THE CESTROUS CYCLE IN THE MAMMALIA 49 



state ; but, if the limit of time for coition is three weeks, there 

 is still just time for the females to undergo two dioestrous cycles, 

 and it is this possibiUty which prevents positive assertion on 

 the matter. 



" Among captive animals, not more than two dioestrous 

 cycles have been observed .in the gnu during one sexual season. 

 Gazella dorcas has two or three ; the giraffe about three ; while 

 the eland, nylghau, and water-buck have a series of dioestrous 

 cycles, each lasting three weeks, during May, June, and July 

 each year. 



" The gayal and bison, the axis and wapiti deer, on the other 

 hand, experience a continuous series of dioestrous cycles all the 

 year round, at intervals of about three weeks." ^ 



Heape states also that with red-deer in the Zoological 

 Gardens there is a very extensive series of dioestrous cycles, 

 and that with wapiti deer in captivity the possibihty of 

 pregnancy at any season is only prevented by the fact that 

 the male does not rut during the casting and growth of the 

 antlers. 



The males of many of the other species referred to experience 

 a definite rutting season, hke the stag in Britain. 



As already mentioned, the male camels in the Zoological 

 Gardens in London experience rut in early spring, or at the 

 same time as the sexual season of the female camels in Mongoha. 

 The period of gestation in the camel is thirteen months, so that 

 in this animal, as in the walrus among carnivores, the recurrence 

 of the sexual season is delayed by pregnancy, and conception 

 cannot take place oftener than once in two years. ^ The same 

 is the case with the wild yak in the deserts of Tibet,^ and also, 

 in all probabiHty, with the musk-ox in Greenland.* 



The sexual season in many Ruminants is a period of in- 

 tense excitement, especially in those cases in which the males 

 experience a definite rut. (See above, p. 27, in Chapter I.) 

 Thus, Catlin,^ referring to the American bisons, says : " The 

 running season, which is in August and September, is the time 



' Heape, loc. cit. 



^ Swayne, Seventeen Trips through Somaliland, London, 1895. 

 ^ Prjewalsky, loc. cit. * Lydekker, loc. cit. 



" Catlin, North American Indiana, vol. i., 2nd Edition, London, 1841. 



D 



