SEXUAL SEGREGATION OF THE SPERM WHALE
IN THE NORTH PACIFIC*SEIJI OHSUMIINTRODUCTION

The whaling for the sperm whale (Physeter catodon) have been operating widely in
the North Pacific. Sperm whales were killed by 7 Russian and Japanese whaling
fleets chiefly in the Bering Sea, coasts of the Aleutian Islands and the Gulf of Alaska
in 1964. And land stations were distributed in the coasts of California, Canada,
Kuril Islands and Japan in the same year. Recently number of sperm whales caught
have been gradually increasmg, especially in the pelagic whaling ground in thenorthern part of the North Pacific. And regulation of sperm whale fishing has
become to be need. However, for reasonable regulation of sperm whale fishing,
we must have more ecological knowledges on the whale.
As one of well-known knowledges on the ecology for the sperm whale, so called" harem " schools distribute in low and middle latitude, and it does not move tohigh latitude even in summer season. On the contrary, males which separate
from harem school distribute in high latitude in summer season (Matthews, 1938).
But, this knowledge is mearly general, and there seems to be no study which deals
the sexual segregation quantitatively. In present paper, I want to discuss on thisproblem.
Japanese whalers have been operating to catch sperm whales in the northernpart of the North Pacific by factory ship whaling, and coastal whaling continues in
the adjacent waters to Japan. And we have relatively many biological materialsof the whale from both waters. I will discuss the sexual segregation of the sperm
whale between high and middle latitudes dealing with the materials as a model.
That is to say, I used the material from the Aleutian waters as a model which represent
the whales distributed in the high latitude, and the material from coast of Japanas another model of the sperm whales distributed in the middle latitude. Of course,
it is a problem that there are some possibilities that they belong to different subpopulation by means of blood typing investigation (Fujino, 1963). However, I used
the materials only as two models in the high and middle latitude, and I consider
my discussion will adapt for each sub-populations.
MATERIALS AND METHODS

Upper teeth of the sperm whale are used for the age determination of the investigated individuals. Preparation and reading of upper tooth growth layers are thesame as the paper of Nish1waki et al. (1958). Age determination is based on the* Brief paper was read in the Meeting of Jap. Soc. Sci. Fisheries in Oct., 1965.

2

S. OHSUMI

paper of Ohsumi et al. (1963), that is to say, one growth layer in the dentine is definedas one year.
As the materials, I used the biological data of the sperm whales which had
been collected by the staffs of the Whales Research Institute in some whaling stations adjacent to Japan from 1960 to 1964 and in the Aleutian waters (Areas IV andV) in 1961 and 1962. The number of materials of which age were determinedwith upper teeth is as follows :Aleutian
Coast of Japan
1,035
1,975
Males
Females
2,237
For estimating the size distribution of the sperm whales which are really distributed in the Aleutian waters, I used data of the estimated body lengths whichwere observed by whale marking vessels in the northern part of the North Pacific.
According to the data, almost of the sperm whales which distribute in the Aleutian
waters are lone or small schools of few individuals, and the distribution of the females
is considered to be very few. Then, I used a length-age key of the male sperm
whale adjacent to IJ a pan for exchange of the estimated size distribution by the marking vessels to estimated age distribution.
SEASONAL CHANGE OF THE SEX RATIO IN THE
WHALING GROUND OF MIDDLE LATITUDE
Sex ratios of the sperm whales changes seasonally in the waters of middle latitude.
Clarke ( 1956) studied on the seasonal change of ratios of the male sperm
whales in the catch in the Azores waters. The ratios are the highest in the winterand the lowest in August.
TABLE 1. NUMBER OF SPERM WHALES INVESTIGATED BY THE STAFFS OF
WHALES RESEARCH INSTITUTE IN THE COAST OF JAPAN FROM
1960 TO 1964
Sex

Males

FemalesMaturityrnnrarure

MatureUnknown
Total

1·=·=
MatureUnknown
Total

Grand total
Sex ratio of males% of{Malesimmature
Females

May
l
19
7
27
2
11
4
17
44
61.4
5.0
15.4

June
2
47
5
54
3
51
25
79
133
40.6
4.1
5.6

July
2
81
14
97
16
88
24
128
225
43.1
2.4
15.4

Aug.
21
297
58
376
26
372
127
525
901
41.9
6:6
6.5

Sept.
66
347
57
470
80
941
201
1,222
1,692
27.7
15.9
7.9

Oct.
51
160
16
227
39
578
116
733
960
23.7
24.2
6.3

Nov.
14
22
3
39
19
99
18
136
175
22.3
40.3
16.l

Total
157
973
160
1,290
185
2,140
515
2,840
4,130
31.3
16.5
8.0

In the coast of Japan, the male ratio is the highest in spring and the lowest
in October in the seasons from 1946 to 1952 when the whaling operated throughout
the year. According to the biological investigations by the Whales Research In-

SEXUAL SEGREGATION OF THE SPERM WHALE

3stitute from 1960 to 1964, the male sex ratios gradually decline from May to November as shown in Fig. 1, and the tendency is almost the same as that from 1946 to
1952. The seasonal changes are seemed to be different about two months eachother between Azores and Japan. This is considered to be caused by the differenceof the oceanographical construction between each waters.
Such seasonal changes of the sex ratio are considered to reflect the change of
migration of sperm whale herds. In the late winter and spring seasons, the malesperm whales which are mainly mature lone bulls begin to migrate from lower latitude, and they leave from the middle latitude to higher latitude, after then, breeding
herds mainly consisted with mature females and baby or juvenile individuals cometo middle latitude waters. Mature male groups begin to migrate from higher latitude by the end of fall. According to Slepzov (1955), the large males appear in the
migration season of spring and fall in Kuril waters. Japanese coastal waters are
considered to be a northward migratory course of lone or male schools in the spring
time.

80
."!
0

E

0

60

c

~
Q;

"-

40

20

JAN.

Mar.

May.

Jui.

Sept.

Nov.

Fig. 1. Seasonal change of male sex ratios in the sperm whales in the northernhemisphere. Open circles and broken line: Coast of Japan, 1946-1952.
Closed circles and solid line: Coast of Japan, 1960-1964. Glosses and chain
line: Azores, 1939, 1954.

Present materials were collected mainly in August, September and October
for the case of coastal Japan, and in June, July and August for the case of northern
Pacific respectively. Therefore, they seem to represent the optimum distributionsof the sperm whale herds in each latitudal areas.

S. OHSUMI4

AGE DISTRIBUTION OF THE SPERM WHALES IN THE
ADJACENT WATERS TO JAPAN
Table 2 and Fig. 2 show the age distributions of the male and female sperm whalescaught in the waters adjacent to Japan from 1960 to 1964 in a semilogarithmic
graph based on number of growth layers in the dentine of upper tooth. There isa remarkable difference between males and females. In younger age, the distribution of the males is almost the same as of the females, but from 12 years of age thetotal mortality coefficient in the females is lower than that in the males. Following
three factors will be concerned with this phenomenon.
1. Difference of natural mortality rate between males and femiles.
2. Difference of fishing mortality rate between males and females.
3. Segregation of males.
Considering the age distribution of the male sperm whales in the Aleutian
waters, the first factor will be abandoned, because the natural mortality rate of themales in this area is not so different from the females as shown in the next chapter.
~~ b."°'l

JOO

80

'!$A'
\/'q R

60

~~

40

Q.dP\

I

I
I

R
9

I
I I ~
I 11

1xJ 1 ,,

0>20

,,,

_g

\1 I
11 I

>-

u

~

" 10
~

1" 8

~

6

I

I

°'ll

4

2

R

Ii I' i
I I I
)J I

I

j

,,

I

I

II

'

0

10

20

30

40

50

~I

9 °'l

I\
11

I

\ ro

l

I

~I

I I
I I
I I

,,

ebb

..

60

I
I
I

0

70

Age !Years I

Fig. 2. Age distributions of male and female sperm whales from the waters
adjacent to Japan in 1960-1964.
Open circle and broken line: Female. Closed circle and solid line: Male.

The second factor may be considerable, because adult males are larger than
the females, and whaling efforts have been added more to the former than to thelatter. But, I consider in the operation system for the sperm whale in the coastal
waters the second factor is seemed to be not so affected on this phenomenon. The

SEXUAL SEGREGATION OF THE SPERM WHALE

5third factor must affect mainly to the difference of present total mortality rate in the
middle latitudal waters. If not, we cannot explain the distribution of adult males
in high latitudal waters.
TABLE 2. AGE DISTRIBUTIONS OF THE SPERM WHALES IN THE COAST OF
JAPAN (1960-64) AND ALEUTIAN ISLANDS (1961, '62).
Coast of Japan
Coast of Japan
Aleutian
Aleutian
~
Age
Age
Males
Males
Females
Males
Females
Males
~

1
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

2
2
5
10
7
22
29
28
45
67
84
90
82
79
72

62
37
44
33
25
26
19
20
18

15
12
7
1
9
11
6
5
4
5
7.
(cont.)

2
1

9
17
26
39
48
63
77
81
105
124
103
92
95
93
71
82
73
68
66
56
50
56
50
40
49
36
39
42
44
41
35
32

3
7
19
21
43
42
60
63
62
78
61
76
75
75
82
60
66
65
71
56
55
51
63

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
65
66
69
71
75
77
Total

5
4
7
4
1
3
3
3

37
38
38
20
20
31
26
11
23
11

1
3

2

8
11
12
7
5
5
3
1
3
2
3
3
2
2
2

57
29
45
51
43
54
41
39
32
25
22
32
31
21
17
22
25
19
18
11
13
13
14
7
7
7
7
9
8
3

2

1,035

2,237

1,975

In present paper, I presume that natural and fishing mortality rates are respectively almost the same for the males and females in Japanese coastal waters,
and the deviation of age distribution in the females minus those in the males presents the age distribution of the males segregated to the high latitude. And thishypothesis will be examined in the following chapters.

S. OHSUMI6

AGE DISTRIBUTION OF THE MALES IN THE ALEUTIANWATERS AND COMPARISON OF IT WITH THE
ASSUMED AGE DISTRIBUTIONIn Fig. 3, solid line shows the age distribution of the male sperm whale caught byJapanese fleets in the Aleutian waters in 1961 and 1962 in a semi-logarithmic graph.
In these years Japanese whalers caught 4,349 sperm whales, and males were 4,305(98.9%) among them. According to Ohsumi & Nasu (unpublished), in the waters
around the Aleutian Islands and Bering Sea, female sperm whales distribute very
few, and they relate with the distribution of warm water mass.

60

,

40

"',I

I\ r-"w~

II

/

IIII

t'

1
1

I I

I

•

I ~I

J

,I 'ii

- 20
OJ
_g

•

I
I

I

I
I
I t

,111,,

4

I
1
I
1
I

-·~
... ~\

+I ,'

2

I

I

I
I

10

20

30

40

50

,
I

I

I

60

70

Age !Years l

Fig. 3. Age distribution of the sperm whales in the North Pacific.
Closed circle and broken line: Females ninus males in the Japanese coastal waters.
Open circle and solid line: Males caught in the Aleutian waters (1961-1962).

Fig. 3 also shows the age distribution which was represented as the deviationof two age distributions between females and males (females minus males) in theJapanese coastal waters, assuming that the deviational part presents in the highlatitude segregated from middle latitude.
Comparing above two age distributions, they are very fittable each other in therange of ages from about 25 to 45 years. But, in younger generations, the possiblly
segregated males are more frequent than the males which were caught. I think
this is caused by the size limitation m the catch by whalers. For the factory ship
whaling, legal size is limitted as 38 feet or more. According to age-length key, the7

SEXUAL SEGREGATION OF THE SPERM WHALE

average age of the male sperm whales which are 38 feet long is 18.6 years. Therefore, in younger generations, the age distribution of the whales caught have a danger
that it does not show the real population which distributes in the waters.
TABLE 3. SIZE DISTRIBUTION OF THE SPERM WHALES OBSERVEDBY INVESTIGATION VESSELS AND CAUGHT BY CATCHERBOATS IN THE ALTUTIAN WATERS
Observed

Body length(feet)

Caught (1957-59)

Whales

%

11
17
61
43
30
43
26
17
2
4
3

0.4
4.3
6.6
23.6
16.6
11.6
16.6
10.0
6.6
0.8
1.5
1.2
0.4

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
Total

Whales

%

87
527
719
772
776
812
719
447
125
12

1.7
10.5
14.4
15.5
15.6
16.2
14.4
8.9
2.5
0.3

4,996

259

25

~

,1

I I

I

I

I
I

20

I

I
I

I

I
I

I
I

I

I

I

I

:
I

'if'.

It.I

I
I

15

I

I

I
I

\

I

\

I

I

>-

I

I
I

g
!!;

I
I

I

CJ"

~

I

I

I

I
I

I

I

'

I

10

I

''

I

I

I

I
/

5

i

I

\

'

I

f/
I

''

I
I

I

I

I

L __.i

35

40

45

50

I

I

I

I

.._ ___ ... _..._......... __55

60

Body length !feet I

Fig. 4. Size distribution of the sperm whales observed and caught in the Aleutian
waters. Open circle and solid line: estimated length by investigation vessels.
Closed circle and broken line: caught whale.

8

S. OHSUMI

For the solution of this problem, I used the records of estimated body lengthsof the sperm whale which had been observed with Japanese research vessels in thecourse of whale marking in the northern part of the North Pacific. Research vessels
recorded all whales which were found in the waters, so we may regard the records
represent the whales distributed in the waters. Table 3 and Fig. 4 shows the size
distributions of the whales observed and caught in Aleutian waters. The size distribution of the whales caught is like normal distribution and has a mode at about
45 feet, on the contrary, that of whales observed does not show a normal distribution,and there is a mode at about 40 feet. Fig. 4 also shows that there are many small
sized sperm whales in high latitude, but they were not caught because of the size
limitation. Of course, this size distribution was based on estimated body lengths,
and they have some errors from true body length. According to the data of the
sperm whales which were recaptured soon after marking, our estimation of bodylength of the swimming whales has a tendency of under-estimation from actual onesin the range of body lengths from 33 to 45 feet long. However, it will be true that
TABLE 4.

AGE-BODY LENGTH KEY FOR THE MALE SPERM WHALESIN THE COASTAL WATERS OF JAPAN
Body length (feet)

Total

Age

6- 7
8- 9
10-11
12-13
14-15
16-17
18-19
20-21
22-23
24-25
26-27
28--2930-31
32-33
34--35
36-37
38-39
40-41
42--43
44-45
46-47
48-49
50-51
52-53
58-59
68-69
74-75
Total

33--4 35-6 37-8 39--40 41-2
2
I
13
3
l
33
29
2
2
38
36
16
18
42
21
5
8
10
29
24
11
19
8
5
5
10
11
8
2
2
l

3

6
6
3

4
6

4

2

2

2

43--4 45-6 47-8 49-50 51-2

53--4 55-6 57-8
3

4
3

18
66
90
86
72
49
35
24
23
14

4

2
3
l

2

2

3

2

12

2

5
6
5
6

2
3

2
2

2
2

2
2

138

189

98

38

18

11

16

7

7

5

3

532

SEXUAL SEGREGATION OF THE SPERM WHALE

9

more small sperm whales distribute in high latitude than the whales caught in thewaters.
It is estimated that the observed sperm whales were almost males from theschool composition of them, because they were almost lone or small numbers in aschool, and it is seldom to find a large school like harem school in Aleutian waters.
TABLE 5. TOTAL MORTALITY COEFFICIENTS FOR THE SPERM
WHALES BY AREAS, SEXES AND RANGES OF AGES
Area
Coast of Japan{

Aleutian

Sex

Ages

Males

12-25
{ 26-50

Females
Males

12-52
24-66

Total mortality
ccefficient
0.165
0.081
0.073
0.071

40

200

20

100
80 60

OJ

,g 10

~
\

8

40 ~
D

\ -1

""1

6

i

-<:

?:
-u

I
I
I

4

20 2

D
Dl

"~

~

0

\

Dl
(!)

l0

2

~

8 0

,.....,_

0.8
\

0.6

I

'\

0.4

I

I
I

~
•-4

>-

0

6 g
~

4 ~
\

~I
\

2

I
I

I

I

I

~

0.2
10

20

30

40

50

60

70

Age !Years I

Fig. 5. Age distribution of the sperm whales.
Open circle and solid line: age distribution of the distributed sperm whale in the Aleutian waters calculated by size distribution and age-length key.
Closed circle and broken line: female ninus males in Japanese coastal waters.

To compare the age distribution of possiblly segregated males with that ofobserved sperm whales which were distributed in Aleutian waters, I calculated theestimated age distribution of the latter from estimated size distribution by meansof age-length key of the male sperm whales from the coastal waters of Japan. I

10

S. OHSUMI

did not use the age-length key of the male sperm whales in the Aleutian waters,
because it is different from that of distributed whales by size limitation.
Fig. 5 shows the age distributions of the sperm whales segregated from middle
latitude and observed in the Aleutian waters respectively. From this figure, we
can find that the both fairly fit each other. Estimation that the deviation of age
distributions between females and males in Japanese coastal waters means thesegregation of males from middle latitude to high latitude may be comfirmed fromthis examination.
There is another difference in the age distributions between the whales caught
and segregated in the older generation (Fig. 3). There is an uncertainty in the latter
distribution because of scarsity of number of samples in older ages. But, it is also
considerable that whalers select large whales for their catch as possible, and the large
sized or old whales apt to be caught by whalers than the actually distributed whales.
THREE MODELS OF AGEAL SEGREGATION AND COMPARISON
OF REALLY AGEAL SEGREGATION OF MALE
SPERM WHALES WITH THEM

When natural mortality coefficient (M) and fishing mortality coefficient are settled
in constant throughout life span for the mother population, the age distribution of
it is shown in Fig. 6A, havmg a total moritalty coefficient" (Z 1 ). If individuals
which are over age of i disperse with a constant dispersal coefficient, the age distribution changes into Fig. 6B. Fig. 6C shows age distribution of a population inwhich the individuals over i years old increase in their dispersal coefficients acceleratively year by year with constant acceleration rate. Fig. 6D is a age distribution
in the third population in which individuals over i ages increase their dispersal coefficient with constar.t accelerate coefficient until the age of j and after then theydisperse with constant dispersal coefficient.
Now, comparing with above three models of ageal segregation, the age distribution of males in the coastal waters ofJapan resembles with Model D (Fig. 6D).
Owing to sl7.e limitation of catch, the age distribution in young generation does
not represent the true age distribution, but in this stage age distributions are almost
the same in males and females. This means that in young stage males distributein the middle latitudal waters in the same way as females. Age distribution of males
changes at the age of about 12 years from that of females. This phenomenon will
mean that segregation of males begins from about 12 years of age, and this age willcorrespond with i-age in Model D. According to Nishiwaki et al. (1958), theage at sexual maturity of the male sperm whale is considered to be about 10 toothlaminations. Then, the segregation of males will begins after sexual maturity.
The total mortality coefficient of males in the range from 12 ages to 25 ages isrelatively high value, and after 25 ages the total mortality coefficient becomes lower
and shows almost the same value of females. This means that in the ageal range
from 12 to 25 years males segregate from middle latitude waters with the almost
constant accelerated rate age to age, and after then individuals segregate with con-

11

SEXUAL SEGREGATION OF THE SPERM WHALE-- --.... _---

A-- -- ---

>-

u

"=>
0)

O"

~
u...

--------c-------------------

D----....
.

Age

Fig. 6. Age distributions in the four cases of which various ageal segregation table plase.
A: Mother population
B: Segregated over i years old with the rate of Z 2-Z 1
C: Segregated over i years old increasing with the constant rates (Z 3-Z 1)
D: Segregated over i years increasing with constant rates (Z 4-Z 1 ) until j years old.

stant segregation rate.
Table 5 shows the calculated total mortality coefficients of the sperm whalesin the coastal waters of Japan and in the Aleutian waters. In these values, thecoefficient of males in the ageal range from 12 to 25 years (0.165) will correspondwith Z 2 in Model D, and that of females (0.073) correspond with Z 2, respectively.
The coefficient of Japanese water males in the ageal range after 26 years (0.081)
is not so different with those of females in the Japanese coastal waters or males in
the Aleutian waters. The deviation between Z 2 and Z 1 (0.092) will mean the accelerated segregation coefficient between one age and the next age until 25 ages.
Age of 25 years corresponds withj-age in Model D. Average body length of males
at 25 years is 45 feet according to the growth curve in the paper by Nishiwaki etal. (1958), and the weight of testis in larger side is 2.8 kg according to Nishiwaki
et al. (1956).
In the sperm whale, although males attain at sexual maturity, it takes someyears until they can join into the breeding activity. This problem is one of theimportant subjects in the ecology of the sperm whale, but I think that 25 years of

12

S. OHSUMI

age or the body length of 43-45 feet will be the minimum age or body length atattainment of full maturity in the male sperm whale. If so, the segregation of males
wih be concluded as follows : In the sexually immature stage, males remains in" harem " school, and after attainment of sexual maturity some of males begin tosegregate from the harem. The rate of segregation of the males increases age toage until they attain at full maturity. After then the segregation rate becomes
constant. For the proof of this assumption, investigat10n of school composition of
the sperm whale will be need.
THE RATIO OF SEGREGATED MALES IN HIGHLATITUDE OF MOTHER POPULATIONIn Model D population, the segregation rate of population size over i age (K) is
shown in following formula :K = 1(1-e-M•) [1-e-(j-i)M,]
1-e-M•

e-[(j-i)M,+M,]

When M 4 =0.073, M 2 =0.165, i=12 and }=25 are set into this formula, thesegregation rate is calculated as 0.416. This means that about 42 per cent ofsexually mature males disperse to high latitude from mother male population.
From Table 2, the actual segregation rate which calculated as the rate of deviation between females and males over 12 age is also calculated as 60.8 per cent.
If the sex ratio is the same for males and fem ales in mother population, thepopulation size of males which segregates from mother population and distributes
in high latitude is about 21 to 30 per cent of mother population size.
However, as mentioned above, in the Aleutian waters the population size as
the object for whaling must be smaller than the population size of whales which
distribute in the high latitude, because the legal size is 38 feet for factory ship whaling.
Comparing two age distributions in Fig. 3, the available rate as the object for whaling
is calculated to be 86 per cent of the real population size of males in high latitude.
CHANGE OF SEGREGATION RATE OF MALE
SPERM WHALE ACCORDING TO THE AGE
The segregation rates in each age were calculated smoosing the age distributions
which were shown in Fig. 2. The change of segregation rates according to theage is shown in Fig. 7. After the age at sexual maturity, the segregation rate increases remarkably accompanying with increment of age until about 25 years of age.
And it becomes maximum at the age of 40 years. After then it decreases accompanying with increase of age. In the middle ages ranging from 25 to 53 years, the segregation rates are over 75 per cent, and at the maximum it attains to about 90 per
cent.
However, there are some males which are fully mature but not segregated tohigh latitude. Most of these males are considered to be so-called harem leaders.

13

SEXUAL SEGREGATION OF THE SPERM WHALE

Not-segregated80

Segregated

g
0

"'40

20

10

Fig. 7.

20

40
30
Age !Years)

50

60

Change of ratio of segregated sperm whale males according to age.

As mentioned above, after about 40 years of age segregation rates decrease accompanying with ages, although there is an uncertainty whether this result istrue or not because of scarsity of number of samples in older age. If it is true,
this phenomenon will mean that old bulls remain in the middle latitude as harem
leaders.
DISCUSSION

One of the questions in the present paper is whether the materials from Japanese
coastal waters represents the mother population from which males segregate tohigh latitude or not. Townsend (1935)'s laboring work on the distribution of
sperm whales by means of logbook data in the American Whaling Age is considered to show seasonal change of distribution and density of the whales in theworld. However, conserning with North Pacific Ocean, there is no catch records
in the waters arround the Aleutian Islands Chain and Bering Sea. I think thismeans that American whalers chased chiefly " harem " schools and did not chasedlone and large bull in the high latitude, because harem school was more easy to
be caught than lone bull by them. If it is true, the distributions of the sperm
whales which were shown by Townsend should represent chiefly those of harem
school. Fig. 8 shows the latitudal frequency distributions of the sperm whales in
the Pacific Ocean in summer and winter seasons drawn from the maps by Townsend.
As mentioned by him, it is clear that there is seasonal movement in the sperm whale.
In Fig. 8, A will be main distribution of sperm whales of northern hemisphere stockin summer, and it will move to A' in winter. If so, the main stock of northernhemisphere sperm whale in the Pacific will distribute in the range from 20°N to 40°N
in summer. Then my materials which were obtained from the adjacent waters toJapan will be able to represent the main population of North Pacific sperm whale.

14

S. OHSUMI

For the solution of the sexual segregation in the sperm whale, we must investigate the composition of school in the sperm whale directly. Clarke (1956)
described on the classification of sperm whale schools in Azores waters. According
to his classification, there are three kinds of schools in the sperm whale. They arejuvenile, bachelor and harem schools. Sex ratio, number of individuals in a school,
age composition, sexual condition etc. are important subjects to study as well as thenumber and ratios of above three kinds of schools in the ocean.

30

*

>~
g 20

B'

U"

~

I

·~

I

I

I
I

10

I

P°'\
I
I
I

I
I

'

\

~\
\

l>0°N

20°N

00

20°S

40"5

\

'

'

60°S

latitudeFig. 8. Latitudal frequency distribution of the sperm whales in the Pacific in summer
and winter season after the log-book records of American whaling ships. Drawnfrom maps by Townsend (1935).
Solid line: Summer (July to September). Broken line: Winter (January to March).

Concerning with sexual segregation of the sperm whale, it is practically needfor the regulation of whaling how many adult males are need at least for the meintainance of reproduction in the sperm whale population. If 25 years is the age
when join into breeding activity in the male, using the age distribution of sperm
whale from the coast of Japan, the tentative ratio of needful adult males can be calculated, when we assume that harem males remain in middle latitude even insummer- autumn season.
The result is that one adult male leads about 16 mature females. However,it will not be solved, unless we have knowledges on the sexual behavior of the sperm
whale.
SUMMARY
Sexual segregation of the sperm whale from middle latitude to high latitude was discussed chiefly by means of the age distributions of the sperm whale caught in coastal
waters of Japan and in the Aleutian waters.
1. There is a seasonal change of sex ratio in the sperm whale caught in the
coast of Japan. The sex ratio of males is the highest in the spring and the lowest in

SEXUAL SEGREGATION OF THE SPERM WHALE

15

October and November. This phenomenon is considered to be caused by changeof the seasonal migration of males in this waters.
2. The age distribution of the male sperm whales is different from that of thefemales in the coastal waters of Japan. And this phenomenon is considered to represent the sexual segregation according to the age in the middle latitude.
3. By means of estimated body lengths of the sperm whales which were observed in Aleutian waters by marking vessels and age-length key for the male sperm
whales, an age distribution was obtained. This age distribution closely fits with thedeviation of age distributions between females and males in the coastal waters ofJapan. This is considered to proove the assumption that the deviation of two age
distributions between females and males in middle latitude segregate to high latitude
waters.
4. Owing to the size limitation for sperm whale catch, the age distribution of
the male whales in the Aleutian waters does not represent the real age distributionof the whales which distribute in the high latitude.
5. Segregation of male sperm whales from middle latitude begins soon after
the age at sexual maturity. And until the age of 25 years, the segregation rate increases with the ages. After then, the segregation rate becomes nearly constant.
The age of 25 years in suggested to be the age at full maturity when males become
to have breeding activity.
6. About 40-60 per cent of mature males of mother population are suggested
to segregate to the high latitude waters. Therefore, the population size of males
which distribute in high latitude will be about 20-30 per cent of mature sperm
whales in mother population.
7. Rates of segregated males in each generation increase rapidly from 12 to
25 years, and it attains to about 90 per cent in maximum at the age of 40 years.
After then, the segregation rates decrease gradually with the age.

ACKNOWLEDGEMENTS

The materials used in the present paper were collected by the efforts of biologists
and inspectors who had investigated the whales in the whaling grounds. My
sincere thanks are due to them. I am much indebted to many members of theJapanese Whaling Companies for the help in investigation and collection of thematerials.
Dr. M. Nishiwaki and Mr. T. Kasuya kindly read the number of growth layers
in the dentine of many maxillary teeth with me for the age determination of thematerials. I should like to thank them.
REFERENCES
Sperm whales of the Azores. Discovery Repts., 28: 237-98.
Identification of breeding subpopulations of the sperm whales in the waters adjacent toJapan and around Aleutian Islands by means of blood typing investigations. Bull. Jap. Soc. Sci. Fisheries,

CLARKE, R.

(1956).

FUJINO, K. (1963).

16

S. OHSUMI

29 (12): 1057-63 (In Japanese).
MATSUURA, Y. & MAEDA, K. (1942). Biological observations on whales caught in the northern Pacific.
Data of Whaling, 9(1): 1-59 (In Japanese).
MATTHEWS, L. H. (1938). The sperm whale, Physeter catodon. Discovery Repts., 17: 93-168.
NrsmwAKI, M., HIBIYA, T. & KIMURA (OHSuMr), S. (1956). On the maturity of the sperm whale (Physetercatodon) found in the North Pacific. Sci. Rep. Whales Res. Inst., 11: 39-46.
NrsmWAKI, M., HIBIYA, 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.
0HSUMI, S. (1965). Reproduction of the sperm whale in the northwest Pacific. Ibid., 19: 1-35.
0HSUMI, S., KAsuYA, T. & NrsHIWAKI, M. (1963). Accumulation rate of dentinal growth layers in the maxilary tooth of the sperm whale. Ibid., 17: 15-35.
SLEPTSOV, M. M. (1955). Biologia i promisel Kitov dalnevostozuih morei (Translated into Japanese).
TOWNSEND, C. H. (1935). The distribution of certain whales as shown by log book records of American
whale ships. <:,oologica, 19(1): 1-50.