FEMALE SEXUAL MATURITY
AND DELAYED IMPLANTATION PERIOD
OF THE KURIL SEAL

SHINICHI HAYAMA*t, MASATSUGU SUZUKI*, HIROYUKI UNO**
AND TADAYUKI YAMASHITA*ABSTRACT

Sexual maturity and period of delayed implantation were studied infemale kuril seal (Phoca vitulina stejnegeri) in Nemuro Peninsula, Hokkaido
from 25 September to 1 December in 1982 and from 14 September to 1December in 1983.
Average ages at the first ovulation and first pregnancy were calculatedat 3. 7 and 4.6 years respectively using the technique of De Master (1978).
Furthermore, pregnancy rate in relation to the age was estimated and it
was found out that 90.9% of the female Kuril seals at 6 years and older
were pregnant.
The period of delayed implantation was estimated by the growth
curve of the fetus and the presence of 3 newly implanted embryos, and thisindicated that implantation occurred primarily in the late part of September.
From this observation, it appears that the period of delayed implantation
is approximately around 3 months because mating takes place in June.
INTRODUCTION

The kuril seal (Phoca vitulina stejnegeri Allen, 1902) is a synonym of Phoca
kurilensis Inukai, 1942 (Shaughnessy and Fay, 1977), ~nd is distributed from theNorthern Kuril Islands to the Pacific coast of eastern Hokkaido. The populationin the Kuril Islands was estimated at less than 2,000-2,500 including those in
Shikotan and Habomai Islands (Belkin, 1964), while that in eastern Hokkaido
at 200-250 (Niizuma, Naito, Itoo, Wada, Abe, Ohtaishi and Nishiwaki, 1980).
The age at sexual maturity in females is important in the conservation ofthis species and from the point view of general biology. However, there areno such data on the eastern Hokkaido population of Kuril seals. The purposeof this study is to determine the average ages at the first ovulation and pregnancy,
and the period of delayed implantation in the Kuril seal.

* Department of Veterinary Anatomy, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido,
080 Japan.

** Institute of Applied Zoolof5Y, Faculty of Agriculture, Hokkaido University, Sapporo, Hokkaido, 060 Japan.
t Present address: Laboratory of Wild Animal Medicine, Nippon Veterinary and Zootechnical College, Musashino,Tokyo, 180 Japan.

Sci. Rep. Whales Res. Inst.,
No. 37, 1986, 173-178

174

HAYAMA, SUZUKI, UNO AND YAMASHITASEA OF OKHOTSK

44°N

HOKKAIDOPENINSULA
42°N
142°E

144°E

146°E

Fig. 1. Sampling was performed in Nemuro Peninsula.

MATERIALS AND METHODS

The field work was conducted at Nemuro Peninsula of eastern Hokkaido
from 25 September to 1 December in 1982 and 14 September to 1 Decemberin 1983. A total of 113 female Kuril seals and 29 fetuses used in this study
were obtained from those caught accidentally by the salmon trap nets (Fig. 1).
The ovaries were collected and preserved in 10% formalin. Using routine
procedures, ovaries were embedded in paraffin, and sections of 10 µm inthickness were taken at every 500 µm interval. These sections were deparaffinized
and stained with aldehyde-fuchsin and Masson's trichrome staining method
modified by Goldner (1938).
The number and size of corpora lutea and corpora albicantia wererecorded. The body length of fetuses was measured from the tip of snout tothe end of tail on straight line along the body axis in millirheter. The age wasestimated by counting cementum annuli in the canine tooth after being
decalcified, and stained with hematoxylin (Mansfield and Fisher, 1960; Bigg,1969; Naito and Nishiwaki, 1972).
De Master ( 1978) presents a technique to calculate the average age of sexual
maturity for marine mammals. The estimate of the average age of sexual
Sci. Rep. Whales Res. Inst.,
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SEXUAL MATURITY OF KURIL SEAL

175

maturity in female is as follows:
j(x) = t(x)ln(x),where x = age of female,f(x) = estimated probability of ovulating at or beforeage x, t(x) = number of females in sample of age x who have ovulated, andn(x) = number of females of age x in sample. With this definition of j(x), theestimated probability of first ovulating at age x is,P(x) = f(x) - j(x-1),
where P(x) = estimated probability of first ovulation at age x. The average ageof sexual maturity is then calculated as,w

x = x~o (x) P(x),where x =average age of sexual maturity and w =maximum age in sample.
In addition to estimating the average age of sexual maturity, this procedurecan be used to estimate the average age of first pregnancy. However, because
the probability of a female being pregnant may never reach unity, there exists
a positive probability that at age x a female will reproduce for the first time,even if the difference between the probability of being pregnant for two
consecutive age-classes is 0. Therefore, the estimate of the average age at firstpregnancy is,z(x) = y(x)/n(x),where z(x) = estimated probability of being pregnant at age x, y(x) = numberof females pregnant at age x in sample, and n(x) = number of females in sample.
For x ~ a, where a equals the age at which z(x) reaches its maximum,r(x) = z(x) - z(x-1),
where r(x) = estimated probability that a female is first pregnant at age x, andfor x >a,r(x) = z(a) (1-z(x))X-a
The average age at first pregnancy is calculated as before, except thatr(x) replaces P(x), and z(x) replaces j(x).
RESULTS AND DISCUSSION

The youngest mature (having experienced ovulation) and the oldest immatureappeared at the age of 2 and 4 years respectively (Table 1). Average age at firstovulation was 3.7 years. Average age at the first pregnancy was 4.6 years andthe apparent pregnancy rate of females at 6 years or older (n=22) was 90.9%.
In Table 2, the average age at sexual maturity of female harbour seal
(Phoca vitulina richardsi) was calculated using the data of Bigg (1969, British
Columbia), Pitcher (1977, Prince William Sound), and Pitcher and Calkins(1979, the Gulf of Alaska). Due to the limited sample size of the present
study, the difference between the two subspecies may not be significant.
The period of delayed implantation in harbour seals was estimatedpreviously as 11 weeks (Fisher, 1954), 2 to 3 months (Harrison, 1960), 2 months(Bigg, 1969), and 11 weeks (Pitcher and Calkins, 1979).
Sci. Rep. Whales Res. Inst.,
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176

HAYAMA, SUZUKI, UNO AND YAMASHITATABLE 1. NUMBER OF OVULATED (NON-PREGNANT) OR PREGNANT FEMALES
1982

Year

1983

Total no.
femalesAge

No. female
samplesOvulated

Pregnant

No. female
samplesOvulated

Pregnant

014II00295242000000234567890012lOl10+3TOTAL45l2312810320000003961368000042l10000004837131019844211212181134220000TABLE 2. AVERAGE AGES (YEAR) AT THE FIRST OVULATION AND PREGNANCY OF
HARBOUR SEALS COLLECTED IN BRITISH COLUMBIA (BIGG, 1969), PRINCE WILLIAM SOUND(PITCHER, 1977), THE GULF OF ALASKA (PITCHER AND CALKINS, 1979),
AND KURIL SEALS IN HOKKAIDOFirst ovulation
First pregnancy

HokkaidoBritishColumbia

Prince WilliamSound

Gulf of Alaska

3.7
4.6

3.3
3.3

3.7
4.4

5.0
5.6

The relationship between fetal length (Y in cm) and the date of catching(X, number of days after the lst of October) for the present 29 data is expressedby the following least squares regression (Fig. 2).
Y = 3.91 X + 28.04
r = 0.898
This regression cuts the axis of time at - 8 (22 September) when implantation
seems to occur. Mating takes place in June in this population (Naito andNishiwaki, 1972), and this was confirmed when 3 mature female kuril seals
collected between 24 September and 29 September had newly implanted embryos.
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

177

SEXUAL MATURITY OF KURIL SEAL

(MM)

3000(/)::::JILLJ

200

u..
u..

00(,.!)

z

LLJ

000_J

a:l

Y = 3, 91 X + 28, QL

0/~:cI->Q0:B/o010000-100<OCT 1)

1020304050<NOV 9)6070<DEC 9)

Fig. 2. The growth curve of prenatal Kuril seal.

ACKNOWLEDGEMENTS
Mr J. Shimazu, Shimazu Fur Co., Mr N. Kanazawa Fur Co. and Mr S. Taguchi,
Kyouei Fisheries Co. helped us in collecting specimens. DrsJ. Yamada and N.
Kitamura, Department of Veterinary Anatomy, Obihiro University ofAgriculture and Veterinary Medicine, gave us valuable advice. Mr N. Hachiya,Faculty of Department of Dentistry, Hokkaido University, helped us in age
determination. Dr A.A. Rayos, Department of Veterinary Obsterics andGynecology, Obihiro University of Agriculture and Veterinary Medicine,made careful revision to the manuscript. The members of the Marine Mammal
Research Group and the Kuril Seal Research Group gave us their help andkind advice. We wish to express our sincere gratitude to all these persons.
This work was primarily supported by Nippon Life Insurance Foundation.

Sci. Rep. Whales Res. Inst.,
No. 37, 1986

178

HAYAMA, SUZUKI, UNO AND YAMASHITAREFERENCES
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Sci. Rep. Whales Res. Inst.,
No. 37, 1986