BIGG: ARRIVAL OF NORTHERN FUR SEALS 



tion was noted in the seasonal pattern of arrival for 

 4-yr-old males, ranging from the typical pattern seen 

 in 3-yr-olds to that seen in 5-yr-olds. Such dif- 

 ferences in the arrival pattern may indicate that the 

 age at which males reach sexual maturity differs 

 between cohorts, a possibility worth further investi- 

 gation. Variations in the age at sexual maturity 

 could result from annual variations in body growth 

 rate caused in turn by fluctuations in food supply. 

 In the second case, pregnant females at >A yr may 

 have arrived slightly earlier with increasing age. 

 This would take place if the first conception resulted 

 in a later date of parturition than in subsequent 

 years. This is a possibility because, according to 

 Craig (1964), the first ovulation appears to be later 

 than subsequent ovulations. The age of primiparous 

 females spans mainly between 4 and 10 yr (York 

 1983), and thus the age at first ovulations presum- 

 ably also spans a similar number of years. Arrival 

 times would tend to be slightly earlier with age from 

 the increased proportion of mature females. 



An alternate explanation for seals arriving in pro- 

 gressively larger numbers, may lie in the energetic 

 costs of the return migration from the North Pacific 

 Ocean to the Bering Sea. For yearlings, the 

 energetic costs may be too large for all but a few 

 individuals to return. With age, the relative costs 

 may be more favorable and permit an increased pro- 

 portion to return. 



For each age, males tended to arrive before 

 females. This situation could result if, through selec- 

 tion or learning, the time of the return migration 

 was ultimately established for each sex by the adults. 

 The mechanism controlling the timing of migration 

 in young seals would gradually shift arrival times 

 with age to eventually synchronize with those of the 

 adults. However, because the arrival times of adult 

 males was earlier than that of cows, the arrival times 

 of immature males would also be before those of im- 

 mature females. The fact that nonpregnant adult 

 females arrived after parous females could be the 

 result of nonpregnant females gaining some advan- 

 tage in the energetic costs of migration. Since pre- 

 sumably competition exists for food around the 

 Pribilof Islands during the summer, perhaps survival 

 of nonpregnant adult females is enhanced by feed- 

 ing elsewhere, thus delaying the return migration 

 by 1 mo. 



ACKNOWLEDGMENTS 



I am grateful to P. Olesiuk for preparing the 

 Probit plots, and I. Fawcett for collating much of 

 the data on kills and pregnancy rates. I thank P. 



Olesiuk, T. Quinn, and two journal reviewers for 

 useful comments on the manuscript. 



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