THE (ESTROUS CYCLE IN THE MAMMALIA 43 



well as 0. vignei, 0. ammon, and 0. canadensis, that these sheep 

 in their wild condition are probably monoestrous, for their 

 annual sexual season is of short duration, and occurs with 

 great regularity. Similarly it may be inferred from Prjewalsky's 

 statements l that 0. poli, 0. burrhel, and 0. argali are monoestrous 

 and breed once a year. Among wild sheep generally the sexual 

 season occurs as a rule in autumn, but it may vary with the 

 locality or climate. Thus with 0. vignei in the Punjab, the 

 sheep begin to breed in September, whereas, with the same species 

 in Astor, the sexual season must be considerably later, since the 

 young in the latter district are produced about the beginning of 

 June. 2 



Scotch black-faced sheep in the Highlands experience two 

 dices trous cycles, each of three weeks' duration, so that the 

 annual sexual season for these animals lasts six weeks. In the 

 Lowlands the sheep of this breed may have at least three re- 

 current dicestrous cycles in the absence of the ram, while flock- 

 masters inform me that, under unusually favourable conditions, 

 there may be as many as five or six, the duration of each cycle 

 varying from about thirteen to eighteen days. It can hardly 

 be doubted that of the two conditions of the Scotch black- 

 faced sheep that of the Highland ewes is the more natural, for 

 sheep, in their wild state, are essentially mountain animals, 

 being almost entirely confined to mountain districts in the 

 Holarctic region, their range only just extending across the 

 border into the far warmer Oriental region. " The immense 

 mountain ranges of Central Asia, the Pamir, and Thian-Shan 

 of Turkestan may be looked upon as the centre of their 

 habitat." 3 



The sexual season in hill sheep in Great Britain is ordinarily 

 from about the middle of November until the end of the year. 

 Under exceptional circumstances individuals may experience 

 oestrus at other seasons, such as in April after an early abortion 

 in the winter. In other British breeds the sexual season is 

 earlier. Thus Hampshire Down sheep are often " tupped " in 



1 Prjewalsky, Mongolia, the Tanyut Country, and the Solitudes of Northern 

 Tibet (Morgan's Translation), London, 1876. 



2 Lydekker, loc. cit. 



3 Flower and Lydekker, Mammals Liviny and Extinct, London, 1891. 



