ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



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Krylov (1967), who observed that the seminiferous tubules in most 7-year-olds 

 taken in spring had an open lumen, and in one of four, sperm were present. He 

 found spermatozoa in four of six 8-year-olds, in 11 of 13 9-year-olds, as well as in 

 all older animals up to 22 years, taken in spring. 



Primarily on the basis of body and tusk size, earlier Soviet investigators esti- 

 mated that male walruses reach "sexual maturity" at 5 or 6 years (Chapskii 1936; 

 Belopol'skii 1939; Freiman 1941). With fewer specimens but more accurate 

 methods. Brooks (1954), Mansfield (1958(3), and Krylov (1967) judged that males 

 are not potent at 5 years but become potent a year or two later. My specimens 

 suggest that incomplete spermiogenesis may begin as early as the fifth and sixth 

 years, but complete spermiogenetic cycles ending in production of spermatozoa 

 seem not to occur generally until the animals are at least 7 or 8 years old. 

 Probably all males are potent by the time they are 10 years old. 



Although the ability to produce spermatozoa is suggestive of potential ability 

 to fertilize ova, the better test of fertility is the presence or absence of sperm in 

 the epididymis. Since spermatozoa occur less often in the corpus epididymis than 

 in the caput or cauda (Ortavant 1959), their presence in the mid-body of the 

 epididymis should be a sensitive index of their abundance, hence probably also of 

 fertility. In my sample, none of the potent animals up to 10 years old had any 

 spermatozoa in the corpus epididymis; the youngest animal in which they were 

 present was 10.8 years. Spermatozoa were present in nearly all of the older, 

 potent specimens taken in winter. Krylov (1967) found some sperm in the epi- 

 didymis of one 7-year-old and in most of the 8-, 9-, and 10-year-olds, as well. 

 Regrettably, he did not indicate in which part of the epididymis he found them 

 or exactly how he did so. 



The findings that male Pacific walruses begin to produce spermatozoa when 

 they are 7 or 8 years old but do not become capable of fertilizing females until 

 they are about 10 years old is in agreement with the reproductive performance of 

 Pacific walruses in captivity. A male reared at Marineland of the Pacific first 

 showed "definite enlargement" of the testes in March and April of its eighth year 

 (J. D. Prescott, personal communication); two others, reared in the same facility 

 with females of the same age, sired their first calves in their 10th year (E. D. 

 Asper, personal communication). In free-living walruses, there is secondary 

 acceleration of growth of the testes, baculum, and body size beginning about the 

 8th to 10th year, as well as a change in the rate of tusk growth and in the 

 development of the thick "bosses" in the skin of the neck and shoulders. Full 

 sexual maturity of the free-living males is probably not attained until they are 

 about 15 years old, when they have essentially completed the secondary growth 

 of body, baculum, and testes, and they are sufficiently large and powerful to 

 compete successfully with the other adult bulls for mates. 



Females 



Krylov (1966/?) reported that eight females up to 4 years old, taken in July to 

 September, had no macroscopic follicles in their ovaries, and that the horns of 

 their uteri were slender and smooth, indicating that they had not been pregnant. 

 I examined 10 specimens 1 to 4 years old, taken in May and June, and found that 

 their uteri also were slender and smooth (1 to 1.5 cm in diameter), but that each 

 had a few vesicular follicles 1 to 6 mm in diameter in both ovaries. Four others 



