204 



NORTH AMERICAN FAUNA 74 



where it swam about nearby, barking incessantly for some minutes thereafter. In 

 those instances, the calf often was lured within the grasp of the hunters, 

 apparently in response to imitation of the adults' barking. Orphaned calves 

 sometimes are "rescued" by other walruses, who grasp them with their fore- 

 flippers and carry them away (Bruce and Clarke 1899; Bel'kovich and Yablokov 

 1961). I witnessed three such "rescues" and was told of two others by reputable 

 observers. In three instances, the calf was carried off by a subadult or adult 

 male; in two others it was taken by an adult female. The probability of survival 

 of those calves is questionable, but there is some evidence of fosterage. J.J. Bums 

 (unpublished data) and I observed seven definite instances of calves that were at 

 least temporarily "adopted" by other cows; the St. Lawrence Islanders (personal 

 communication) and Eley (1978) reported two others. Burns' records were of 

 three barren cows and one newly impregnated cow, none of which had given 

 birth less than 1 }'ear previously but each of which was accompanied by a young 

 calf, no more than a few days or weeks old. My records were of one cow 

 containing a dead, partly autolyzed fetus but accompanied by a living calf, 

 obviously of other parentage, and two in which the uterus had just begun to 

 involute from parturition not more than 48 hours previously but were 

 accompanied by calves that were certainly several weeks old. The Islanders 

 report was of another cow with dead fetus and accompanied by a living calf. 

 Eley's record was of a cow with a normal fetus and new calf, not her own. 

 Unfortunately, neither Bums, Eley, nor I determined whether any of these 

 adopted calves were being suckled by their foster parents. At least, none could 

 have been considered "starvelings," inasmuch as they were not weakened or 

 deficient in blubber relative to other, normal calves taken at the same time. 



Postpartum Involution of Uterus and Corpus Luteum 



The uterine horn of pregnancy rapidly diminishes in size, probably reaching 

 its normal progestational length within about 2 months after birth (cf . Gier and 

 Marion 1967). Apparendy, a low amplitude, annual cycle of uterine size is 

 established thereafter, even in the absence of further pregnancies (Fig. 122). 



The corpus luteum undergoes a significant reduction in volume about the time 

 of parturition. In specimens taken 2 months after birth, it was no more than half 

 as large as the corpus luteum of pregnane}' (Fig. 123) and was made up mostly of 

 fibrous tissue; lutein cells were scarcely evident. The corpora albicantia of 

 females about 7 months postpartum seemed to be entirely of grayish connective 

 tissue, tv'pically having a stellate organization. The stellate structure was 

 apparent in some instances up to 2 years after parturition. Corpora albicantia 

 3 years old and older were more consolidated and whitish, with no apparent 

 stellate organization. 



Evidently the rate of diminution in size of the corpora albicantia from 

 pregnancies varies widely. Some declined to less than 10 mm in diameter within 

 2 years, but others were nearly 20 mm in diameter after 3 years (Fig. 123). 



Reproductive Performance 



Rate of Ovulation 



The age-specific fertility rate of 205 individuals in which the age at first 



