Statistic Study of Foetuses of \iVhales
Kazuhiro Mizue and Hisako Jimbo
Introduction
The material of the present study regarding foetuses of whales is based
upon the monthly reports forwarded to the Whaling Society by various
whaling companies from 1911 to 1948. Most of such reports were destroyed by the fire during the War. Thanks to Mr. Ohmura's efforts,
however, all the numbers issued in the following sixteen years, -1911,1914, 1919, 1921, 1922, 1926, 1932, 1934, 1941, 1942, 1943, 1944, 1945, 1946,
1947 and 1948- have been recollected so as to be available for us atpresent. Moreover, six numbers of both 1910 and 1940, too, have beenrestored.
These whaling reports record particulars of individual animales of themonthly catch with such items as date of the catch, species, sex, bodylength, foetus, stomach contents, and locality of the catch.
As for the older records of foetuses, they seem to have been madeaccording t? the rough. estimation by the eye. Yet there may be no grossforgery about them, as in the case of body-lengths which are subject. tobody-length limitation and ratio money. Since 1946, however, the records
have become authentic, because in that year whaling inspectors began tobe employed in each landstation along our coasts, who, as in factory-ships,
make biological investigation of the catch as well as superintend whaling
operations. As anyone that has been on board a factory ship and atpresent in the scene of dissection of a carcass, can well suppose, foetueses.
below 5 inches are very apt to be overlooked. In fact, either for this.
reason or for the ship in recording, description of such small foetuses areextremely rare before 1945. Neither is it probable that before 1945 eachovaries with functional corpora lutea should have been operated, upon soas to ascertain the presence of a foetus. The descriptiop. of foetuses areconfined to such large sized species as Sperm, Sei, Fin, Blue, Hump-back,Right and Grey whales. Of these the foetuses of Blue, Hump-back, Rightand Grey whales are s.o few in number that it is hardly possible to study
them with any statistic result.

Statistic Study of Foetuses of Whales

120Sex Ratio
As for Sperm whales, the ratio of males and females are 46.2% and53.8% respectively : so it is roughly half and half. Since early days it
has been said of Sperm whales, which have the habit of forming Harem,that females greatly outnumber males. · This is by no means correct.
According to the sex ratio of foetus of the same species, females slightly
outnumbe.r males, but this relation is quite reversed in the sex ratio of thecaught whale. (male:-i are 52.4%, females are 47.6%)
The sex ratio of the foetuses of Sei whales is 45.5% and 54.5%, showing · the ratio of either sex is almost the same. This relation is notchanged in the sex ratio of the caught whale. (males are 49.0%, females
51.0%)
Table 1.
Species

Foetus of Fin

Foetus of Sei

Foetus of Sperm

YearTotal
Male Female Unknown

T t 1
Male Female Unkna
own

I!Unkn- Total
Male !Female in

1910
1911
1914
1919
1921
1922
1926
1.932
1934
1940
1941
'1942
1943
1944
,1945
1946
1947
1948
T·>talSexratio

----0
1
2
0
0
0
0
4
2
4
6
6
4
3
1
1
1
3
38
51.4

00
2
0
0
00
1
4
2
2
8
6
2
1
6
20. 3648.6·O020000
50000000000
701
60000
10668
14105273381100- - -°- - - - - ·' -ow

12l

11l0ll21513156602
91421914
6734804
7178145.5

i'

12
1197
54.5

2011531113
123842702
60
302
160120
28
0280
1428
2002
12
170161
194030822045
19'
1 II 2956I
21 I 199
247
46.2
100!
II

53
74
9932251338
41421
9293360!

'IIII011000
303·2166
6314200I

l222288
i
53.8'

18
3545

00
0
00
2i

I

6718
1216I

1351
79118557
100-

So is the sex ratio of Fin whales, males being 51.4%. while females
48.6%. According to Mr. Matsuura, female foetuses are less in number
than male ones in Mystacoceti. But it cannot possibly be correct, for boththe sex ratio of foetuses shown in ·Table 1 and that of the caught whale,indicate either in Mystacoceti or. in Odontbceti the sex ratio is half andKazuhiro Mizue and Hisako Jimbo

121

half.
Relation between Pregnancy and Body-length
Fig. 1 shows graphi~ally the numbers of pregnant whales of the speciesof Sperm, Sei and Fin whales, respectively, according to their body-lengths.
For lack of material, we had to omit the observation of pregnancy in otherspecies. As for Fin whales, the minimum size of pregnant whales is 50
feet and the maximum size 71 feet, while the greatest number of pregnant
cases, are seen in the body-lengths of 64-66 feet. In Sei whales, theminimum size is 38 feet, the maximum size 56 feet and the peak of pregnancy is formed between 45-50 feet. In Sperm whales, the maximum size,of pregnant animales is 46 feet but females measuring 30-37 feet, especially
32-35 feet ·are most frequently pregnant. In the adjacent waters of Japanthe catch of Sperm whales below 30 feet is prohibited by body-length
limitation, while Sperm whales measuring 30 feet are ·already sexuallymature and therefore liable to pregnancy. For this reason the minimum
size of pregnant of Sperm whales cannot be determined.
In recent years the size of the whales in the catch has been reducedFig.
100.1~

llO/,'\'\/,

70.~ ! \90- - - - - S'Pent ..'halo- - - - - - - - - - Sei Whsh

- - - - F i n \'hale

I' I \'60

5oi40!
}OIl\' 'I \!i \I'\\Statistic Study of Fo~tuses of Whales

122

as js clearly seen in any body-length frequency curve. So in the stock ofwhales as they are in the sea, this curve of pregnancy ~ay shift a little
to the side of larger sizes.
Growth Curves
Now we are going to determi~e the pairing period, the duration ofpregnancy, the delivery period and the body-length at delivery according
to such species as Fin whales, Sei whales and Sperm whales.
(A) Regarding ·Fin whales, Fig. 2 shows coordinate with the transversal
axis indicating months and the vertical axis indicating body-lengths offoetuses. The black dots representing the foetuses in our material, areplaced on the coordinates according to their size and the time of the catch.
Each signs of. CD represents an average body-length per ten days. Due tothe scantiness of our material, tracing these signs of CD would produce too
rugged a curve. So we have shown an average body-length per month bysmall circles, and prepared a growth curve based on them. We have no
foetus in our material for the month of November and of December. SoFig.

2..
10'·-. .. . .

.. .
'h••

I~monthf Ja;.""""~h~b.-'--:-:---'-A-~-.-'-~-,-1..~J-un-.-'--,J~ul-.J....-,A-ug-.'---,..-p.-'-~~--'-~No--'-~~.-. .i. . . ,.~~.
••••

•

••

"Kazuhiro Mizue and Hisako Jimbo

123

we are obliged to prolong the growth curve which has. already shown aclear tendency by the time it reaches the month of July-so as to complete
the year's curve. If we are to apply the body-length of Fin whales atdelivery, in the South Semisphere, 21-22 feet, determined by Mackintosh& Wheeler and Matthews, to our growth curve, the size will find its placeon the curve at the beginning of January next year. The pairing period
according to our growth curve comes in January. Therefore, if we areto assume,oand we have some reason to believe,-that the boby-lengths atdelivery in both the semispheres are the same, the duration of pregnancy istwelve months and one decade. There is six months' variation in both theperiods of pairing· and of delivery between the South and the North Semispheres. Our growth curve has roughly two months' breadth.
· (B) The coordinates in Fig. 3 have been prepared in the same way as
those of Fin whales, and show the growth of foetuses of Sei whales. The
signs of CD represent average body-lengths per ten days, and the circles
those per month. The records of foetuses in the months of March, April
and May are exclusively those of the Bonin Islands sea area because there
is no foetus found in other sea areas duripg that period. The season forFig.

3124

Statistic Study of Foetuses of Whales

Sei whales in the Bonin Islands sea area lasts only these three months andthe foetuses found then ,are still very small in size as is shown in .thecurve. In July, August and September pregnant whales are captured mainlyin the Sanriku sea area, and in October, mainly in the Hokkaido Padic
sea area. These shifts in seasons and whaling grounds as well as thedifferent growth stages of foetuses of the catch lead us to suppose that theSei whales in our adjacent waters migrate according to the course andseason above mentioned. If we had whaling landstation at the south of theBonin Islands sea area we should get still amaller foetuses.
During the whaling operations in this season, some marking weredischarged in the Bonin Islands sea area. The result of this experiments
will clarify the course and the season of migration in several years.
The growth curve of Sei whales was shown in Fig. 3, run smooth
upwards as far as the tenth month, after that the dots are hardly traceable .
. So we have prolonged the curve already drawn as in the case of Fin
whales. According to this curve, foetuses measuring 13 feet are seven in
number while those of 14 feet and ot 15 feet number one and two respectively. From this we assume foetuses at delivery measure 14 to 15 feet aridthat delivery time is at the end, of November. Like Fin whales, Sei whales
have their pairing period in January, so that the duration of pregnancy isless .than 11 months. Our growth curve of Sei whales has three month~'breadth.
Sei whales are to be subdiv:ded into totan whales (there are many
designs like the surface. of galvanized iron plate in abdomer.) and Sei whales
proper. Totan whales are larger sized and supposed to belong to a Northern stock while Sei whales are of a Southern stock and mostly captured
off the coast of Wakayama Prefecture. The ct:fferences of these two kindsof Sei whale have been a subject of many d:scussions but not yet put toany morpholog~cal investigation. Fig. 3 show four dots placed far apartfrom the main .group. These four foetuses are all found in animals captured off Wakayama Prefecture. This will give rise to the question whether
the Sei whales in that neighbouring sea are of different species from thatof Totan '*hales or of the same species taking different course in theirmigration. The problem will be clarified by the further study of the localityof the catch, of the d:stribtion of body-lengths according to the sea areas"and 'by experiments 'by marking as well as by morpliolog:cal investigations.

Kazuhiro Mizue and Hisako Jimbo

125(C) In order to ascertain the courses of migration of Sperm whales in
the adjacent waters of Japan we marked the locality of the catch of eachindividual whale on a chart, wh:ch has led us to suppose that Sperm whalesin those waters l~ve in several different groups. So we have made our studyof their foetuses according to the four different sea areas, the BoninIslands, the Kinan, the Sanriku and the Hokkaido-Pacific sea areas. In
other respects, the method of our study is the same as in the case of Fin
and Sei whales; we have drew the growth curves of the foetuses of Sprem
whales by the average body-length per ten days and that per month.
In Fig. 4 you will find a growth curve of foetuses of Sperm whales inthe Kinan sea area though the cruve is imperfect for lack of material.
The curves in Fig. 5 shows the growth of the foetuses .in the Bonin Islands
sea area. The whaling season here lasts only three months,' ~arch, Apriland May, and yet we can dearly discern that foetuses group themselves
along the two growth curves. Figs .. 6 and 7 show the growth curves ot
the Sanriku and the Hokkaido-Pac~fic sea areas respect:vely, either of whichshow two groups of foetuses with different tendencies.
Fig.

15toreet

4_]___

126

Statistic Study of Foetuses of Whales

Fig.

5Fig.

615

105re et127

Kazuhiro Mizue and Hisako· Jimbo
Fig.

7-.

15..

••10II>..

5feet

lb: . .

V.ar.

Apr.

MeyJun.

Jui.

Au._g.

Sep.

·, Oct.

Nov.

Dec.

month

Comparison of the curves of these four sea areas has proved that there
is no variation between the tendencies of each· curve. So, upon the combined data of these curves we have formed body-length distribution curves
per months. Then tracing the vallegs appearing in the monthly curves,
we calssified the foetuses into two groups, upper and lower. We have·
prepared a growth curve for each group of the foetuses based upon average
body-lengths per ten days and per month in the same way as before.
When we arrange the curve of the lower group to the left of the curveof the upper one we can complete the growth curve of the foetuses ofSperm whales. (Fig. 8)
According to this curve the pairing peribd of Sperm whales in ouradjacent waters is March. As for the body-length at delivery, Wheeler
made a report of a Sperm whale immediately after birth measuring 13.4 feet,captured in the neighbouring waters of Bermuda Island, the Atlantic, in .
1933. So did Bennett of the one measuring 14.2 feet in 1840, and reportedby Matsuura two measuring 17 feet. Figs 5, 6 and 7, foetuses measuring
13 feet are found in num?ers but those of 14 feet are comparatively few
while those of 15 feet are rare. From this we assume that the body-length

Statistic Study of Foetuses of Whales

'128

Fig.

8

•!'eet.

l~~ ~en. ~r.
.. onthat delivery is 14-15 feet. The g~owth curve in Fig. 8 shows that the sizeof. 14-15 feet is attained to at August, so that the delivery time of Spermwhales is to be determined as August. However, this growth curve whose
composing members are rather homogeneous at first, grows in breadth andat the time of delivery it has the breadth of two months and a half. So
the delivery period of Sperm whales may practically be determined as
lasting from the end of July to the beginning of October.
If we are to determine the dura~ion of pregnancy of Sperm whales
from the delivery period shown on the curve of the foetuses, which isAugust, it lasts 17 ·months. The pairing period of Sperm whales differsfrom that of Baleen whales by two months, and the duration of pregnancy
here determined is much longer than theirs, which naturally brings about
a different delivery period.
(D) In this present study, we have determined the pairing period, theduration of pregnar..cy, the delivery period and the body-lengtl;i at del:veryof Fin whales, Sei whales and Sperm whales. Fig. 9 shows the distributionof the foetuses of Right, Blue and Hump-back whales in our material. The
growth curves coi:r:cic;le, with six months' variation; with those of Blue andKazuhiro Mizue and Hisako Jimbo

129

Hump-back whales in the Antarctic prepared by Mackintosh & Wheelerand Matthews. The pairing period of Baleen whales in the adjacent watersof Japan and perhaps in the North semisphere at large is January.
Fig.

9Blu" <fheJ..,•

Hump t:ie.olt "l'he.le

Right l.tiaL

I15

IQt"eetl.rsn.

•eb.

~--)

~ar.

TI'llOnthIn Figs. 10, 11 and 12, you will find the seasonal distribution of foetus'body-length of Fin, Sei and Sperm whales in successive eight years.

Statistic Study of Foetuses of Whales

130Fig.

I1941.II

1942

I-~---II•//>· nonit_,,. /~./
12;

/9'l?

I

1945

IIl.94f

II----+.~

••~

II
I.•'

'·

I

12'

1942

6'

I

I

II

• I./
II

J

9;

12/'

I•;

II/
6912'll

194,I•- - - - •7I - -I.;/

12I
I

91948

III•

6.

-·/

IFig.

1941

129-~---;--~

9I••
/I/,

/66,1I~;

IIt'eet.;···.

IIII•ItO - - - - ; - - -I

j_ ----r-I -

\947

I,.

IIII./!',.

.,,.

.

I/III1944I•j~---;yI

feet-' I..

I----,--- --I

194,

I

•/

1010; 1944

0912,.

·'

Kazuhiro Mizue and Hisako Jimbo
Fig.

131

12//
10-~-.
···.
•. .
:i~::.;ci•"•. i:.
... ~··e .

_,,,,,,.

12'6l~(Mar. 7, 1949)/I'