Temte: Birth timing of captive Zalophus cslifornianus 



715 



have been suggested as agents of premature pupping 

 in wild populations (Delong et al., 1973; Odell, 1970, 

 1981), the high rate of stillbirth and non-viability in 

 captivity may be influenced by the level of surveil- 

 lance inherent to captive populations. 



Male California sea lions were reported to be 3.37 

 cm longer and 1.41kg heavier than female pups at 

 birth (Le Boeuf et al., 1983). Year-to-year differences 

 in mean birth lengths and weights have been related 

 to differences in the mean birth date of harbor seals 

 (Boulva, 1975). No such differences were found be- 

 tween the birth timing of male and female pups in the 

 present study. Hence, discrepancy in birth size is likely 

 due to greater growth velocity of fetal males. 



Northern sea lion 



Only limited data from two locations were available 

 for captive northern sea lions. Although these animals 

 appeared to have a temporal pattern of birth similar 

 to, but 30 days later than, that of the closely related 

 California sea lion, no significant latitudinal trend was 

 found. 



The northern sea lion has a wide distribution of 

 breeding (see Fig. 1 in Loughlin et al., 1984), but little 

 evidence exists for latitudinal variation in birth tim- 

 ing in the wild. For example, a review of median birth 

 dates at rookeries from 37°N to 60°N by Merrick ( 1987) 

 failed to demonstrate any latitudinal variation in this 

 species. 



Northern fur seal 



A high rate of stillbirth was noted for the fur seal. 

 This phenomenon was previously reported by Bigg 

 ( 1984) during studies indicating that contact with sub- 

 strate (e.g., arrival on shore) may stimulate parturi- 

 tion. Captive environments, with shallow pools and 

 access to platforms, cannot adequately reproduce the 

 pelagic environment which female fur seals inhabit 

 during most of active gestation. Hence, premature par- 

 turition may be induced by enclosures. 



The mean date of pupping for viable pups in captiv- 

 ity of 10 July was the same as that derived for St. 

 George Island, Alaska (Temte, 1985). Unlike wild north- 

 ern fur seals, however, the captives demonstrated no 

 significant latitudinal variation. The sample set 

 may have been too small to detect the estimated 0.6 

 days/iatitude trend reported by Temte ( 1985). 



Pacific harbor seal 



Captive harbor seals had a high rate of viable birthing 

 with pups born over a pupping season comparable to 

 that of wild counterparts (Temte et al., 1991). This 



species demonstrated stronger latitudinal effect than 

 that of any other North Pacific pinniped covered in 

 this report, with an average shift of 4.1 days/°latitude. 

 Whereas this relationship is similar to that previously 

 described for wild Pacific harbor seals on the North 

 American west coast south of 47°N, the larger captive 

 data set allows better definition of a curvilinear 

 relationship. 



Temte et al. ( 1991) separated the Pacific harbor seal 

 into three subgroups based on birth timing. The north- 

 ernmost group, from northern British Columbia and 

 Alaska has no latitudinal variation in birth timing. 

 The late-birthing group inhabits Puget Sound, Wash- 

 ington, and pups two months later than coastal seals 

 at the same latitudes. The southern group, as noted 

 above, has highly significant latitudinal variation. Mor- 

 phometric analysis of skulls from each of these popu- 

 lations supports the hypothesis of discrete populations 

 (Temte, unpubl. data). No individuals from populations 

 originating in Puget Sound, northern British Colum- 

 bia or Alaska were included in this study 



The sex of the pup, maternal age, and the previous 

 maternal cycle (pregnant vs. non-pregnant) had no ef- 

 fect on birth timing. These findings are from captive 

 animals with dependable food supplies. In contrast, 

 Boyd ( 1984) has suggested that maternal condition af- 

 fects implantation timing in grey seals. 



Latitudinal variation and photoperiod 



For captive California sea lions, a smooth and con- 

 tinual temporal change of pupping dates across lati- 

 tude occurred despite wide diversity of climatic condi- 

 tions. Nevertheless, as almost all the captives could be 

 traced to a single wild population on the California 

 Channel Islands, a strong environmental component 

 appears to influence birth timing. This pattern is highly 

 suggestive of a response to a predictable, latitudinally 

 dependent seasonal cue. The 6-month shift in birth 

 timing between hemispheres, as demonstrated by Cali- 

 fornia sea lions in New Zealand, strongly supports a 

 photoperiod hypothesis. Furthermore, the decreasing 

 variance to the north (in the Northern Hemisphere) is 

 as expected if photoperiodism occurs (Bronson, 1985). 

 At higher latitudes, organisms experience a greater 

 rate of change in photoperiod. Consequently, responses 

 to specific cues should occur over a compressed time 

 period. 



Latitudinal variation in birth timing of the northern 

 fur seal, a species with delayed implantation (Daniel, 

 1981), can be explained by a response to photoperiod 

 occurring between ovulation and implantation (Temte, 

 1985). This sets the time of implantation and birth 

 while allowing flexibility in estrus timing. For example, 

 arrival at breeding grounds and mating occur as much 



