofcorpora. If the similar calculation is made for S. coeruleoalba in the early l 970's,
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

103

it gives approximate figure (49-9)/2.4=16 calves. The extremely higher numberof production of calves in the latter species must be the sum of the above
mentioned reaction of the population to the exploitation.

Estirnation of the population of S. coeruleoalba
In view of the present study indicating the minor change of the parameters, the population analysis made by Kasuya and Miyazaki (1976) shall bereconsidered based on the improved parameters concerning total mortality rateof post-war period, annual change of mean calving interval, and calving interval
at initial population level taken from that of S. attenuata. Furthermore, thecatch statistics at Taiji in 1973 and the total catch of the same year shall becorrected to 1,000 and 8,200 dolphins respectively instead of 7,500 and 14,700
used by Kasuya and Miyazaki (1976), and the total catch of the species in
1974, 13,000 animals, was added.
If the analyses of the population is made by the same principle as used by
Kasuya and Miyazaki (1976), the relationship between net annual recruitment
rate (R in %) and the population (P in thousands) estimated for each year
between 1960 and 1975 based on the recruitment and catch is shown by thefollowing least squares regressions.
R= -0.01190P+4.437,
R= -0.01598P+6.462,
TABLE 8.

when µ 2 =0.07
when µ 2 =0.08.

COMPARISON OF POPULATION ANALYSIS OF S. COERULEOALBABETWEEN PREVIOUS AND PRESENT ESTIMATES
Kasuya and Miyazaki 1l

0.07
0.05354
Mean annual catch around 1960
14,000Total mortality coefficient
0. 1365
Fishing mortality coefficient
0.0665
Fishing mortality rate
0.0643Population around 1960
218,000
Population at 1974
129,000
R 4l at 1974 level
3.09%
SY 5 l at 1974 level
3,980
Initial population;?::321,000
Population, MSY 6l level
161,000
R at Msysi level
2.57%
MSY
4, 130Assumption, µ 2
Presumed µ 1 3J

2l

0.08
0.03870
14,0000. 1365
0. 05650.0549255,000
186,000
3. 21;';]'
5, 960;?::340,0'.JO
170,000
3.53%
6,010

Present estimation

0.07
0.0606
14,0000. 1258
0.05580.0543258, OOO
176,000
2.34%
4, 120;?::373,000
186,000
2.22%
4, 1400.08
0.045814,0000. 1258
0.04580.0448313,000
252,000
2.44%
6, 140;?::404,000
202,000
3.23%
6,530

1) Kasuya and Miyazaki 1976, 2) Annual natural mortality coefficient of adult female,3) Mean annual natural mortality coefficient of immature female calculated from µ 2 , sex
ratio 0.476, and calving interval of S. attenuata at initial level, 4) Net annual recruitment
rate, estimated from the relationship between P and R in the above, 5) Sustainable yield,
6) Maximum sustainable yield.
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

104

KASUYA

These equations give reproductive rate slightly lower than that in Kasuya andMiyazaki (1976). The minimum initial population is estimated as the valueof P corresponding to R=O. Other revised population parameters based on
the above equations are shown in Table 8. Though, the population is suspected
to be continuously decreasing since 1970 at an mean rate of 2, 700 (µ 1 =0.08) or
4,800 (µ 2 =0.07) dolphins per year, it is unnecessary to alter their conclusion
saying that the present status of the population is "close to the level producing
the maximum sustainable yield of 4,000 to 6,000 dolphins per annum". This is
the result of the interference of the higher population estimate lead from thelower fishing mortality rate in the post-war period and the lower recruitment
rate calculated from the new estimate of mean calving interval. However it isalso important to note that the population parameters lead from the assumptionof the natural mortality coefficient µ 2 =0.07 might be closer to the truth. The
reason is that, if we consider that the length of lifetime of S. coeruleoalba isprobably longer than that of S. attenuata by about 10% (see Table 7), thenatural mortality coefficient of adult female of the former species is suspected
to be about 10% lower than that of the latter species µ 2 =0.0779.
The annual net recruitment rate at the instance of extinction is extraporated
from the above equations as 0.04437 (when the natural mortality coefficient of
adult female µ 2 =0.07) or 0.06462 (µ 2 =0.08). The mean calving interval giving
these recruitment rates, under the assumption that other parameters stays
unchanged, are calculated with the following equations modified from those inpage 92 of this study (Kasuya and Miyazaki 1976),

10 =A·L·s/C
A= 10e-Cµi+f)m/(µ 2+f)
R=l-e-f
where f indicates sustainable fishing mortality coefficient. They are 1.59 (whenµ 2 =0.07) or 1.18 (µ 2 =0.08) years. If the ratio of females which start the next
conception increase linearly after the preceding parturition and the length ofgestation of this species is about 12 months (Kasuya 1972), all the females must
start the conception within 1.2 or 0.36 years respectively after the last parturition.
Though they are not impossible, the latter seems to be difficult to be accomplished if the mating peaks of two (Kasuya 1972) or three (Miyazaki 1975)
times in a year are retained unchanged. However, it is possible to expect at
the lower population level, the non linear change of recruitment rate affectedby the change of other reproductive parameters (age at the attainment of sexual
maturity, sex ratio, or mortality rate of weaning calves) which are not detected
at present.
ACKNOWLEDG MENTS

I am greatly indebted to Mr. S. Matsui of the Taiji Whale Museum and to
Dr. N. Miyazaki of the Ocean Research Institute, University of Tokyo, for
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

LIFE HISTORY PARAMETERS OF STENELLA

105

collecting the data and samples. Dr. R. L. Brownell of the Smithsonian
Institution kindly read the manuscript and criticized. The preparation of
tooth sections and the analysis of the data were made with the assistance ofMiss S. Wada and Miss M. Oya of the Ocean Research Institute. These persons
are deeply acknowledged.

REFERENCES
BEST, P.B., 1970. The sperm whale (Physter catodon) off the west coast of South Africa. 5. Age,
growth and mortality. Invest!. Rep. Div. Sea Fish. S. Africa, 79: 1-27.
CHRISTENSEN, I., 1973. Age determination, age distribution and growth of bottlenose whale,
Hyperoodon ampullatus (Forster) in the Labrador Sea. Norwegian]. Zoo!., 21 (4): 33-340.
GAMBELL, R., 1972. Sperm whales off Durban. Discovery Rep., 35: 199-358.
HIROSE, K., T. KASUYA, T. KAZIHARA and M. NISI-IIWAKI, 1970. Biological study of the corpus luteum and the corpus albicans of blue white dolphin (Stenella caeruleoalba). ]. 1\famm.
Soc. Japan, 5(1): 33-39.
KASUYA, T., 1972. Growth and reproduction of Stenella caeruleoalba based on the age determination by means of dentinal growth layers. Sci. Rep. Whales Res. Inst., 24: 57-79.
KASUY A, T., 1976a. The stock of striped dolphin, part 1 and 2. Geiken Tsushin, 295: 19-23,
296: 29-35. (In Japanese)
KASUYA, T., 1976b. Preliminary report of the biology, catch and populations of Phocoenoides in
the western North Pacific. ACMRR/MM/SC 21, 20p.
KASUYA, T. and N. MIYAZAKI, 1976. The stock of Stene/la coeruleoalba off the Pacific coast ofJapan. ACMRR/MM/SC 25, 37p.
KASUYA, T., N. MIYAZAKI and W.H. DAWBIN, 1974. Growth and reproduction of Stenella
attenuata in the Pacific coast of Japan. Sci. Rep. Whales Res. Inst., 26: 157-226.
LA ws, R.M., 1961. Reproduction, growth and age of southern fin whale. Discovery Rep., 31:
327-486.
LOCKYER, C., 1972. The age at sexual maturity of the southern fin whale (Balaenoptera physalus)using annual layer counts in the year plug. ]. Cons. Inst. Explor. Mer., 34(2): 276-294.
MITCHELL, E. and V.M. KOZICKI, 1975. Autum stranding of a northern bottlenose whale(Hyperoodon ampullatus) in the Bay of Fundy, Nova Scotia. ]. Fish. Res. Board Canada, 32:
1019-1040.
MIYAZAKI, N., 1975. School structure of Stenella coeruleoalba. Doctoral thesis, Faculty of Agriculture
Univ. Tokyo (Manuscript, in Japanese).
OHSCMI, S., T. KASUYA and M. NISHIWAKI, 1963. Accumulation rate of dentinal growth layers
in the maxillary tooth of the sperm whale. Sci. Rep. Whales Res. Inst., 17: 15-35.
OHSUMI, S., 1965. Reproduction of the sperm whale in the North-West Pacific. Sci. Rep. Whales
Res. Inst., 20: 1-16.
SERGEANT, D.E., 1959. Age determination of odontocete whales from dentinal growth layers.
Norw. Whaling Ga;;.., 48(6) : 273-288.
SERGEANT, D.E., 1962. The biology of the pilot or pothead whale, Globicephala melaena (Traill)in Newfoundland waters. Bull. Fish. Res.· Bd. Canada., 132: 1-84 .
.SERGEANT, D.E., D.K. CALDWELL and M.C. CALDWELL, 1973. Age, growth, and maturity ofbottlenosed dolphin (Turiops truncatus) from Northeast Florida. ]. Fish. Res. Board Canada,
30: 1009-1011.

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

106

KASUYA

EXPLANATION OF PLATESPLATE I
Fig. I. Decalcified and stained tooth section of S. attenuata, 167 cm male (school 5,
no. 54), pulp cavity at the right. White spots indicate 3 stainable cementa! layers,
and numerals stainable postnatal dentinal layers. Scale indicates 0.1 mm.
Fig. 2. Another part of the same tooth section in Fig. I, cusp at the left and pulp
cavity at the bottom. 0 indicates prenatal dentine. For other marks see Fig. I.
Fig. 3. Cementa! part of a decalcified and stained tooth section of S. attenuata, 19.5
cm female (school 3, no. 35), dentine at the right. 46 cementa! layers are counted.
Scale indicates 0.1 mm.
PLATE II
Fig. I. Decalcified and stained tooth section of S. coeruleoalba, 232 cm male (school
44, no. 337), cusp at the lower left and pulp cavity at the lower right. 28 dentinal
layers are counted. Scale indicates 0.1 mm.
Fig. 2. Cementa! part of the same tooth section in Fig. 1, dentine at the bottom.
28 cementa! layers are counted. Scale indicates 0.1 mm.
Fig. 3. Cementa! part of a decalcified and stained tooth section of S. coeruleoalba,
227 cm male (school 44, no. 166), dentine at the right. 40 cementa! layers arecounted. Scale indicates 0.1 mm.
PLATE III
Fig. I. Cementa! part of a decalcified and stained tooth section of S. coeruleoalba,
241 cm male (school 44, no. 295), dentine at the lower left. 49 cementa! layers arecounted. Scale indicates 0.1 mm.
Fig. 2. Comparison of a tooth section before and after the decalcification and staining. S. coeruleoalba, 161 cm male (school 38, no. 161), pulp cavity at the right.
Scale indicates 0.1 mm. Top: After decalcification and staining. Bars indicate
stainable dentinal layers. Bottom: Before decalcification. Bars indicate opaquedentinal layers.
PLATE IV
Decalcified and stained tooth of S. attenuata, 183 cm male (school 7, no. 27). 10 stainable layers are counted in both cementum and dentine. Numerals indicate prenatal
dentin (0) and postnatal dentinal layers (1-10). Pulp cavity at the lower right.
Scale indicate 0.1 mm.

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

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