716 



Fishery Bulletin 91(4). 1993 



as two months earlier for parous cows than for virgin 

 northern fur seals (Craig, 1964; Bigg, 1986); this dis- 

 parity is not reflected in birth timing (Trites, 1992). 

 Gentry (1981) noted, however, that individual parous 

 females have fairly rigid year-to-year estrus timing, 

 and Bigg (1984) has suggested that the breeding 

 synchrony found in this species may be due to arrival 

 on shore and on social factors. 



The harbor seal has been shown to have delayed 

 implantation (Fisher, 1954; Bigg, 1969b) and the Cali- 

 fornia and northern sea lions are thought to have this 

 reproductive pattern as well (Odell, 1975; Boshier, 1981; 

 Schusterman, 1981). Hence, a similar mechanism may 

 exist to signal implantation in these species. Temte 

 ( 1985) proposed that northern fur seals responded to a 

 photoperiod of 12.5 hours per day (prior to the autum- 

 nal equinox), thus explaining the positive slope in lati- 

 tudinal variation. Likewise, the Pacific harbor seals in 

 this study have a positive slope and may respond to 

 long (>12. Oh/day) photoperiods. In contrast, California 

 sea lions have a negative slope in latitudinal varia- 

 tion, which suggests a response to a photoperiod slightly 

 less than 12.0 hours per day (following the autumnal 

 equinox), and occurring prior to implantation. 



Acknowledgments 



I thank the many dedicated personnel of the zoos, 

 aquaria, and marine zoological parks for cooperation 

 in data collection and verification, and am most grate- 

 ful to Ann Terbush and Wanda Cain of the Office of 

 Protected Resources, National Marine Fisheries Ser- 

 vice, for providing Marine Mammal Inventory Reports 

 on these species. M. D. Sibley of the Auckland Zoo 

 made available birthing records from the Southern 

 Hemisphere. I also thank Dan Odell of Sea World, Ted 

 Garland of the University of Wisconsin, Department 

 of Zoology, and two anonymous reviewers who pro- 

 vided critical and highly valuable comments and sug- 

 gestions. Partial support was provided by Lutheran 

 Brotherhood through a Medical Research Fund M.D./ 

 Ph.D. Scholarship to the author. 



Literature cited 



Asper, E. D., L. H. Cornell, D. A. Duffield and N. Dimeo- 

 Ediger. 



1988. Marine mammals in zoos, aquaria, and marine 

 zoological parks in North America: 1983 census re- 

 port. Int. Zoo Yearb. 27:287-294. 

 Bigg, M. A. 



1969a. Clines in the pupping season of the harbour 

 seal, Phoca vitulina. J. Fish. Res. Board. Can. 

 26:449-455. 



1969b. The harbour seal in British Columbia. Bull. 

 Fish. Res. Board Can. No. 172:1-33. 



1973. Adaptations in the breeding of the harbour seal, 

 Phoca vitulina. J. Reprod. Fert. (Suppl.) 19:131-142. 



1984. Stimuli for parturition in northern fur seals 



(Callorhinus ursinus). J. Mammal. 65:333-336. 

 1986. Arrival of northern fur seals, Callorhinus ur- 

 sinus, on St. Paul Island, Alaska. Fish. Bull. 84:383- 

 394. 

 Boshier, D. P. 



1981. Structural changes in the corpus luteum and en- 

 dometrium of seals before implantation. J. Reprod. 

 Fert. (Suppl.) 29:143-149. 

 Boulva, J. 



1975. Temporal variation in birth period and charac- 

 teristics of newborn harbour seals. Rapp. P.-v. Reun. 

 Cons. int. Explor. Mer 169:405-408. 

 Boyd, I. L. 



1984. The relationship between body condition and the 

 timing of implantation in pregnant grey seals 

 (Halichoerus grypus). J. Zool., Lond. 203:112-123. 



1991. Environmental and physiological factors control- 

 ling the reproductive cycles of pinnipeds. Can. J. 

 Zool. 69:1135-1148. 

 Braham, H. W. 



1974. The California sea lion on islands off the coast 

 of San Luis Obispo County, California. Calif. Fish 

 Game 60:78-83. 



Bronson, F. H. 



1985. Mammalian reproduction: an ecological perspec- 

 tive. Biol. Reprod. 32:1-26. 



Brownell, R. L., R. L. Delong, and R. W. Schreiber. 



1974. Pinniped populations at Islas de Guadelupe, San 

 Benito, Cedros, and Natividad, Baja California, in 

 1968. J. Mammal. 55:469-472. 

 Coulson, J. C. 



1981. A study of the factors influencing the timing of 

 birth in the grey seal Halichoerus grypus. J. Zool., 

 Lond. 194:553-571. 

 Craig, A. M. 



1964. Histology of reproduction and the oestrus cycle 

 in the female fur seal, Callorhinus ursinus. J. Fish. 

 Res. Board. Can. 21:773-818. 

 Daniel, J. C. Jr. 



1981. Delayed implantation in the northern fur seal 

 {Callorhinus ursinus). J. Reprod. Fert. (Suppl.) 

 29:35-50. 

 Delong, R. L., W. G. Gilmartin and J. G. Simpson. 



1973. Premature births in California sea lions: asso- 

 ciation with high organochlorine pollutant residue lev- 

 els. Science 181:1168-1170. 

 Fisher, H. D. 



1954. Delayed implantation in the harbour seal, Phoca 

 vitulina L. Nature 173:879-880. 

 Gentry, R. L. 



1981. Northern fur seal Callorhinus ursinus (Linnaeus, 

 1758). In S. H. Ridgway, and R. J. Harrison (eds.), 

 Handbook of marine mammals. Volume I: The wal- 

 rus, sea lions, fur seals and sea otter, p. 143-160. 

 Academic Press, London. 



