182 



NORTH AMERICAN FAUNA 74 



Table 27. Age at first ovulation and age-specific fertility rate 

 of the Pacific walrus.^ 



Age class (years) 





4 



5 



6 



7 



8 



9 



10 



11 



Total 



No. ovulating 





















first time 



22 



68 



49 



43 



14 



8 



1 



0 



205 



Percent 





















ovulating 





















first time 



10.7 



33.2 



23.9 



21.0 



6.8 



3.9 



0.5 



0 



100 



Age-specific 





















fertility 





















rate (%) 



10.7 



43.9 



67.8 



88.8 



95.6 



99.5 



100 



100 





'^From J. J. Burns and F. H. Fay (unpublished data). No ovulation observed at age 3. 



up to 4 years old examined by J. J. Burns (personal communication) in May and 

 June had numerous small vesicular follicles; one 4-year-old specimen had one 

 ripening follicle 13 mm in diameter. Gol'tsev (1975) found that one of six 4-year- 

 olds taken in the Chukchi Sea in July to September was pregnant for the first 

 time. The oldest animal in its first pregnancy reported by Gol'tsev (1975) was 9 

 years old; the oldest at first ovulation reported by Krylov (1966^?) was 12 years 

 old. 



The distribution of ages at first ovulation in 205 specimens examined by J. J. 

 Burns (unpublished data) and me (Table 27) indicates that at least 10% of the 

 females ovulate for the first time at 4 years of age; about 68 % have ovulated at 

 least once by the time they are 6 years old, and 100 % are fertile by the age of 10 

 years. 



It should be understood that the data in Table 27 relate only to the ability to 

 ovulate. Ovulation does not necessarily mean that the animal also has the ability 

 to produce offspring, for first ovulations frequently do not result in successful 

 fertilization, implantation, gestation, or birth. Sexual maturation in female 

 walruses is a prolonged process. The animals are not sexually mature in the most 

 conservative sense until they have attained full physical development in their 9th 

 to 10th year, at which time they also attain the peak of their reproductive 

 performance, as will be described further in the following parts of this report. 



Annual Reproductive Cycle 



Testis Cycle 



An annual cycle of changing size of the testes probably begins in the Pacific 

 walrus in the fifth or sixth year after birth and intensifies thereafter until 

 adulthood (Table 28). Its course, in terms of histological events in the 

 seminiferous epithelium and in the ducts of the corpus epididymis, was studied in 

 117 specimens more than 4 years old. For the purpose of assigning a rank to each 

 of these, the principal cellular associations seen in the seminiferous tubules were 

 divided into eight general types. In sequence, beginning with the onset of 

 recrudescence and ending with retrogression, they were as follows (see Fig. 113): 



