206 



NORTH AMERICAN FAUNA 74 



ovulation could be determined with certainty, was shown in Table 27. A fertile 

 female is one that is capable of ovulation. Fertility of a given female was 

 diagnosed by the finding in its ovaries of at least one ripe follicle, a corpus 

 luteum, or a corpus albicans. The age at which ovulation first took place, in 

 those with a corpus luteum or corpus albicans, was determined from the size and 

 condition of the corpus and of the placental scar or fetus in the adjacent uterine 

 horn. The resultant data indicate that about 10 % of the females were capable of 

 ovulation in their fourth year, two-thirds by the sixth year, and practicalK" all by 

 the eighth or ninth year. 



In a sample of 23 females, first o\'ulations occurred 11 times from the right 

 ovary and 12 times from the left. Second, third, and later ovulations in 52 

 females occurred 40 times from the right ovar\- and 37 times from the left. 

 Successive ovulations occurred more often (39 times) from alternate ovaries than 

 from the same one (15 times) (X^ ^ = 5.6, P <0.02), indicating that follicular 

 de\ elopment usually is suppressed for a year or more in the o\'ary that contained 

 the corpus luteum of pregnancy. 



Whereas most pregnancies are spaced at least 2 years apart, Popov (1960a), 

 Krylov (1962), and Gol'tsev (1975) indicated that 4 to o^c of 474 females that 

 they examined in late summer were nursing a new calf and were pregnant as 

 well. From those findings the>' concluded that some animals become pregnant in 

 successive years, but they failed to present comincing evidence of each female's 

 actually having given birth to the calf. Apparentiy, their conclusion was based 

 onh' on the presence of the calf and fetus and of milk in the female's mammar>' 

 glands. As noted earlier, J. J. Burns and I found that 7 of 144 females (4.9%) 

 accompanied by cah'es in May and June were with foster mothers. One of those 

 foster parents also was newly pregnant. Of the other 137 animals in this series, 

 1 was newly pregnant and accompanied by its own calf; in that instance (Bums 

 1965), however, the calf had been born unusually early, and the female 

 apparently had been able to ovulate in her postpartum estrus, before the end of 

 the males' period of fertility. Ordinarily, the postpartum estrus occurs "too late" 

 for fertilization, since it is 3 to 5 months after the fertilit\- period of adult males 

 has ended. 



Because the duration of pregnancy in walruses ordinarily is longer than 

 1 year, the probability that a healthy, fecund individual will ovulate annually 

 and be fertilized appears to be extremely low\ The individual's usual inter\'al 

 between o\'ulations and conceptions should be about 2 > ears. The work of 

 Krylov (1962). Burns (1965). and Gol'tsev (1975) has confirmed that prediction, 

 as have my findings. In 160 fertile females whose recent reproducti\'e histor\' I 

 was able to trace with certaint>-, the latest tw^o ovulations had taken place once 

 (0.6%) in the same year, 29 times (18%) 1 year apart, 100 times (62%) 2 years 

 apart. 25 times (16%) 3 years apart, and 5 times (3%) more than 3 years apart. 

 In general, there was a positi\'e correlation between the age of the animal and 

 the length of the inter\-al between the latest two ovulations (Fig. 124). 



Since the sample represented in Fig. 124 was drawn from the St. Lawrence 

 Islanders' selective harvest of mainly cows with calves, I assume that it is not 

 representati\'e of the population as a whole but probably is representative of the 

 most producti\'e age classes. That it is biased mainly in the oldest classes is 

 indicated by the fact that half of my specimens o\-er 23 years of age had recently 

 given birth to a calf and showed signs of regular biennial pregnancies, whereas 



