240 



NORTH AMERICAN FAUNA 68 



in an air breathing mammal born under water (Essapian, 1953). 

 Slijper (1956), however, does not subscribe to this theory. 



If caudal presentation is a survival factor in aquatic birth and 

 it were the rule in the sea otter, it might be supposed that birth 

 would occur in the water. Among 43 fetuses for which fetal 

 orientation were recorded, delivery would have been cephalic in 

 22 (51 percent) and caudal in 21 (49 percent). These data indicate 

 that selective evolution for caudal presentation at birth, and by 

 inference for birth in water, has not taken place in the sea otter. 

 By comparison, the fur seal is more specialized for life in the 

 marine environment than is the sea otter but emerges from the 

 water before parturition. Peterson (1965, p. 136) recorded that 

 in 61 percent of 112 observed births the young emerged head 

 foremost. It is thus indicated that birth in the sea otter, as in 

 the fur seal normally occurs on land. 



Period between births 



Figure 91 and table 47 demonstrate that during the winter and 

 spring when the maximum number of sea otters are pregnant, 

 one-third of the tracts in our sample were nonpregnant. I strongly 

 suspect that all reproductive tract samples are biased toward a 

 low frequency of nonpregnant animals. This assumption is based 

 on three observations: (1) that no female known definitely to be 

 accompanied by a pup was found to be pregnant, (2) that females 

 with young are more wary and secretive than females without 

 young and are thus less available to hunters, and (3) that during 

 hunting operations I noted a degree of reluctance in hunters to 

 shoot a female accompanied by a pup if animals without pups were 

 present. Thus, an unknown degree of bias in favor of single ani- 

 mals, i.e., a high percentage of pregnant animals, is included in 

 all samples. The July to August 1963 sample is particularly biased, 

 since single animals only were selectively killed. At this season the 

 number of mothers accompanied by pups is approaching the annual 

 maximum (see table 52). Thus, the percentage of nonpregnant 

 animals in the adult female population would also approach maxi- 

 mum numbers at this season. This is shown by the sample (fig. 91), 

 but it would be much more apparent had the sample been 

 unselective. 



Because the summer sample of reproductive tracts was taken 

 selectively to exclude mothers accompanied by pups it is, in some 

 respects, not comparable with the spring and fall samples. Never- 

 theless, the following conclusions concerning nonpregnant female 

 sea otters in our samples are indicated. (1) Females accompanied 



