REPRODUCTION OF THE SPERM WHALE
IN THE NORTH-WEST PACIFIC
SEIJI OHSUMI
It is need to establish the countermeasure for the control of the sperm whale population as well as some baleen whales. For reasonable control of the population, wemust make a correct diagnosis of the population size. The problem on the reproduction is one of the needful factors in the stock assessment.
On the reproduction of the sperm whale, there are some reports-Matsuura(1936, 1940), Matthews (1938), Omura (1950), Nishiwaki & Hibiya (1951), Clarke(1956), Nishiwaki, Hibiya & Ohsumi (1969), Chuzhakina (1961), etc. However,on the sexual cycle which is need to stock assessment, we have only few knowledges
yet.
Recently, age studies of the sperm whale have been developing (Nishiwaki,
Hibiya & Ohsumi, 1958; Ohsumi, Kasuya & Nishiwaki, 1963), and many materials
for the reproduction of the sperm whales in the adjacent waters to Japan have beencollected with connection to age. Present report is chiefly based on the materials,
and the results will be added new informations in some points to poor knowledges
on the sperm whale.
To our regrets, we have not yet succeeded to keep sperm whale to study reproductive physiology, and it is very deficult to get true knowledge. We must develope
the study on this problem furthermore.

MATERIALS AND METHODS

Whales Research Institute has continued to investigate the whales in adjacent waters
to Japan, and present paper is based on the biological data and materials collectedin the investigations during the seasons from 1960 to 1962.
Age of sperm whale was shown as the number of growth layers in dentine ofmaxillary tooth which was prepared with the same method as described by Ohsumi
et al. (1963).
Sexual maturity of the female was judged with ovaries, that is to say, the female
which has one or more than one corpus luteum or corpus albicans in ovaries was judged
as sexually mature. Pregnant female was also judged with ovaries, because inJapanese coastal whaling, whales are opened their belly before they are towed to aland station and foetuses are often lost at that time. The female with corpora lutea
in ovaries was judged as pregnant, although there will be a possibility to exist somefemales which is not pregnant with corpora lutea in the ovaries. Lactation of the
sperm whale was determined by examining mammary gland. Cutting the mammary grand, secretion of white milk was judged as lactation. Number of corpora
lutea and corp. albicantia in the ovaries was counted for the purpose to know number

S. OHSUMI

2

of ovulation of a whale by naked eye, and on some individuals, diametre of thecorpora were measured to study the involution of corpus albicans.
The records of foetuses in International Whaling Statistics 1937-1961 wereused to count the sex ratio of the sperm whale in foetal stage and to draw growth
curves of foetuses.
BREEDING SEASONThe breeding season is here defined as the period of pairing. because, in the sperm
whale, gestation period extends for 16-17 months, and pairing takes plase in different season with the parturition season as shown in the following chapter.
FrequencylNo.)

0,,..-,--,
10

-E 40
.§

c
0
·;::
0

>

0

0 20
2(])
E
0

i5 10

June

July

Aug.

Sept.

Fig. 1. Frequency distribution of diametre of ovarian follicles in resting and pubertal
sperm whales caught in the waters adjacent to Japan.
Cross and broken line : mean diametre.

Seasonal change of genital organ
Males: Matthews ( 1938) reported that there was no definit reproductive activity in the males examining histologically the testes of the sperm whale in thesouthern hemisphere. That is to say, testes are active in the all seasons. Nishiwaki &Hibiya (1951, 1952) examined the testis tissue of the sperm whale in Japanese coastal
waters, and described the possibility of a periodicity in the activity of testis. Clarke( 1956) examined the seasonal change of diameter of semineferous tubules in the teste~of the sperm whale in the Azores waters, and stated that there was sexual cyclein the male of sperm whale, and the breeding season existed during October tc
June.

REPRODUCTION OF THE SPERM WHALE

3

According to Chittleborough (1955), there is quite difference in the weight
between testes in feeding season and breeding season for the humpback whale.
Meek (1918) reported that weight of testes of Phocaena communis increased remarkably in summer. On the contrary, Mackintosh & Wheeler (1929) describe thatamount of sperm is more in breeding season than in feeding season, but there isno seasonal change in the weight of testes for the southern blue and fin whales.
Comparing the relations between age and weight of testes for the sperm whalescaught in adjacent waters to Japan during the months April-June and SeptemberNovember, there is no difference between them, although it is not yet concluded
because of scanty of the materials of the former.
Further investigation will be need on this problem in the sperm whale.
TABLE I. FERTILIZED DATES (MONTH/DECADE) OF SPERM WHALE FOETUSESACCORDING TO THE FOETAL LENGTHS AND MONTHS KILLED
Body length(feet)

Gestation
month

Jan.

Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec.

0
I
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

1.0
2.3
3.4
4.5
5.8
6.9
8.0
9.2
10.3
11.5
12.6
13.8
14.9
16.1
17.2
18.4
19.5
20.7

12/2
11/1
10/1
8/3
7/3
6/2
5/2
4/1
3/1
2/1
12/3
11/3
10/2
9/2
8/1
7/1
5/3
4/3

1/2
12/1
11/1
9/3
8/3
7/2
6/2
5/1
4/1
3/1
1/3
12/3
11/2
10/2
9/1
8/1
6/3
5/3

2/2
5/2 6/2
7/2 8/2
3/2 4/2
I/I
2/1
3/1
4/1
5/1
7/1
6/1
12/1
I /I
2/1
3/1
4/1
5/1
6/1
10/3 11/3 12/3
1/3
2/3
3/3 4/3
1/3
2/3
3/3
9/3 10/3 11/3 12/3
1/2
2/2
8/2 9/2 10/2 11/2 12/2
7/2 8/2 9/2 10/2 11/2 12/2
1/2
6/1
7/l
8/1
9/1 10/l 11/1 12/l
5/1
9/1 10/1 11/l
7/1
6/1
8/1
4/1
6/1
8/1
9/1 10/1
5/1
7/1
7/3 8/3
2/3
3/3
4/3
5/3 6/3
4/3
5/3 6/3
7/3
1/3
2/3 3/3
12/2
1/2 2/2 3/2 4/2 5/2 6/2
11/2 12/2
1/2
2/2
3/2 4/2 5/2
I/I
2/1
3/1
10/1 11/1 12/1
4/1
I/I
2/1
3/1
9/1 10/1 11/1 12/1
1/3
7/3 8/3 9/3 10/3 11/3 12/3
6/3 7/3 8/3 9/3 10/3 11/3 12/3

9/2
8/1
7/1
5/3
4/3
3/2
2/2
1/1
12/1
11/1
9/3
8/3
7/2
6/2
5/1
4/1
2/3
1/3

10/2 11/2
9/1 10/1
9/1
8/1
6/3
7/3
5/3 6/3
4/2 5/2
3/2 4/2
2/1
3/1
1/1
2/1
12/1
1/1
10/3 11/3
9/3 10/3
8/2 9/2
7/2 8/2
7/1
6/1
5/1
6/1
3/3 4/3
2/3
3/3

Females: Diameters of the largest graafian follicles were measured for 89
sperm whales in resting and pubertal stages. They were caught in the adjacentwaters to Japan during the months June-November. Monthly frequencies of thediameters are shown in Fig. 1. Mean diameter has a tendency to decrease with
the increase of month. Although there is no data on the measurement in othermonths, it is estimated that there is a periodic change in the diameter of graafianfollicle. If the season when the whale has the largest graafin follicle agrees withbreeding season, it is estimated to be before June. In the other hand, in the frequency distribution of the size of graafian follicles there are two modes. If the ovarian follicle over 25 mm in diameter is near the maturity, such follicles are observed
during June-Ocotober. This phenomenon will show a possibility that there aresome individuals which ovulate during the seasons.

4

S. OHSUMITABLE 2.

Foetallength(feet)Jan.

RECORDS OF FOETAL LENGTHS OF THE NORTHERN HEMISPHERE
SPERM WHALES GIVEN IN THE INTERNATIONAL WHALINGSTATISTICS 1937-61
Feb.

Mar.

May

Jun.
2

2

4

3

8
2

23
9
3
4

22
47
27
11
5
9
3
5
7
6
9
18
14
10

5

0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
20
Total

Apr.

2

2

2
4
1
2

1

3
6

3
l
6
2
9
4
6

4
5
6
16
16
18
19
4

I
7
2
5
7
8
12
20
14
19
13
3

3
4
6
3
4
6
3
5
6
10
4

2

Jui.

3
2
3
4
8
10
17
16
11
2

Aug.

2

Sept.

Oct.

Nov.

22
45
50
30
11
7

8

2

16
25
29
30
7

7
16
13
14
8
7

2
2
4
5
11

13
9
4
4

5
2
3
2
1
3

Dec.

3
2
I

7
3

3

Total

14
89
131
126
100
76
56
36
40
60
81
75
IOI
68
35
13
2

II

16

35

98

117

68

117

199

220

133

68

21

1, 103

TABLE 3. RECORDS OF FOETAL LENGTHS OF SOUTHERN HEMISPHERE
SPERM WHALES GIVEN IN THE INTERNATIONAL WHALINGSTATISTICS, 1937-61
Foetallength(feet)

Jan.

Feb.

Mar.

0

I

I

2

5
6

6
11
22
13
7

3
4
5

6
7
8
9
10
11
12
13
14
15
16
17
20
Total

2
5
2

7
5
3

2
2

3

5

2

I

5

I

5

2

8
14
13
3
3

3
3
4

1

4

Apr.

May

7
14
22
21
19
14
7
4

3
13
23
38
34

Jui.

Aug.

Sept.

Oct.

2

4
2
4

Nov.

Dec.

5

2
2
2

2

Jun.

22

14
2

12
29
27
19
12
16
8

2

5

3

5

3

2

2

4
8
13
22
17
21
11
7
4
2

3
II
10
29
24
44
26
14
2
6

4
3
14
19
24
41
36
19
20
7
3
3

2
2

v
8
29
24
45
35
37
26
27
13
4
4

8
10
14
32
20
35
17
27
19
6

2
3
4
8
8
6
6
5

Total
8
29
53
100
134
145
170
148
213
148
133
91
103
67
27
10
I

2

3

48

1,592

I

71

47

82

1

115

160

126

111

170

196

267

199

REPRODUCTION OF THE SPERM WHALE

5

Estimation of pairing season from body lengths of foetuses
In the following chapter, growth curves in the gestation period will be shown.
In Fig. 7, the growth curves of foetuses in northern and southern sperm whales agreewith each other sliding the time axis to 6 months. This means that the pairing
season in the northern and southern hemispheres are opposite in time but the samein season of the year as shown by Clarke ( 1956).

m.--~~~~~~~~~~~~~~~~~~~~~~~~~~~~~6.0

18

0

20

rr-i

5.0

16
-14

a;

4.0

~ 12
-.-=
gi 10

3.0

~

> 8

"

0

OJ

2.0

6
4

1.0

2
Jan.

Feb.

Fig. 2.

22

0

Mar.

Apr.

May.

Jun.

Monthly foetal length frequencies of the northern hemisphere sperm whale.

20

r-r-1

m

20

6.0

18
5.0

16

a;

~ 14

4.0

-C

0, 12
c

"'

:. I 0

"

0

OJ

3.0

8
2.0

6
4

1.0

2
Jan.

Fig. 3.

Mar.

Apr.

May.

Jui.

Aug.

Sepf.

Oct.

Nov.

Dec.

Monthly foetal length frequencies of the southern hemisphere sperm whale.

S. OHSUMI

6

Conception date will be calculated with the method of Huggett & Widdas( 1951) in the following chapter. According the result, peak of conception date is
middle-late decade of April in the northern hemisphere, and middle-late decadeof October in the southern hemisphere. However, as shown in Figs. 2 and 3, breeding will last fairly long time. Basing the growth curve, conception dates andgestation months are calculated in each foetal length of each month as shown in
Table 1. And Tables 2 & 3 are the frequency of foetal size distribution in eachmonth for the sperm whales of which data were given from the International
Whaling Statistics, 1937-1961.
Month of birth
MAMJ JASONDJ., --. ,.
...

/
I

15
I
I

I
I

:

I

0

ONDJ

F

MAM

. .

....

,~-

\\

I

I

\
\

I

\

I

..

\I

,'

I

AS

~

I

I

I

\
\

I

/
I

~

.....\

\

•

\

\

\
\

\

\

~

\

\

5
........

AS

..

,,

I

I

/
I

\
\

\

\I

I

/

M A M J

I

I

\
I

/

,.

I

I

I

~

0

\

'

_ _ •.,/.
....

0

__..

ON DJ
Month of pairing

M A M J

I
I
\

I

I

~

_
' ........... ____ •.,,,,/•//A S 0 N D

Fig. 4. Pairing and parturition frequencies calculated from foetal lengths for the southernand northern sperm whales.
Open circles and solid line : southern hemisphere, Closed circles and broken line:
northern hemispnere.

Frequency of conception dates were calculated from Tables 1, 2, and 3. Pairing extends all seasons of the year as shown in Fig. 4. But the mode of pairing is
April and October in the northern and southern hemisphere respectively, and mostconception extends between February to June in the northern hemisphere, and between August to December in the southern hemisphere. The months are late
winter to late spring in both hemispheres, and during the months, 71 and 67 per centof total conceptions of the year are considered to take plase in the northern andsouthern hemisphere respectively. Bimodal frequency distribution of the pairing
season is not seen in the sperm whale as reported by Naaktgeboren et al. (1960) for
the fin whale.
Change of pairing season according to the age of the mother whale

Laws (1961) reports that pairing seasons are different between primiparous andREPRODUCTION OF THE SPERM WHALE

7

multiparous fin whales, the former is about one month later than the latter, andpairing season becomes earlier with the increase of age of the mother whale.
Body lengths of foetuses were measured and maxillary teeth of the mothers
were collected for 110 sperm whales in Japanese coast during 1960 to 1962 seasons.
The conception dates were calculated from foetal lengths, and the ages were counted
on the growth layers in the dentine of maxillary teeth. Fig. 5 shows the average
conception date in the age classes of the whales. Average conception date in under
10 years class which is estimated to be almost primiparous is 6th of June, on thecontrary, average conception dates of over 11 years classes which are estimatedto be almost multiparous are early decade or middle decade of May. Furthermore,
there is a tendency that pairing date becomes gradually earlier accompanying with
the increase of age in the sperm whale.

J"" I {
Moy

31

Mo, 2{
Moy I],___---,->,,---~-~~----=-.,,...____.
10

20

30

40

50

Age in years

Fig. 5. Change of average conception dates according to the age of
sperm whales in the waters adjacent to Japan.

In Japanese coastal whaling, composition of younger female sperm whales in
the catch increases gradually from spring to fall. This phenomenon means older
group migrates northward ealier. And it is estimated that whales having earlypairing season leave their breeding ground earlier than the others. Further investigation will need on the ecology of schools especially " harem " in the sperm
whale for the solution of this problem.

Yearly fluctuation of pairing season
Conception dates were calculated from the records of foetal lengths of the sperm
whales caught in adjacent waters to Japan during 17 years from 1946 to 1962, andaverage conception date was calculated in each year. Fig. 6 shows the result.
During these years conception took plase earliest in 1947, and average date was
6th of April, and the latest year was 1958. The average date was 20th of May.
Range of yearly fluctuations is one month and a half.
Recently pairing season seems to become later from Fig. 6, but we must re-S. OHSUMI

8

member that recently the catch season removes to fall in Japanese coastal whaling.
And as described in the previous section, composition of younger whales in thecatch increases from spring to fall. The younger females have later pairing season.

May 2

(J)

o May 1-uO>E

°8_ April 3
(J)

O>
0

Q) April 2><{

April 1

'46

'48

'50

'52

·54

'56

'58

'60

'62

Fig. 6. Yearly fluctuation of conception dates for the sperm whales in adjacent waters
to Japan during 1946 to 1962, based on foetal lengths in each month.

GESTATION, PARTURITION AND LACTATION

Monthly foetal length frequency
Using the records on the foetal lengths given in the International Whaling
Statistics, 1937-1961, monthly foetal length frequencies are summarized in Tables
2 and 3 for the northern and southern sperm whales respectively. In Figs. 2 and3 the monthly foetal length frequency distributions are set out. Comparing the two
figures, we can find that monthly frequency distributions are different each otherbetween the northern and southern sperm whales. It is clear that they are separated
to different stocks. On the contrary, in the northern sperm whales, the monthly
frequency distributions do not show significant difference among the whales takenin the waters of Japan, Kuril Islands, Pacific coast of North America and theNorth Atlantic. In the same way, the monthly length distributions do not show
the significant difference among the whales caught in the waters of Chile, Peru andSouth Africa. That is to say, if there are several stocks in the same hemisphere,
they will show the same growth pattern in their gestation periods.
In some months there are two peaks of the length distribution. Bimodal
distributions are evidently seen in July, August and September in the northernhemisphere and January, February and March in the southern hemisphere. The
monthly peaks move continuously to the peak in the next month. These phenomena
suggest that modal period of conception is one a year, and the gestation period
lasts over one year.

REPRODUCTION OF THE SPERM WHALE

9Growth curve of the foetus

Fig. 7 shows the relation between the mean foetal lengths in each decade forthe northern and southern sperm whales. The data are due to the International
Whaling Statistics, 1937-1961. In this figure, growth curves of both hemispheresperm whales fit each other when one of them slide the axis of abscissa to 6 months.
This means that the growth rates are almost the same between the both hemispheresperm whales. Excluding the earlier and later stages, the given figures are recognized as straight lines. In early stage the figures are seemed to be higher than theactual one, for very small embryos are apt to be overlook and are not recorded occasionally. On the contrary, in the late stage of gestation we must consider thatthe given figures are seemed to be lower than the actual, because in this season
there are some individuals which are already born, and they bring down the true
figure.

14

m

4.0
12

10

3.0

Q;
~

"

8

-"

g>
~

>-

2.0
6

-0
0

"'
4
l.O

2

.........._,,__,..,'-r-'~~,__..__,.....__..._,.....,M A M J J
S 0 N D J.......,...,.......,._,._,_,'-v-'._,_,..._,...., ,_,_,'-v-'

A S 0 N D
F M A M JJJ

M A M J J
A S 0 N D J
f'-y-1'--,,.J-....,,...J--.,-)~A S 0 N D Nor1hern hem.
F M A M J Southern hem.

Fig. 7. Mean foetal lengths in each decade and foetal growth in length of both hemispheresperm whales.
Broken line and closed circles : Northern hemisphere, Solid line and open circles :Southern hemisphere, Broad line: growth curve.

Two regression lines are calculated using the mean foetal lengths in each decadeduring the months September to June in the northern hemisphere and March to
December in the southern hemisphere. Cross point between the regression line
and the axis of abscissa is the last decade of May and the last decade of November
for the northern and southern hemisphere respectively. And the cross point between

10

S. OHSUMI

the body length at birth (405 cm or 13 feet 3 inches, as shown in the following
section) and the regression line is the last decade of August and the last decade ofFebruary in the northern and southern hemisphere respectively. Therefore, theperiod between the two points is 15 months and one decade (468 days) in the bothhemisphere.
Laws (1959) showed that the foetal growth in toothed whales could be desscribed by a linear plot of length except non-linear early part of pregnancy. And
I agree with his theory for the case of the sperm whale.
The mean conception date was calculated by means of the method of Huggett & Widdas ( 1951) and Laws (1959) :Lt 0 = 468 days x 0.07 /0.93= 35 days
Therefore, the conception date is estimated to be middle to last decade of April andmiddle to last decade of October in the northern and southern hemisphere sperm
whales.
Thus, the growth curve of the foetus of the sperm whale is drawn as broad line
in Fig. 7, and the both hemisphere sperm whales will show the same growth curvein gestation period.
TABLE 4.

Gestation
month

2
3
4
5
6
7
8

FOETAL LENGTHS OF SPERM WHALE IN EACHGESTATION MONTH
Foetal length,--------------------...
feet

cm

0.2
0.8
I. 7
2.5
3.4
4.2
5.1
6.0

6
24
52
76
103
127
155
183

Foetal length
Month
9
10
II

12
13
14
15
16

~

feet

cm

6.9
7.7
8.6
9.5
10.3
11.2
12.1
12.9

210
234
262
289
314
341
369
393

Table 4 shows the average foetal length in each gestation month.
Growth rate of foetus in males seems to be slightly greater than that in females,
but the difference will be very small.
Sex ratio in gestation period

Sex ratio in foetal stage is one of the needful factors for the stock assessmentof sperm whale.
Summarizing 2677 foetal records in the International Whaling Statistics,
1937-1961, calculated sex ratios of males are shown in Table 5. They are 48.1,
56.2 and 45.6 per cent in the waters of the North Pacific, Chile & Peru and Africa
respectively. The average sex ratio of males is calculated as 51.03 per cent. There-

REPRODUCTION OF THE SPERM WHALE

11

fore, it will be concluded that the sex ratios of males and females are almost equalin the sperm whale.
Body length at birth

On the body length birth in the sperm whales, several authors described asshown in Table 6.
TABLE 5.

SEX RATIOS OF THE SPERM WHALE IN THE FOETAL STAGE
North PacificMales
Females
Both sexes% of males

514
554
1,068
48.13

Chile & Peru

Africa

Total

628
490
I, 118
56.17

224
267
491
45.62

1,366
1,311
2,677
51.03

After the International Whaling Statistics 1937-1961
TABLE 6.

DESCRIPTIONS ON THE BODY LENGTHS AT BIRTH IN THE
SPERM WHALE BY SEVERAL AUTHORS

Authors
Bennett ( 1840)
Melville (1851)
Matsuura (1936)(1940)
Matthews (1938)
Mizue & Jimbo (1940)
Clarke ( 1956)
Laws (1959)
TABLE 7.
Body length
3.71 m
3.86"
3.89"
4.04"
4.04"
5.00"
5.2
"
5.2 ,,
5.5"(12' 8")(12' 8")(12' 9")(13' 3")(13' 3")(16' 5")(I7' )(17' )(18' )

Body length at the brith
14 ft
14-15 ft
13-14 shiaku
14-15 shiaku
4 m or little more
14-15 ft
3.92m
4.15 mLocality

Japan
Japan
Southern hemisphere
Japan
AzoresSouthern hemisphere

RECORDS OF NEWLY BORN SPERM WHALES
AuthorDate

Locality

14, VIII12, VIII12, VIII
II, VIII? IX
27, VII19, VIII27, x

AzoresClarke (1956)

BarmudaJapan

Wheeler (1933)
Ohsumi et al (1963)
Matsuura (1940)

North Atlantic

Harmer (1933)

Observation of parturition for the cetacea is very rare chanses for us, and someasurement of body length at birth is almost impossible. Neonatal body lengthmust be determined with larger foetal lengths and body length of calves which are
estimated as soon after birth. However, whale calves are protected from catch,
and we have only few data on the measurements of the calves as shown in Table 7.
The shortest calf was 3.71 m (12' 2") in this Table. According to Ohsumi
et al. (1963), a calf stranded in a Japanese beach on 27th July, 1957. This calf

12

S. OHSUMI

was 5.00 m (16' 5") in body length, and had no growth layer in dentine of teeth.
Examining the deposition of dentine, the calf was at least 4 months old, then the
neonatal length is clearly estimated to be under 5 m.
The maximum foetal length is 19 feet 6 inches in the records on the sperm
whale foetuses in International Whaling Statistics. But it is exceptional, and I
wonder whether it was foetus actually or not. In the records of 2,695 sperm whale
foetuses, there are 6 (0.22%) foetuses of which body lengths are 17 and 16 feet.

12

l0

5

l0
Foetal length !feet J

Fig. 8. Size distributions of the sperm whale foetuses, and estimation of the length atbirth.
Open circles and solid line : Southern hemisphere, Closed circles and broken line :Northern hemisphere, Crosses and broad line: Both hemisphere, Straight Chain
line : Estimated change of frequency from broad line, Broad chain line: Estimatedsize distribution of calves (Straight line minus broad line).

Fig. 8 shows the size distributions of sperm whale foetuses. Pattern of the
distribution in the northern hemisphere is different with that in the southern hemisphere. It is caused with the difference of whaling season between the both hemisphere (foetus data are many in summer-fall seasons in the northern hemisphere,
and they are many in spring season in the southern hemisphere). Summing up
the data in the both hemisphere, the combined frequency distribution gradually
decreases from 3 to 12 feet, after then the frequency decreases suddenly. Comparing
size distributions over 8 feet long in the summer season (July-September in northernhemisphere and January-March in southern hemisphere) when is the parturitionseason of the sperm whale as described in the following section, the distribution patterns of the both hemisphere are nearly the same each other, although modal lengths
are 13 and 12 feet in the southern and northern hemisphere respectively as shownin Fig. 9. Average foetal lengths are 11.67 and 11.78 feet in southern and northen
hemisphere sperm whales. These results mean that there is no difference of body

REPRODUCTION OF THE SPERM WHALE

13

lengths at birth between the both hemisphere sperm whales. The neonatal length
should be larger than the average foetal length at the season, because the individuals
which have been born at that season are of course excluded from the size distribution of foetuses, and they drop the really average body length in this season.

20
#-

->u
c::

<ll'

"'~

()"

~

10

8

10

12
FoetalFig. 9. Foetal size distribution of the sperm whales in parturition
months.
Open circles and solid line: Southern hemisphere in January,
February and March, Closed circles and broken line: Northernhemisphere in July, August and September, Crosses and broad line:
Both hemisphere.

Regression line was drawn in Fig. 8, and estimating that the distance between
the line and smoothed broad curve is the frequency of calves, the size distributionof calves are shown as a broad chain line in Fig. 8. Cross point between chain line
and smoothed foetal line is 13.6 feet (4.15 m). It is estimated the length is theaverage body length at birth in the sperm whale. However, considering the records
that there are 5 calves ofwich body langths are under 4.15 m among the few data
in Table 7, the given neonatal length will be little larger than the real one. The
body lengths of the 5 calves are between 3.71and4.04 m, and the average is 3.98 m.
In conclution I estimate that 4.05 m (13 feet 3 inches) is the average body length atbirth in the sperm whale of both hemispheres.

Parturition season
Table 8 shows the parturition season in the sperm whales described by several
authors.
In the previous section, the average body length at birth was given as 13 feet
3 inches. The mean parturition date will be got from a cross point between thegrowth curve and the body length of 13 feet 3 inches in Fig. 7. It is last decade ofAugust in the northern hemisphere, and last decade of February in the southernhemisphere. They are both late-summer season. Catch dates of the calves which

S. OHSUMI

14

are considered to be soon after birth are during middle decade of August to September as shown in Table 7. This will support the above result.
Fig. 4 shows periodical change of parturition dates in addition to the periodical
change of conception dates calculated by means of growth curve and foetal size
frequencies in each month. In the northern sperm whales, 71 per cent of the
year's parturition takes plase during June to October, and in the peak month August
20.8 per cent of parturitoion. Parturition lasts through the year, but during January
to April the proportion of parturition is only 11 per cent. In the southern hemisphere, 67 per cent of parturition will take plase during December to April,· and 9
per cent during July to October.
TABLE 8.

DESCRIPTIONS ON THE PARTURITION SEASONS IN THE
SPERM WHALE BY SEVERAL AUTHORS

Authors
Northern hemisphere
Beale ( 1839)
Bennett (1840)
Tago (1922)
Harmer (1933)
Matsuura (1936)(1940)
Mizue & Jimbo (1950)
Clarke (1956)
Chuzhakina (1961)
Southern hemisphere
Matthews (1938)

Parturiton season
Not limited
Ditto
Ditto
June-December
May-August
May-September
July-October
May-November(peak: July-August)May-October(peak: July-August)
September-April(peak: February)

Location

Japan
Japan
Bormuda Is.
Japan
Japan
Japan
Azores Is.
Kuril Is.
AfricaGestation period
On the gestation period for the sperm whale, Scammon (1874) described as
10 months. Harmer (1933) and Matsuura (1936) estimated to be 12 months.
Later authors all reported longer gestation periods. Matthews (1938), Matsuura(1940) and Clarke (1956) got a result of 16 months, Mizue & Jimbo (1950) 17
months, and Chuzhakina (1961) 16-17 months respectively for the gestation periodof the sperm whale. Laws ( 1959) studied foetal growths for several cetaceans, andcalculated the gestation period of the sperm whale to be 491 days or 16 months.
As described in the former sections, average conception date is middle decadeof April, and the average parturition date is last decade of August in the next year
for the northern sperm whale. For the southern sperm whale, they slide just 6
months from those in the northern hemisphere. Therefore, gestation period is calculated to be 16.4 months (about 503 days). Of course we must estimate thatthere isindividual variation in the gestation period in the sperm whale.
Body length at weaning
We have scanty knowledges on the lactation of the sperm whale.
Matsuura (1936) estimated the lactation period to be about 6 months, but heREPDODUCTION OF THE SPERM WHALE

15

did not describe on the body length at weaning. Matthews (1938) thought lactating period was 7 months, and length at weaning was estimated with growth curveand lactating period to be 6.5 m (21 feet). Clarke (1956) examined the stomach
contents of 15 small sperm whales caught in the waters to adjacent Azores Islands.
And he found that milk was contained in the stomach of whales under 6.6 m inlength, but food animals were found in those of whales over 6.8 m in length. Thenhe determined weaning length of the sperm whale would be between 6.6 and6.8 m, probablly 6. 7 m (22 feet).

30

w25

~

-E

Dl

c

Q)

>- 20

-0

0

cO

15

7
Age

8

9

(years I

Fig. JO. Growth curves of male and female sperm whales based on thegrowth layers in dentine of maxillary tooth, and estimation of weaning
period from body length at weaning.
Open circle and broken line: Females, Closed circle and solid line:
Males.

There was nothing in the stomach of a 5.0 m long sperm whale which stranded
on Japanese coast. This whale was estimated to be suckling, although milk was
not confirmed in the stomach.
According to some observations on feeding of small sized toothed whales inaquariums, infants begin to eat food simultaneously suck mother's milk after somemonths from birth (Tavolga & Essapian, 1957; etc.).
At the present time, there is no datum to be added to the information by Clarke(1956) on the problem of body length at weaning for the sperm whale, and so I
also employ his theory for the length at weaning in the present paper.

Weaning season and lactation period
Matsuura (1936) estimated lactation period was 6 months, and in his paper

16

S. OHSUMI

parturition season was May-August. Then weaning season is December-February
in his theory. Matthews (1936) estimated that weaning season was August forsouthern sperm whale. Clarke (1956) determined the season was September, for
the whales of weaning body length appeared during July-October in Azores. And
he also estimated the lactation period to be 13 months after correction with weaning
season of the calculated 15 months from the composition of lactating and pregnant
whales.
As described in former sections we have few chanses to investigate calves, andso to pursue a growth in length seasonally is impossible for the sperm whale. Another method is to use growth curve as examined by Matthews ( 1938).

25

20
Qi

J£
~ 15

°'
c

-----------

<lJ

>

-g 10
£l'.)

5

MMJSNJ

MM
Month

SNJMMJSN

Fig. 11. Growth curve of foetuses and calves for the sperm whales in the northernhemisphere, and the records of date and body length of calves.
Open circle : Female, Closed circle: Male.

Recently age determination of the sperm whale has developed by reading
growth layers in the dentine of maxillary teeth (Nishiwaki et al., 1958; Ohsumi et al.,1963). Fig. 10 shows a growth curve based on reading the growth layers in young
sperm whales. According to Ohsumi et al., dark band in dentine appears in winter,and mean parturition season is August. Therefore, a time from birth to formationof the first dark band is about 4 months in average. In Fig. 10, graduations on theaxis of abssisa show winter (January lst).
If the weaning length is 22 feet, lactation period is estimated from Fig. 10 as
1.6 years (2.3-0. 7=1.6). If lactation period is 1.6 years ( = 19 months), weaning
season becomes March-April in the northern hemisphere. However, Clarke states
weaning season is September considering the appearence of weaning whales, andas shown in later chapter, proportion of resting whales increase after August. There-

REPRODUCTION OF THE SPERM WHALE

17

fore, I determine weaning season is August or September in the northern hemisphere. Then the lactation period is more suitable to estimate as 24-25 months
than 19 months.
Fig. 11 shows growth curve in gestation and calving ages and some plots ofcalves by date and length. As there are individual variations in growth rate andconception date, plotted points ride not always on the growth curve.
In the present paper, lactation period becomes much longer than the previous
theory. On this problem, bottle-nosed dolphin (Tursiops truncatus) which were
kept alive in an aquarium suckle for 18 months (Tavolga & Essapian, 1957), anda mother bottle-nosed dolphin ( Tursiops gilli) which died at 2 years and 6 months
after parturition secreted milk still at its death (Nakajima, 1963). According toSergeant (1962), pilot whale (Globicephala melaena) has 22 months oflcatation period.
Above reports seem to support my result.
OVULATION

Relation between the number of ovulation and age
Age of the sperm whale is present in the dentine of the maxillary tooth as a
growth layer (Nishiwaki et al., 1958). And the annual accumulation rate of thegrowth layer is studied by Ohsumi et al. (1963).
The relation between the age (number of growth layers in the dentine) andthe number of corpora lutea and corpora albicantia in the ovaries were given for
892 female sperm whales which were investigated in the coast of Japan during theseasons from 1960 to 1962. Fig. 12 shows the above relation on each individual.
Individual variation is relatively remarkable, but correlation coefficient is calculated
as +o. 76 for the ages more than 6 years old. There is an intensity that the deviation spreads accompanied with the increase of the age. This seems to show that
there is individual variation in the ovulation cycle of the sperm whale. On theother hand, the ages at one ovulation distribute from 6 to 18 years. This shows that
there is an individual variation in the age at sexual maturity. Then, the varianceof the relation between age and number of ovulation is caused by the above two
factors.
Fig. 13 shows the mean number of ovulations in each age (solid line) and themean age in each number of ovulation (broken line). Two lines are both likely
straight. This shows that there is close relation between age and number of ovulation, and the ovulation cycle is constant as the avarage. However, examining indetail, the gradient of the line is larger in the young ages than that in the older
ages, and in old ages, the deviation between solid line and broken line spread eachother. The ovulation is seemed to become gradually to lessen in the relation withthe age.
Average annual number of ovulation and average ovulation cycle
Average annual number of ovulation was given calculating the coefficient ofregression line from relation table. In above section we found that gradient of the

18

S. OHSUMI

25

/

20

.,,

()

c:

.2

/

0 15

/

/

,/

·-· ...

,f+ /

=>

>

~

0

0 10
ci

z

L
,1....

///(/:• AA••&•:.::: :;~~i::--·..
......................

•.: ••: ..
0.. _____!--• • .... -II&&&••. A. - - • . . . • . . . . . . . .
••• •-------/••
au.a... a\611••=11 .,. ••••• ••• --------,,... •• ••••lllll•A• I•• •••1111-••- !.•~---'I""' 0 . \ 4J<.
/·••--·••• - . ........
• __ _.________

50<.:>· I

177.351'0_:.------:::2.0_,?:::!.:t•• •• . . .----------. · ·~vi • • ------5

10

15

20

25

30

35

40

45

50

60

55

Age !no. of growth loyersJ

Fig. 12. Relation between age and number of ovulations for the sperm whales caught in
the waters adjacent to Japan (1960-1962).
Small circle: One whale, Large circle: Ten whales, Broken lines: Upper andlower limits.

12

10
~

c

."?

0

"

8

>

0

0
ci

6

z

4

2

5

45

50

Age (no. of growth layers J

Fig. 13. Mean number of ovulations in each age and mean age m each number ofovulations for the sperm whales in the waters adjacent to Japan (1960-1962).
Open circles and solid line : Mean number of ovulations in each age, Crosses andbroken line : mean age in each number of ovulations, Triangle : Result in the paperby Chuzakina (1961).

REPRODUCTION OF THE SPERM WHALE

19line in younger ages is different from that in older ages, then two coefficients were
given separated at the age of 20 years as shown in the following:
Age
6-20
21-48
6-48

Average annual number of ovulation
0.308
0.255
0.265

In the older ages average annual number of ovulation is low, and 3 factors will beconsidered on this subject. The first is a possibility that ovulation cycle becomesprolonged as the increment of the age. The second factor is a ploblem whether
old corpus albicans is not able to be distinguished or not. On this proble~ I will
examine in the following section. The third factor is a artificial error on the
counting of corpora albicantia. As the increase of accumulation of corpora albicantia in the ovaries, we are apt to miss counting the corpora number.
TABLE 9.

Mark no.

J2871
J2883, 2984
J3166
J3237

BIOLOGICAL DATA ON FOUR MARKED SPERM WHALES
RECAPTURED IN THE NORTH PACIFIC BY 1962

Elapsed time(A) No. of growthlayers at(Years-Months)
recapture(B)10-1 1/3
10-1 1/3
9-1
8-2 1/2

18

20
30
14

No. of corp
Jut. & alb.
in ovaries(C)0-2, 0-1
1-2, 0-3
0-3, lost
0-0, 0-2

Estimated
Estimated
Annual no.
age atage fromof ovula·
marking(B-A) sexual mat.(D) tion(C/D)

8
10
21
6

9
11

21
5

0.33
0.54
(0.29)
0.40

As shown in Fig. 12, the deviation of corpora number are relatively large, but
the ranges are limitted within two broken lines. Average annual number of ovulalation given from the upper limitted line is 1.00, and this value is considered to show
the maximum vulation rate. On the contrary, annual ovulation number given fromthe lower limitted line is 0.14, and this will show the minimum ovulation rate of
the sperm whale. Therefore, annual ovulation number is included within therange from 0.14 to 1.00, and the average annual rate is considered to be 0.27 in the
sperm whale caught in the waters adjacent to Japan.
Calculating the ovulation cycle from the annual number of ovulation, it distributes from one year to 7.1 years, and the average ovulation cycle is 3. 7 years.
Then, the ovulation cycle of the sperm whale is considered to be fairly long, andconnecting with the reproductive cycle as shown in the following chapter, the sperm
whale is estimated to be mono-oestrous.

Examination ef annual number ef ovulation by means ef whale marking
Amoung the marked sperm whales recaptured in the North Pacific by the endof 1962, there were 4 individuals of which maxillary tooth was collected for the age
counting and in the same time the corpora number in the ovaries were counted.
The biological data on the 4 individuals are shown in Table 9. We can not know
the age at the time of marking of the recaptured whales, but it is able to be estimated20

S. OHSUMI

by the elapsed time from marking till recapture and the growth layers in the dentineof upper tooth.
Now, the age at sexual maturity was estimated to be 9 years (Ohsumi et al.,1963). In the present paper also shows the similar result, for cross point of solid line
at !ovulation level in Fig. 12 is 9.2 years. Age at marking ofJ 2871 whale is estimatedto be 8 years, then this individual was near the age at sexual maturity at that time.
Using the average age at sexual maturity, it will be estimated to become sexual
maturity after I year from marking. As elapsed time is I 0 years, this individualis estimated to be 9 years from sexual maturity. Finally, average annual numberof ovulation is calculated from the number of ovulations devided by the age fromsexual maturity, and it is 0.33. In the same way, average annual number ofovulation on other 3 recaptured whales are 0.54, 0.29, and 0.40 respectively forJ 2883, J 3166, and J 3237. The average of 4 whales is 0.39. This value is similar
to those which calculated in the above section.

Persistence ef corpus albicans
It is believed that the involution speed of corpus albicans is very slow in the
cetacea, and it does not vanish for the life span. On this problem, Laws (1961)
studied thouroughly for the ovaries of the fin whale, and he supported the theory.
-E
5
0
0

-" 100
.D
0 80
~

I

60

0

0

2 40
~

e.
0

u

20

0
Q;

a;
E

0

Ci

10
8
6
p

l

l

2

3

4

5

6

7

8'

'
9' 101112131415161718

Number of corpora albicantia

Fig. 14. Regression of mean diametre of corpora lutea and albicantia withincrement of corpora numbers.
P : Corpora lutea of pregnancy, L : Corpora albicantia of lactation,
Open circle : Mean value, Bar: Range of diametre.

Nishiwaki, Hibiya & Ohsumi (1958) measured the diametre of corpora lutea
and corpora albicantia in the ovaries of 18 sperm whales, and reported preliminallythat corpus albicans persisted for life span in the sperm whale. Then, Chuzhakina(1961) examined the ovaries of the sperm whale histologically, and got the same
result as Nishiwaki et al., because a corpus albicans which was disappearing partly
or completely in the ovary was not observed at all.

21

REPRODUCTION OF THE SPERM WHALE

In the present paper, diametres of corpora lutea and c. albicantia were measured
for 29 sperm whales caught in the adjacent waters to Japan, and they were arrenged
according to their size for each individual, to get the change of diametre accompaniedwith the increase in number of corpora albicantia in the ovaries. Corpus albicansof lactation was separated from other corpora albicantia.
As shown in Fig. 14, corpora lutea of pregnancy were 95-120 mm in diametrefor 10 individuals, and the mean diametre of them was 105.0 mm. On the changeof size in corpus luteum during gestation period, Chuzhakina ( 1961) reported thatdiameter of the body did not involute during the period. Laws (1961) describes
the same result in the fin whale.
40

E 20
E

~ 10

e- 8
0

u

0
~
(J)

E
0

0

6
5
4
3

2

0

5

l0

15

20

25

30

35

40

45

50

55

60

Order number of corp. albicantia

Fig. 15. Arrangement of diametre of 37 corpora albicantia in the ovaries
of a sperm whale caught in the coastal waters of Japan.
Open circle : Actual diametre, Broken line : Regression curve.

After parturition corpus luteum recluses suddenly its size into corpus albicans.
Corpora albicantia oflactation were 20 to 53 mm in diameter for 18 lactating sperm
whales, and the mean diameter was 33.6 mm. The diameters of corpora albicantia
except corpora albicantia of lactation were 3-37 mm. Arrenging them according totheir size, it is found that the speed of involution gradually decreases, and the mean
diameter of the corpora is 12 mm in the corpora number of 16. This size is easily
recognized by naked eyes. However, the size of corpora albicantia in old stage is
not constant as described by Laws (1961) for the fin whale, and seems to continue todecrease as shown in Fig. 14.
Fig. 15 shows the arrengement of diameters of 37 corpora albicantia in theovaries of an individual which has the most corpora number in our materials. According to the Figure, the process of involution of corpus albicans is drawn as ahypabola, and the smallest corpus was 3 mm in diameter. Ifwe are difficult to distinguish the size under 2 mm by naked eyes, the number of corpora will be 42 fromthe Figure. Considering from the frequency distribution of corpora number in the

22

S. OHSUMI

sperm whale, the composition of individuals of which number of corpora are over 20
is very small, and the maximum is 37. The age of the individual in Fig. 15 is 56+
from dentinal growth layers, and it is estimated to be very old.
It will be concluded that corpus albicans in the ovaries of the sperm whale
continues to decrease in its size, but persists in ovary through the life.

Multiple-ovulation and multiplets
More than one ovum ovulate in the same time or simultaniously to the previous ovulation in an oestrus and at least one ovum furtilize in the oestrous, all
Graafian follicles which have ovulated into oestrous should remain as corpora lutea
during the gestation period. Then, the number of corpora lutea in the ovaries ofa pregnant whale is considered to show the number of ovulations in the sameoestrous period, although there are rare cases in which more than one ovum ovulate
from one graafian follicle in cetacea (Bannister, 1963).
Of the total of 446 sperm whales which were investigated their both ovaries
and had corpora lutea in the ovaries, 442 whales (99.11 %) had only one copus luteum, 3 whales (0.67%) had 2 corpora lutea and one whale (0.22%) had 3 corpora
lutea in the ovaries. Therefore, the frequency of multiple ovulation is considered
to be extreamly low.
A frequency of multiplets is also an indication of multiple ovulations. However, the frequency of multiplets should be lower than that of multiple ovulation,
because all ovulations do not always succeed to the furtilization. On the occurrenceof twins in the sperm whales, Bennett (1840), Beneden & Gervais (1880), Matsuura( 1936) and Matthews ( 1938) reported. And the frequency of twins is calculated
as 0.66% from the data in the papers of the latter two authors. Using the records
of foetuses of sperm whale in the International Whaling Statistics, of total 2,664
pregnant whales only 12 whales (0.45%) had twins. There is no record on the multiplets more than twin in the sperm whale. The frequency occurrence of multiplets
will be lower than that of baleen whales (Kimura, 1956).
The frequency of the multiplets (0.45%) is lower than that of multiple ovulations (0.8%). But, it is dangerous to connect the figures to the fertilization rate ofovums, because the sperm whales are usually flensed in land stations where equipments are often worse than those in factory ships, and it will be apt to miss thefinding of foetuses in the flensing at land stations.
PREGNANCY RATE

Seasonal change ef pregnanry rate and the true pregnanry rate
Pregnancy rate is expressed as the rate of pregnant whales in the sexually mature. females which have one or more than one corpus luteum or corpus albicans in theovaries. Matsuura (1936) described that the occurrence of pregnant sperm whales
varied with seasons in Japanese coast, but the total pregnancy rate was calculated
as 34 per cent through the year. Matthews (1938) reported on the sperm whales
in South Africa that of the total of 14 females which had been investigated during

REPRODUCTION OF THE SPERM WHALE

23

the seasons June to September 7 were pregnant and 5 were desided as soon after
ovulation or early stage of pregnancy. Then the pregnancy rate is calculated as
86 per cent. Clarke (1956) stated that the pregnancy rate was 27 per cent during
the seasons June to December in Azores Islands area, although he suggested thepossibility of the phenomenon that some pregnant and lactating sperm whales did
not approach to the area.
TABLE 10. MONTHLY CHANGE OF PREGNANCY RA TES OF THE SPERM WHALFSCAUGHT IN THE ADJACENT WATERS TO JAPAN, 1960-62, AND THE
CORRECTION FOR THE TRUE PREGNANCY RATE
Pregnant whales
Total mature whales
Pregnancy rate (%)
Exchange ratio 1
True pregnancy rate 1 (%)
Exchange ratio 2
True pregnancy rate 2 (%)

May &June

July

Aug.

Sept.

Oct.

Nov.

Total

26
41
63.5
0.29
18.4
0.50
31.8

31
48
64.6
0.38
24.5
0.50
32.3

117
221
53.0
0.65
34.5
0.50
26.5

280
658
42.6
0.77
32.8
0.67
28.4

98
346
28.3
0.91
25.8
1.00
28.3

21
85
24.7
1.00
24.7
1.00
24.7

573
1,399
40.9
26.8
28.7

In the sexually mature females caught in the coastal waters of Ja pan from May
to November during 1960-1962, the number of whales of which both ovaries were
examined and number of whales which had corpus luteum in the ovaries among
the above whales are shown in Table 10. Cutting the abdorminal part of whales
for the purpose of cooling the carcas, foetuses are often lost in Japanese coastal
whaling. However, I think that we have no objection to regard the whale which
has corpus luteum in ovaries as the pregnant whale. It is because that pregnant
whale has always corpus luteum, and when an ovum does not furtilize, corpus luteum
involute fastly into corpus albicans. As the peak of pairing season is April in thenorthern hemisphere, the cases of occurrence of corpus luteum without furtilization
will be scarse in the season of our investigation.
As shown in the Table, the pregnancy rate varies remarkably according toseason. During the months from May to August, the rates are over 50 per cent,but it becomes to 42 per cent in September, and the rates are under 30 per cent in
October and November.
Now, in the former chapter, gestation period of the sperm whale was estimatedto be 16.4 months, and two groups exist in some seasons of the year. They are individuals which have been pregnant since the last year, and the other group includes
the individuals which conceived in the next year during the seasons April to August,and after then, most pregnant whales were conceived in the year. Therefore, thepregnancy rates which were got according to each month during April to August
are pretence ones.
For the purpose of calculation of the true pregnancy rate, the following two
methods were employed. The first method is to get the ratios of the pregnant whales
conceived in the year to total pregnant whales from size frequency of foetuses inTable 3, and the pretent pregnancy rates are recounted into the true rates by theabove ratios. The ratios and the true pregnancy rates are shown in Table 10 as

24

S. OHSUMI

exchange ratio 1 and true pregnancy rate 1. As the result, the calculated pregnancy
rates in each month become nearly constant and ranged between 18 and 35 per cent,and the average is 26.8 per cent.
However, in Table 10, small embryos are often difficult to be found and so it
is considerable that some of the small embryos are not recorded by fishermen.
Then, the second method is employed for calculation of the true pregnancy rate.
In the northern hemisphere, the peak of parturition season is about August, and thepeak of pairing season is considered to be April. Then, during 5 months from
April to August, half of the pregnant whales should be conceived in the last year.
In September I considered that 2/3 of the total pregnant whales are conceived in theyear, because the large foetuses exist fairly in this month. The pregnant whales
which appear in October and November are considered to be conceived in thesame year. This method was preliminally emploied in the paper of Ohsumi et al.
(1963). True pregnancy rate 2 is given in Table 10, using the above exchange ratio
2. The exchanged pregnancy rate 2 are almost constant in each month as similaras the true pregnancy rate 1, and they distribute between 24 and 32 per cent, and theaverage is calculated to be 28. 7 per cent. In conclution, the true pregnancy rateof the sperm whale in the waters adjacent to Japan is estimated to be 26-29 per cent.
Previous reports on the pregnancy rate of the sperm whale did not consider
the ratio of the whales which had been conceived in the last year. For instance,
Matsuura ( 1936) reported the pregnancy rate as 34 per cent, but in his paper the
average pregnancy rate during the months September to March was 28.6 per cent.
This figure is very near to my result.
On this subject, a question is occured whether the materials which had beencaught in the coast of Japan represent the total population of mature females in theocean or not. Clarke (1956) suggested a segregation in the migration of pregnant
and lactating females in Azores Islands waters. Ecological study on the schools of
the sperm whale has not been developed, and the investigation on this problem
should be done in future, but I want to pay attention on the result that the pregnancy
rate fairly coincides with the annual number of ovulations which obtained in previous
chapter. I consider that this coincidation shows the figures obtained as the true
pregnancy rate representing those which in the mother population.

Change of pregnancy rate accompanied with age
Change of pregnancy rate accompanied with age is known in general mammals. But there are few reports on this subject for the cetacea.
Table 11 shows the age distribution of the pregnant whales (individuals whichhave corpus luteum in the ovaries) and the total females (individuals of which bothovaries were examined) caught in the coastal waters of Japan during 1960-1962
seasons. The ages are based on the number of growth layers in the dentine ofmaxillary tooth. Fig. 16 shows the change of pregnancy rate accompanied withage. These rates are of course pretent as described in previous section. Pregnancy
rate increases suddenly after the age at sexual maturity, and it attains to maximum
at 15-20 years. After then, the rate begins to decrease gradually with increment

REPRODUCTION OF THE SPERM WHALE

25

of age. However, the rate at 50 years of age is still 75 per cent of the figure at maximum rate. In Table 11, there is no pregnant whale over 61 years old. But the materials are few, and so it is not concluded from this phenomenon that there is menopause in the sperm whale.
TABLE II. AGE COMPOSITION OF FEMALE SPERM WHALES, PREGNANT SPERM
WHALES AND THE PERCENTAGE OF PREGMANT WHALES IN THE
TOTAL FEMALES, JAPANESE COAST, 1960-62
Age (year)
I- 2
3- 4
5- 6
7- 8
9-10
11-12
13-14
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
41-42
43-44
45-46
47-48
49-50
51-52
53-54
55-56
57-58
59-60
61-62
63-64
Total

Pregnant whales

2
6
22
46
40
35
32
41
23
16
18
12
14
12
13
12
7
8
7
5
4
2
2
2

Total whales
I
3
29
52
69
JOI
119
82
81
76
67
43
54
33
35
35
37
31
29
19
19
13
13
6
4
3
3
l

383

4
2
3
2
1,069

% of pregnant
whales
0.0
0.0
6.9
11.5
31.9
45.5
33.6
42.7
39.5
53.9
34.4
37.2
33.4
36.4
40.0
34.3
35.2
38.8
24.1} 31 1
42.1.
36.8} 37 5
38.5.
30.8} 31 6
33.3.
} 28.6} 50.0} 33.30.0
0.0

Number of corpora albicantia in the ovaries is also one of age characters in the
cetacea, and as shown in Fig. 12, there is close relation between the age based on
number of growth layers in the dentine and number of corpora in the ovaries.
Table 12 shows two frequency distributions of corpora numbers. One is thewhales with corpus luteum and the other one is the total whales of which bothovaries were examined. And Fig. 17 shows the pregnancy rate in each corpora
class calculated from Table 12. The given curve has the same tendency as Fig. 16,

26

S. OHSUMITABLE 12. NUMBER OF PREGNANT SPERM WHALES AND THE PERCENTAGE
OF THE PREGNANT WHALES IN EACH NO. OF CORPORA IN THE
OVAVIES. COAST OF JAPAN, 1960-62
No. ofcorporaPregnant whales

Total whales% pregnant

36
42
66
78
62
43
39
28
23
15
10
5
4
2
3

103
108
188
157
156
99
97
68
57
39
29
18
16
12

35.0
38.8
35.1
49.1
39.7
43.4
40.2
41.1
40.4
38.5
34.5
27.8
25.0
16.7

2
3
4
5
6
7
8
9
10
II

12
13
14
15
16
17
18
19
20
21
22
23
24
25
30
35
37
Total

]]

} 21.43
42458} 16.7

1,174

<:

g 30
Ol

~

Q_20

0

*- 10
10

20

30

40

50

Age in years

Fig. 16. Change of pregnancy rates according to the age in the female
sperm whales.

REPRODUCTION OF THE SPERM WHALE

27

that is to say, in earlier stage, pregnancy rate increases with increment of corpora
number, and the rate attains to maximum value at 4 of corpora numbers. Afterthen the rate decreases gradually, but the tendency of the decrement is more remarkable than the case in Fig. 16. In Table 12, there is no pregnant whale over 25 ofcorpora numbers. However, an individual with 37 corpora in the ovaries waslactating. This example shows that the sperm whale can conceive in older age.
50
~

Ql

0 40

-"

;<;

~c 30
Ol

~

~20
0

?R
0

10

5

10

15

20

Number of corpora in ovaries

Fig. 17. Change of pregnancy rates according to the number of corpora
in the ovaries of the sperm whales.

The difference of the tendency of decrease between Figs. 16 and 17 is able
to be explained that annual number of ovulations decreases accompanied with increment of age as shown in the previous chapter, and the older ages based on numberof ovulations concentlates the true ages, on the other hand, the individuals of whichannual number of ovulations is higher than the normal have a tendency to haveType D of reproductive cycle as examine in the following chapter. The whales
which have this type of reproductive cycle have low pregnancy rate.
REPRODUCTIVE CYCLEPatterns of reproductive cycle and a review on the previous papers
The sperm whale is mono-oestrous as discussed in the previous chapter of thepresent paper. In general, oestrous period is classified as follows:
I. Post-partum oestrus.
2. Inter-lactation oestrus.
3. Post-lactation oestrus.
4. Post-resting oestrus.
The first case of oestrus is considered to exist in the sperm whale. However,as shown in the following section, the frequency of this case will be low. Clarke(1956) reported on this kind of oestrus for the sperm whale in Azores. Chittleborough (1958) and Laws (1961) also described on this case for the baleen whales.
The second case has a possibility to exist in the sperm whale, for the lactation
period is considered to be very long in this kind of whale. The fact that there are

28

S. OHSUMI

some individuals which are simultaniously pregnant and lactating will support theexistence of the second case as well as the first case. Ch uzhakina ( 1961) mentionedon the possibility of this case in the sperm whale.
Matthews (1938) described that ovulation occured immediately at the end of
lactation for the sperm whale, and resting period was short even if it was existed.
However, as Clarke pointed out, the season when Matthews investigated was theformer half of the pairing and it is proper that there is no resting whales in thisseason. Clarke did not recognize the post-lactation oestrus. If the sperm whale
ovulate immediately to weaning, the season will be mostly August or Septemberin the northern hemisphere. But, the frequency of conception is the lowest of theyear in this season. Laws (1961) showed a theory that most of the female fin whale
ovulate at weaning, but they do not conceive in this time because of the decline ofsexual activity in the males in the season. Laws's theory will not be applied to thesperm whale. Because, the occurrence of mature ovarian follicles or corpora lutea
just after ovulation in the ovaries is not recognized in the sperm whales caught insummer season in Japanese coastal waters. On the contrary, Sergeant (1962)
reported that in the pilot whale (Globicephala melaena) there is no resting period,and it generally ovlates just after the weaning.
Clarke considers that post-resting oestrus is the most normal case in the sperm
whale. I agree with his consideration from the investigation of reproductive cycle
for the sperm whale.
Concerning to the reproductive cycle of the sperm whale, Matsuura (1936)
first estimated two-years cycle (gestation period: 12 months, lactation period: 6months, and resting period: 6 months). However, today this theory is not given
our approval, for it is clear that his estimation on the gestion period is not true.
Matthews ( 1938) also showed two-years reproductive cycle for the southern sperm
whale. He estimated gestation period as 16 months and lactation period as 7months. But, his theory is contradictory to the real phenomena in two points that
lactaing period is too short and there is no resting period in his theory. Clarke(1956) advocated 3 years, reproductive cycle for the sperm whale in Azores. It
includes 16 months gestation, 13 months lactation and 7 months resting periods.
And Chuzhakina (1961) reported that the reproductive cycle was 2.5 years, thatis, gestation was 16-17 months, lactation lasted to 10-11 months, and resting was
3-4 months.

Calculation of reproductive rycle
From the relation between age and number of ovulations, mean number ofovulations per year decrease slightly with the increment of age, and it is 0.31 by
20 years of age, 0.26 between 21 and 48 years, and 0.27 in total ages. From thesefigures, the ovulation cycle is calculated as 3.2, 3.9, and 3. 7 years respectively inabove ages. That is to say, the reproductive cycle is considered to be 3-4 years
calculated with number of ovulations per year.
In the previous chapter, the true pregnancy rate was given as 26-29 per cent.
Then the pregnancy cycle is calculated as follows:

REPRODUCTION OF THE SPERM WHALE

29

1 year/0.26-0.29 = 3.4-3.8 years
This value coincides fairly with the reproductive cycle calculated from annual rateof ovulations. I consider that pregnancy cycle supports that reproductive cycle is
3-4 years for the sperm whale.

Establishment of several types of reproductive patterns
Pattern of reproductive cycle and the number of ovulations in each pattern
are shown in Fig. 18, considering from gestation and lactation periods. Type A
is a case that ovulation does not occure after parturition and weaning, and theresting stage lasts for more than one year. In this case mean annual rate of ovulation is less than 0.25. The possibility of existing of such individuals is deemed by
the result that the lower limit of annual ovulation is 0.14 in the relation between
age and number of ovulations. The longer resting stage is, the smaller annual
ovulation becomes. The longest reproductive cycle is calculated as 7.2 years fromthe lower limit of above relation. Considering the ageal change of pregnancy
rate, the frequency occurrence of Type A will increase with age.
OvulationType

per year

A 0
B

c
C1

D
E

<; 0.25

0
0
0

0.25

0;

0.50

0

0.75

0

0.75

0
0

0

-I

0

0

0

0

0

-i

0

=

::)

0.33 or 0.66

0

0

G

0
-I

0

0

0.61

H ~---------~ ________ Q_________O_________ j
0

0.50

0

2

3

l.00

4

Years

Fig. 18. Types of reproductive cycles in the female sperm whale.
Square : Pregnant period, Solid line: Lactating period, Broken line:
Resting period, 0 : Ovulation.

Type B is a 4 years reproductive cycle which ovulate after 8 months' resting
period. Annual rate of ovulation of this type is 0.25, it is the most near the mean
number of annual ovulations, and it is regarded as the most normal reproductive
cycle for the sperm whale, estimating with other reproductive phenomena. Type
C is a reproductive cycle in which an unsuccessful ovulation occures immediately
to the weaning, and the whale conceives in the time of the next ovulation after
resting period. The mean number of ovulations per year is 0.50. In Type C', thefirst ovulation takes plase immediately to parturition in addition to Type C. Annual

S. OHSUMI

30

rate of ovulations is 0. 75 in this case. However, according to the investigation of
the ovaries during August to October when is considered as the season of parturition and weaning in Japanese coastal waters, appearence of corpora lutea of
ovulation or mature graafian follicles is rare in this season, and Clarke ( 1956) got
the similar result for the sperm whales in Azores. Therefore, the possibility ofexisting of Types C and C' is considered to be small in the sperm whale.
TABLE 13. REPRODUCTIVE STAGES OF FEMALE SPERM WHALES IN
EACH MONTH IN THE WATERS OF JAPAN, (1960-62)
Actual figure
Pregnant
Pregnant and Lactating
Lacating
Resting
Total

May and
June

July

Aug.

Sept.

Oct.

Nov.

Total

8

13
2
11
3
39

54
14
28
28
124

107
26
100
104
337

41
15
72
78
206

7
1
8
12
28

225
59
223
225
732

37.5
12.5
50.0
0.0

44.9
6.9
37.9
10.3

43.6
11.3
22.6
22.6

31. 7
7.7
29.7
30.9

19.9
7.3
35.0
37.9

25.0
3.6
28.6
42.9

30.7
8.1
30.4
30.7

3
4

Percent
Pregnant
Pregnant and Lactating
LactatingRestingType D is a 4 years reproductive cycle in which unsuccessful ovulation istaken plase in a pairing season and the next ovulation lead to conception after the
resting period. The reproductive cycle of Type D has a possibility to remove into
Type E or F explained in the following, and it is considered to exist according to theseasonal change of graafian follicles. Average number of ovulations per year in thistype is calculated as 0. 75, and it is under the upper limit of the relation between
age and number of ovulations. It is considered that there is a possibility to exist sucha type of sexual cycle, especially in younger ages. Type E is a 3 years reproductive
cycle in which whale does not ovulate or does not conceive if it ovulates at the firstbreeding season during lactating period, and it conceives at the next breeding season.
The possibility of existing of this type will be supported by the fact that there areindividuals which are pregnant and lactating. Annual number of ovulations is 0.33
or 0.67. In the reproductive cycle of Type F, whale conceives at the first breeding
season, and the next parturition takes plase at weaning. The existence of this typeis supported with the same fact as shown in Type E, and a phenomenon that there
is a record that a sperm whale with a foetus of 340 cm in length was still lactating is
explained more easily by Type F than by Type E. Type F is a 2 years reproductive
cycle, and annual ovulation is 0.50. The whales of about 20 years of age haveTypes E and F of sexual cycle more frequently than other ages judging with ageal
change of pregnancy rate. Type G is a 16-17 months sexual cycle which conceives immediately to parturition. This type has the same contradiction as explainedin Types C and C', and a whale of this type must continue to suckle the last calf
after the next parturition. Therefore, the possibility of existing of this kind of typewill be negligible. Type H is a sterile sexual cycle in which whales repeat an

31

REPRODUCTION OF THE SPERM WHALE

nusuccessful ovulation in every breeding season. The annual number of ovulations
is 1.00, and this agrees with the upper limit of Fig. 12. This type of reproductive
cycle will be more frequently in pubertal or young ages.
Summarizing the above considerations, Type B will be typicl reproductive cycle
for the sperm whale, and Type A will be also common in the species. Types D, E,
F and H also have possibility to exist, especially Types E and F will be 8 per cent of
the reproductive cycle in the sperm whales in adjacent waters to Japan as shown in
the following section.
Ovulation
0

====

0

per year

0.25
0.25

o:c:::============-~~~~~ 0.25
0

Jan.

Jan.

Jan.

Jan.

0.25

Jan. 0.25

Fig. 19. A pattern of reproductive cycle for the female northern sperm
whale (A), based on Type B in Fig. 18. Marks are the same as in Fig.
18.

Type
A

0
0

B

Ovulation
per year0.20- 0.25

0

B

0.25
0

B

0.25

0

B

0.25

0

B

- 0.25
0

B

D
D'

0

0

Jon.

0.50

0

0

0.50
0

0

F

0.25

Jan.

Jan.

0.50
Jan.

Jan.0.32

Fig. 20. A combined pattern of reproductive cycles for the female northern
sperm whale (B), based on Types A, B, D and F in Fig. 18.
Marks are the same as in Fig. 18.

Examination of reproductive cycle by means of seasonal change in reproductive conditions
Reproductive conditions were grouped into pregnant, simultaniously pregnant
and lactating, lactating and resting stages for 732 female mature sperm whales ofwhich both ovaries and mammary gland were examined in the coast ofJapan. The
result is shown in Table 13. And seasonal change of the compositions of the above
reproductive conditions is also shown in Fig. 21-C.

32

S. OHSUMI

The composition of individuals which were simultaniously pregnant and lactating is 3.6-12.5 per cent, and 8.1 per cent in average. The frequency does not vary
so remarkably, although there is slight tendency to decrease with season. The ratioof resting individuals is low until August, and then it increases. This phenomenon
is concerned with that weaning season is about August in the northern hemispheresperm whale. It was already studied that the frequency of pregnant individuals
decreases after August. Lactating individuals are almost constant in the composition seasonally, and the ratio is about 30 per cent.
Averagefrequency t % I
Rest.

A 12.5

lact.

50

50.0

~

Pre g.

37.5
Jun.

Jui.

Aug.

~

~

Lact.

>u
c

~

er

50

Erfilj. and Lact.

~

LL

Pre g.
Jun.

Sept.

Jui.

Nov.

B 20

Rest.

40

~

Aug.

Sept.

Lact.

50

Oct.

10
30
Oct.

Nov.

Rest.

c

~Lact.~

30
8

Pre g.
May& Jun.

31

31
Jui.

Aug.

Sept.

Oct.

Nov.

Fig. 21. Seasonal change of estimated and actual composition of reproductive stages for the female sperm whales caught in adjacent waters toJapan.
Data are based on Figs. 19, 20 and Table 12.

Fig. 19 shows a pattern of reproductive cycle for the female northern sperm
whale. This pattern is composed with only Type B which is considered as the
typical reproductive cycle in the sperm whale. And the seasonal change of ratiosof reproductive conditions is shown in Fig. 21-A. Comparing the seasonal change
of Pattern A with the actual one (C) in Fig. 21, the ratio of resting stage of the

REPRODUCTION OF THE SPERM WHALE

33

former is smaller than the latter, on the contrary, ratio of lactating stage is larger
than the latter. Furthermore, Pattern A can not explain the actual composition,
for there is no pregnant whales with lactation in Pattern A. Therefore, the reproductive cycle for the sperm whale is not composed of only single type of Type B,
but it will be composed of complex types of reproductive cycles.
Making a combined pattern (B) of reproductive cycle with Types A, B,D, andF as shown in Fig. 20, the seasonal change of composition of reproductive conditions
is calculated as shown in Fig. 21-B. This seasonal change fairly agrees with theactual one (Fig. 21-C). The combination of reproductive types in this pattern is
60 per cent of Type B, 20 per cent of Type D (D'), 10 per cent of Type A and 10
per cent of Type F. And the annual rate of ovulations is calculated as 0.32. Thisrate is near that of young ages.
In practice, the reproductive cycle in the sperm whale will be a complex
pattern which is composed more types than the pattern which was shown in Fig.
20. In Type A, there will be Types A', A", ... in which resting period is longer
than that in Type A, and it is estimated that Type E will add in the actual reproductive cycle in the sperm whale. Accompanying with the increment of the age,
proportion of Types A, A', A", ... is considered to increase, on the contrary, theproportion of Types D and F will decrease in the reproductive cycle in the sperm
whale.
From the above consideration, it was found that the types of reproductive cycle
which were established in the previous section are able to explain the seasonalchange of composition of reproductive conditions by a combination of the types.
And the estimated pattern is supported to be near the actual eombination ofreproductive cycle in the sperm whale in the adjacent waters to Japan.
SUMMARY

1. Some problems concerning to the reproduction of the sperm whale chiefly
in the north-west Pacific were studied.
2. Diameter of graafian follicle changes seasonally, and it is estimated to become largest in spring season in average.
3. Pairing season was estimated from monthly frequency of foetal lengths.
Pairing extends all season of the year, but the mode of pairing is in April and Octoberin the northern and southern hemisphere respectively. There is a tendency thatpairing date becomes gradually earlier accompanying with increase of age.
4. Sex ratio of the males is 51.0 par cent in gestation period.
5. Body lengths at birth is estimated as 4.05 m (13 feet 3 inches). There isno difference of the lengths between the both hemisphere whales.
6. Parturition chiefly takes plase during June to October, and December
to April in the northern and southern hemisphere respectively.
7. Gestation period is calculated to be 16.4 months in both hemisphere.
8. Lactation period is suitable to estimate as 24-25 months, and the weaning
season is determined to be August or September in the northern hemisphere.

34

S. OHSUMI

9. Annual rate of ovulation is calculated from the relation between age andnumber of ovulations. It is 0.27. The rate decreases gradually accompaniedwith increase of ages.
10. Process of involution of corpus albicans is drawn as like a hypabola, but
it persists in ovary through the life.
11. Frequency occurrence of multiple ovulations and multiplets are very low
in the sperm whale. It is estimated to be 0.89 and 0.45 per cent for multiple ovulation and multiplets respectively.
12. Pretence pregnancy rate varies remarkably according to season, and thetrue pregnancy rate is calculated to be 26-29 per cent for the sperm whale in thewaters adjacent to Japan.
13. Pregnancy rate changes accompanied with the age, and the age ofmaximum pregnancy is 15-20 years and 4 corpora.
14. Average duration of reproductive cycle is calculated to be 3-4 years forthe sperm whale. Eight types of reproductive cycles were established considering
gestation and lactation periods and annual rate of ovulation.
15. Examining the reproductive cycle by means of seasonal change in reproductive conditions, it is found that the actual reproductive cycle is composed ofmany types of the cycle.
ACKNOWLEDGEMENT

The materials used in the present paper were collected by the efforts of biologists who investigated, collected and examined the material under the leading by
Dr. H. Omura, Director of the Whales Research Institute. My sincere thanks
are due to them.
I am much indebted to many members of the Japanese whaling companies
for the help in investigation and collection of the materials.
REFERENCES
BEALE, T. (1839). The natural history of the sperm whale. (Cited by Matthews, 1938).
BENNEETT, F. D. (1840). Narrative of a whaling voyage round the grove. (Cited by Matthews, 1938).
CHITTLEBOROUGH, R. G. ( 1954). Studies on the ovaries of the humpback whale, Megaptera nodosa (Bonnaterre),on the western Australian coast. Aust.]. Mar. Fresh-water Res., 5 (1): 35-63.
- - (1955). Aspects of reproduction in the male humpback whale, Megaptera nodosa (Bonnaterre). ibid.
6 (1): 1-29.
- - (1958). The breeding cycle of the female humpback whale, Megaptera nodosa (Bonnaterre). ibid. 9 (1):
1-18.
CHUZHAKINA, E. S. ( 1961). Morfologicheskaya kharakteristika yaichnikov samok Cashalota (Physeter catodon)v svyazi s opredeleniem vozraska. Trudi Inst. Nor. :(hivotnilth A. N. SSSR., 34: 33-55.
CLARKE, R. (1956). Sperm whales of the Azores. Discovery Reps. 28: 237-98.
COMMITTEE FOR INTERNATIONAL WHALING STATISTICS (1938-1962). International Whaling Statistics 1937-1961.
HuGGETT, A. St. G. & WmnAs, W. F. ( 1951). The relation between mammalian foetal weight and conception
age. J. Physiol., 114 (3): 306-17.
KIMURA, S. (1956). The twinning in southern fin whales. Sci. Rep. Whales Res.Inst., 12: 103-25.

REPRODUCTION OF THE SPERM WHALE

35

LAws, R. M. (1959). The foetal growth rate of whales with special reference to the fin whale, Balaenoptera
physalus Linn. Discovery Reps., 29: 281-308.
- - (1961). Reproduction, growth and age of southern fin whales. ibid., 31: 327-486.
MATSUURA, Y. (1936). Breeding habits of the sperm whale in the adjacent waters of Japan. Z,ool. Mag.
(Japan), 48 (5): 260-6 (In Japanese).
- - (1940). Statistical study on whale foetuses-III. Sperm whales in the adjacent waters of Japan. Bull.
Jap. Soc. Sci. Fish., 9 (4): 142-4.
MATTHEWS, L. H. (1938). The sperm whale, Physeter catodon. Discovery Reps., 17: 93-168.
MELVILLE, H. (1851). Moby Dick, or the whale. Collins Classics, London 1953, 478 p.
M1zuE, K. & JIMBO, H. 1(950). Statistic study of foetuses of whales. Sci. Rep. Whales. Res. Inst., 3: 119-31.
NAAKTGEBOREN, C., SL!JPER, E. J. & VAN UTRECHT, W. L. (1960). Research on the period of conseption,
duration of gestation and growth of the faetus in the fin whale, based on data from International Whaling
Statistics. Norsk Hvalfaugst-tid., 49 (3) : 113-9.
NAKAJIMA, M. (1963). Death of Echo, a female bottlenose dolphin. Geikentsushin., 145: 6-12 (InJapanese).
NAKAJIMA, M., TAKAHASHI, K., 0GURA, M. & SAWAURA, K. (1963). On the growth of infants of small
size toothed whales. J. Jap. Soc. Z,oo Aquarium, 5 (1): 16-22 (lnJapanese).
NrnmwAKI, M. & HIBIYA, H. (1951). On the sexual maturity of the sperm whale found in the adjacent watersof Japan (I). Sci. Rep. Whales Res. Inst. 6: 153-66.
- - (1952). Sexual maturity of the sperm whale found in the adjacent waters of Japan (II). ibid., 7:

121-4.
NisHIWAKI, M., HrnIYA, T. & 0HSUMI, S. (1958). Age study of sperm whale based on reading of tooth laminations. ibid., 13: 135-53.
OMURA, H. (1950). Whales in the adjacent waters of Japan. ibid., 4: 27-113.
OHsUMI, S., KAsuvA, T. & NrnmwAKI, M. ( 1963). Accumulation rate of dentinal growth layers in the maxillary
tooth of the sperm whale. ibid., 17: 15-35.
SERGEANT, D. E. (1962). The biology of the pilot or pothead whale Globicephala melaena (Traill) in Newfoundland waters. Bull. Fish. Res. Board Canada, 132 : 1-84.
SLEPTsov, M. M. (1955). Biologia i promise! kitov dalnevostozuih morei (Translated into Japanese by H.
Sakiura).
TAGO, K. (1922). [On the whales in the adjacent waters to Japan]. Z,ool. Mag. (Japan) 34 (401): 446-79(In Japanese).
TAVOLGA, M. C. & EssAPIAN, E. S. (1957). The beheavior of the bottle-nosed dolphin (Tursiops truncatus):
Mating, pregnancy, parturition and mother-infant beheavior. Z,oologica, 42 (1): 11-31.
WHEELER, J. F. G. (1933). Note on a young sperm whale from theBermude Islands. Proc. Z,ool. Soc. London,

1933: 407-10.