
The difference between pregnancy rates of Pribilof and Asian 
fur seals is striking in age 4, the earliest year of puberty: Pribilof, 
7 percent (in 101 specimens); Asian, 27 percent (in 288 specimens). 
Furthermore, the pregnancy rate is lower in Pribilof females of all 
other age classes. A breakdown by age classes is available only for 
Callorhinus in the four groups above. The extent to which the samples 
represent an entire breeding population is open to question, and further 
investigation is planned. 
We conclude that the present ratio of cows to bulls on the 
Pribilofs has increased beyond the point of maximum reproductive success. 
As a gregarious animal, the fur seal would be expected to follow ina 
general way the breeding behavior of other gregarious species. Of those 
that have been studied, for example, gulls, red deer, and Atlantic seals, 
it is said "reduction of gregariousness, and therefore of males, to a 
low level results in much less thorough overt, pre-coital sexual 
behavior, and females are apt to be left barren. Indeed, in some species 
of animals there is a threshold of numbers below which breeding will not 
take place" (Darling, 1951, p. 250). The Pribilof herd is the only group 
of seals studied with respect to pregnancy from which a large and consis- 
tent portion of the male life is removed annually. Mortality among males 
up to age 5 from natural and artificial causes combined is 98.6 percent. 
Therefore, significance may be attached to the low rate of preg- 
nancy here as compared to that among Asian fur seals and Atlantic harp 
seals. The reproductive success on the Pribilofs would presumably rise 
if the number of adult males were allowed to increase or if a portion of 
the female population were removed through commercial killing. The decrease 
in the number of pups born because of the decrease in the breeding stock 
should be compensated for by the increase of reproductive success and 
the decrease in pup mortality, provided the killing of cows were carried 
on within conservative and reasonable limits. Perhaps an annual kill of 
cows should be considered. Since the reservoir of physically mature 
males in the Pribilof herd appears to be at a relatively low level, the 
male breeding capacity may be substantially spent during the early and 
niddle parts of the breeding season. This would explain why most young 
females, habitually arriving on the breeding islands late in the season, 
are neglected. Even among older cows the optimum breeding potential is 
not realized. Hanna (appendix B) was unable to find marked 7~year-old 
males on the breeding ground. Today, 7-year-olds are observed holding 
harems. The fact that a half-grown bull is allowed on the breeding 
ground today and was not in the twenties may be additional evidence 
that the population of fully mature bulls is not sufficiently large to 
serve the present cow population. 
It appears that today the number of bulls is the determining 
factor for the number of pups born each year on the Pribilofs. If the 
killing of cows should be undertaken on a modest scale and under careful 
control, the following three results might eventually be expected: (1) 
A rise in the pregnancy rate, especially among young cows. (2) A 
reduction in the overcrowding that now exists (and is known to exist in 
‘other wild-mammal populations that have reached ceiling level). The 
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