ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



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some ovulations occur in successive years. The questions, "How do they occur?" 

 and "Of what significance are they in terms of productivity?" have several pos- 

 sible answers. In my sample, annual ovulations occurred most often where the 

 first shed ovum failed to result in a conceptus (i.e., either was not fertilized or 

 the blastocyst failed to implant), or the fetus was rejected (aborted) early in 

 pregnancy. In each instance, the animal had ovulated again in the following 

 year, but in none had this resulted in two successful pregnancies. Thus, the 

 ability to ovulate in successive years appears to be usually the result of unsuccess- 

 ful fertilization, implantation, or gestation in the first year. 



Burns (1965) reported one instance in which ovulation in successive years 

 apparentiy resulted in two successful pregnancies. In that instance, the calf of the 

 first pregnancy apparently was born unusually early (probably in February), 

 and the female was impregnated again during her postpartum estrus, probably 

 in April. That this was unusual should be evident from the fact that birth usually 

 takes place from late April to early June, and the postpartum estrus usually 

 occurs in July or August, well after the period of fertility of most adult males. 

 This unusual observation was the only one of its kind among 204 specimens for 

 which Burns and I had obtained sufficient diagnostic data for comparison. 



Fecundity 



The reproductive efficiency of the female depends not only on the frequency 

 and proper timing of estrus but also on the success in fertilization, implantation, 

 and gestation. The female that achieves each of those steps and successfully pro- 

 duces a calf is considered here as being fecund. Hence, fecundity is the rate of 

 production of calves. 



Several investigators, notably Chapskii (1936), Burns (1965), and Gol'tsev 

 (1975), attempted to assess the fecundity of walruses by counting the numbers of 

 corpora lutea and corpora albicantia in the ovaries, assuming that each of those 

 bodies could be equated with one pregnancy completed. The development of a 

 corpus luteum or corpus albicans, however, is not necessarily indicative of 

 production of a calf. For various reasons, the ovum may not be fertilized, the 

 blastocyst may not implant, or the conceptus may be rejected, resorbed, or born 

 dead (Hafez 1967). 



Most earlier investigators did not clearly distinguish in walruses between 

 corpora of pregnancy and those formed from unfertilized or even unruptured 

 (atretic) follicles. Occurrence of reproductive failure was nearly ignored, 

 although Krylov (1966fc) recognized that all ovulations do not automatically 

 result in pregnancy and success in gestation. Gol'tsev (1975) observed that some 

 pathological conditions in the uterus and ovaries occasionally result in sterility. 



In each instance of reproductive failure or success, a corpus albicans will be 

 formed. Retrogression of corpora lutea of pseudopregnancy, as well as of corpora 

 accessoria and corpora atretica also will result in formation of a corpus albicans. 

 Although some of these corpora may tend to be smaller and for that reason less 

 persistendy visible than the corpora albicantia from pregnancies, those that do 

 persist cannot be identified with certainty by visual inspection. 



For these reasons, the numbers of corpora in the ovaries of walruses are not 

 reliable indices of fecundity, although they should tend to approximate the 

 number of ovulations in an individual's recent reproductive history. Apparently, 



