194 



NORTH AMERICAN FAUNA 74 



Each display lasts for 2 to 3 min, and consists of a subsurface portion, in which 

 series of pulses ("clicks" or "knocks") and bell-like sounds are made (Ray and 

 Watkins 1975), and a surface portion, in which the bull raises his head above the 

 water and emits a series of sharp "clacks" (apparently with his teeth) and a short, 

 harsh whisde. Each adult bull in the water adjacent to the herd of females 

 performs these displays continuously, for as long as the females remain at rest. 

 Presumably, the displays serve as advertisement to the females of the male's 

 sexual readiness and as a deterrent to other males in the vicinity. We saw females 

 leave the herd and join a displaying male in the water. After some preliminary 

 play (nuzzling, mounting), they dove beneath the surface, where copulation may 

 have taken place. 



When more than one bull was attending a herd of females, the bulls 

 maintained a distance of about 7 to 10 m apart, and each performed his displays 

 in a fixed location. Invasion of a male's display locus by another male resulted, 

 first, in visual agonistic displays similar to those described by Miller (1975a), 

 then in violent fighting. The "winner" of the ensuing battle continued to display 

 in the same locus; the "loser" retreated. We frequently saw bulls with bleeding 

 wounds, which suggested that the fights between bulls often result in physical 

 injury to one or both of the combatants. 



The sex ratio of males to females of breeding age in the two main wintering 

 areas where those observations were made ranged from 1:5 to 1:17 in March and 

 April (Table 1). The ratio of displaying males to potential mates, however, 

 ranged from 1:5 to 1:67 (average, about 1:23). Subadult and adolescent males 

 were scarce to absent in the vicinity of the displaying bulls. The few that were 

 seen were situated within rather than at the periphery' of the female herds and 

 were engaged only in sleeping or feeding, not in displays. One of these, 6.8 years 

 old, was taken on 21 March and found to be impotent and infertile. 



I sighted herds of females attended by one or more displaying bulls often in 

 March, infrequently in early April, and not at all after that time. Of some 200 

 groups of females in mid-April to mid-June, none was comparable to the mating 

 groups seen in March, inasmuch as none contained a displaying male. 



The mature bulls that I saw in March and early April mostly hauled out 

 separately. After the mating season, however, mature bulls form all-male herds, 

 many of which resort in large numbers to hauling grounds, such as Round Island 

 and Rudder Spit, where they simply rest, heal their wounds, and molt. 

 Probably, Round Island (and formerly, Walrus Island in the Pribilof area) is the 

 traditional summer haven for males of the Bristol Bay wintering concentration; 

 the Rudder, Meechken, and Arakamchechen hauling grounds along the Soviet 

 coast probably are the traditional summering areas for males from the St. 

 Lawrence concentration. That is, the all-male herds found on the Bering Sea 

 hauling grounds in summer probably are not outcasts or "old gentlemen's clubs" 

 so colorfully reputed by Dall (1902), but are made up of most of the mature, 

 breeding males from the winter concentrations. 



Pregnancy 



Within a few days after fertilization, the mammalian ovum becomes a fluid- 

 filled, spherical vesicle, the blastocyst. In some mammals, the embryo may 



