Temte: Birth timing of captive Zalophus cahfomianus 



713 



California Sea Lion 



- -o-  



Latitude (°N) 



Figure 4 



The birth timing of viable California sea lions as a function of 

 latitude. See Table 2 for regression model. 



Northern sea lion 



The captive births of nine northern sea lions occurred 

 at 32°45'N and 41°21'N (latitudinal range=8°36). All 

 births were of viable pups. The mean date of birth was 

 9 July ± 16.5 days (± SD; Table 1). Although the plot of 

 pupping date vs. latitude was similar to that for the 

 California sea lion, a simple linear regression model 

 (Table 2) failed to demonstrate significant latitudinal 

 effects in this species (/- 2 =0.271; F ll7i =2.60; P=0.23); 

 the lack of significance was possibly due to the small 

 sample size. However, when northern sea lion data 

 were combined with those of the California sea lion in 

 a multiple linear regression model, no significant dif- 

 ferences were found between the latitudinal slopes of 

 the two species. Analysis of covariance identified sig- 

 nificantly later birthing (P<0.001) for northern sea 

 lions, occurring 30 days after California sea lions. 



the range of data. Weighting of the regression to cor- 

 rect for the unequal variances did not appreciably 

 change the parameter estimates. 



Eleven pups were born at the Auckland Zoo 

 (Auckland, New Zealand: 37° S) from 28 November to 

 23 December and had a mean birth date of 11 Decem- 

 ber. The polynomial model predicted that birthing at 

 37°N should occur on 6 June. Therefore, California 

 sea lions translocated to the Southern Hemisphere ex- 

 perienced an approximate 6-month shift in birth 

 timing. 



Climatic conditions did not appear to significantly 

 affect the timing of birth. For example, colonies at 

 similar latitudes (Vallejo, California: 37°47'N and Kan- 

 sas City, Missouri: 39°07'N) maintained similar pup- 

 ping schedules despite having vastly different seasonal 

 temperature cycles. Following an adjustment for lati- 

 tude, no significant difference was found between the 

 birth timing of male and female pups. 



Northern fur seal 



The captive births of 27 northern fur seals were re- 

 ported from four locations between 32°45'N and 49'07'N 

 (latitudinal range=16°22'l. Slightly more than half of 

 the pups were stillborn (Table 1). The mean date of 

 birth of viable pups was 10 July ± 14.8 days (± SD). No 

 significant latitudinal variation was detected in birth 

 timing (Table 2: r~=0.174; F a „|=2.32; NS). 



Pacific harbor seal 



The birth dates of 127 captive-born Pacific harbor seals 

 were identified from 14 locations between 32°45'N and 

 49°18'N (latitudinal range=16°33'). The birth outcomes 

 of two pups were not known and they were excluded 

 from further analysis. Of the remaining 125 births, 

 88.0^ were of viable pups (Table 1). The mean date of 

 birth for viable pups was 20 May ± 36.7 days (± SD). 

 No significant differences in the annual birth timing 



