THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 245 



45, while the average of the three seasons, if January, February, March, and 

 April are pooled into one value, is 50. 



If the length of the gestation period is about 13 months, the first average 

 above, i.e., 45 percent, would imply an 11-month period for lactation and rest- 

 ing and a 2-year total cycle. The other average, 50 percent, would imply a 

 13-month lactation and resting period or a total 26-month cycle. This seems 

 less probable, though it could be the average of a breeding group in which 

 some proportion of the animals miss being impregnated at the first oppor- 

 tunity after the end of lactation and thus lengthen the average cycle. It is 

 also possible that the estimates of the length of the gestation period are high 

 due to sampling errors and that both gestation and lactation plus resting 

 periods are 12 months in length. 



It thus seems that the best estimates available at present are as follows: 



Lactation plus resting 11-12 months. 



Unimplanted period 7-8 months. 



Implanted fetus 4^ to 5V2 months. 



Gestation period 12-13 months. 



Total length of reproductive cycle 2 years. 



Average birth rate mature female 0.50. 



IMPLICATION OF OTHER OBSERVATIONS 



Recruitment and mortality rate: The observations on the proportion of 

 primiparous animals among pregnant animals (17 percent, table 48) taken 

 together with the 2-year cycle indicates that the recruitment rate among 

 mature females is 0.085 per year. If the mature female herd is nearly stable 

 in size, this will also be the mortality rate of this component of the popula- 

 tion. This is relatively low but only slightly lower than the corresponding 

 rate among fur seals. 



Proportion of juveniles and length of juvenile stage: If 50 percent of the 

 mature females give birth each year and nurse the young animal for nearly 

 a year (say 10 months), this implies the ratio of dependent young to mature 

 females is V2 X W12 or 0.42. Extensive field observations (table 52, cf. also 

 discussion) show the ratio of dependent young to independent females is 



1670-"-^^- 



The independent females include both mature and juvenile animals. If it is 

 assumed that the survival rates of both of these groups is 1-0.085 (=.915) 

 after the initial transition from dependence to independence, then a com- 

 parison of the two ratios given above shed light on both the proportion of 

 juveniles in the herd and the length of the juvenile stage. 

 For denote by 



Y=dependent young (of both sexes) 

 J= juveniles (females) 

 A=adults (females) 

 then the above two ratios are: 

 Y 



^=0.42 



