200 



NORTH AMERICAN FAUNA 74 



15 20 25 30 5 10 15 20 25 30 5 10 15 20 25 30 5 10 

 March April May June 



Fig. 120. Frequency of occurrence of near-term fetuses (upper), newborn calves less 

 than 48 h old (center), and older calves of uncertain age (lower) during mid-March to 

 mid-June. (From Brooks 1954, Krylov 1969, and J. J. Burns, F. H. Fay, K. W. Kenyon, 

 and R. A. Ryder, unpublished data) 



22 May. Krylov (1969) recorded six full-term fetuses between 8 April and 

 24 May and obtained the first newborn calves on 5 May. 



During an aerial survey of the Pacific walrus population, Kenyon (1972) 

 sighted and photographed a female with a calf, apparently just born (judging 

 from blood on the floe), on 15 April. R. A. Ryder (personal communication) 

 recorded a fresh ("steaming") placenta among a group of females and calves on 

 the ice near St. Lawrence Island on 2 May and observed the birth of a calf on 

 21 May in the Chukchi Sea. Near St. Lawrence Island on 12 May, I examined a 

 cow with new calf, together with the still-warm placenta and fetal membranes. 



A graphic summary of all available data relating to time of birth is presented 

 in Fig. 120. These findings indicate that (1) near- and full-term fetuses occur 

 commonly as late as the last 10 days of May and occasionally in the first 10 days 

 of June, (2) newborn calves, certainly less than 48 h old, occur commonly during 

 late April and May, with the earliest record being 15 April, and (3) other, older 

 calves have been recorded in abundance throughout May and June. The 

 duration of the birth period implied by these data is about 55 days, from 

 15 April to at least 8 June, with a probable mode between 30 April and 25 May. 

 From this, I estimate that the mean birth date is about 12 May. 



Although most births take place from mid- April to mid-June, a few apparently 

 occur at other times during the year. Freuchen (1935) reported two "newly born" 

 calves taken in Frozen Strait, Foxe Basin, Canada, on 31 January, and Burns 

 (1965) obtained a calf in Bering Strait in May that he judged had been born in 

 January or February. I examined one calf in May that was unusually large and 

 had a fully healed umbilicus, comparable in all respects to those taken in July by 

 Nikulin (1941), hence I assumed that it might have been born as early as March. 

 During an aerial survey flight on 1 March, K. W. Kenyon (1960g) sighted a 

 small animal, possibly a newborn calf, in the midst of a group of cows on a 



