ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



207 



Fig. 124. Time intervals between the two most recent ovulations in Pacific walruses 

 in relation to age. Each point represents one specimen. Dashed line connects the means 

 for each age class at latest ovulation. Solid line is fitted curve (least squares) of means for 

 age classes 6 to 20. (Based on specimens provided by J. W. Brooks, J. J. Burns, K. W. 

 Kenyon, and on the author's data) 



Krylov (1966^?) found no evidence of recent reproductive activity in any of 18 

 specimens more than 23 years old that were taken nonselectively. The principal 

 effect of selection in my sample probably was deflection of the mean ovulation 

 interval strongly downward in old age, toward 2 years. My findings in the age 

 classes from about 6 to 20 years were comparable to Krylov's (1962) and 

 Gol'tsev's (1975); hence, I assume that these are representative of those age 

 classes. On that basis, I fitted a cur\'e to the data from age classes 6 to 20 by the 

 least-squares method and projected it into the older ages, assuming that this 

 would provide a better estimate of the actual means for those older animals than 

 did the data from the sample. 



If the fitted curv^e (Fig. 124, solid line) value for each age class is taken as the 

 estimated mean interval between ovulations for females in that class, then the re- 

 ciprocal of that value is the estimated rate of ovulation for females in that class 

 which have ovulated at least once before. For example, the curve indicates a 

 mean ovulation inter\'al for females in their ninth year of life as 1 .73 years; hence 

 the probability that a given female which ovulated previously will ovulate again 

 in that year is 58 % . Because 99.5 % of females at age 9 had ovulated at least once 

 before (Table 27), the number ovulating for the second (or more) time in age 

 class 9 will be 554 per 1,000. The sum of that value and the number ovulating for 

 the first time in that age class (39 per 1,000) is the estimated mean ovulation rate 

 for all females in age class 9 (Table 34) . 



Although the probability of annual ovulation in walruses would seem to be 

 low because of the long duration of pregnancy, the foregoing data indicate that 



