THE (ESTROUS CYCLE IN THE MAMMALIA 53 



one another. These observations, therefore, are in a general 

 way confirmatory of those of Haldane. 



According to Guldberg and Nansen, 1 the porpoise copulates 

 at any time between June and October, the period of gestation 

 being ten months or longer. The white-sided dolphin is said 

 to copulate in late summer, pregnancy being about ten months, 

 and the white-beaked dolphin is thought to be similar. 2 



Humpbacked whales and other Cetacea have been 4escribed 

 as indulging in amorous antics at the breeding time, rubbing 

 against one another and patting one another with their long 

 fins. 3 



CARNIVORA 



In the female of the dog the average duration of the complete 

 cestrous cycle is six months, there being two annual " heat " 

 periods, typically in the spring and in the autumn. It 

 follows, therefore, that the bitch is moncestrous. Bitches be- 

 longing to the smaller breeds tend to come " on heat " more 

 frequently than those of the larger varieties. Thus, in Irish 

 terriers, the cycle may recur after four months, though in this 

 breed six months is the more ordinary time. 4 On the other 

 hand, in Great Danes the duration of the cestrous cycle is often 

 as much as eight months. It would appear that in those cases 

 where " heat " recurs as often as every four months, this is 

 only when pregnancy is prevented, for more than two litters of 

 pups are seldom if ever produced in a year. Stonehenge 5 says 

 that there is much individual variability in the periodicity of 

 the cycle, and that " heat " may recur at any interval from 

 four up to eleven months, but that six, five, and four months 

 are the most usual periods. Each bitch as a rule has her own 

 peculiar period to which she remains constant, unless systema- 

 tically prevented from breeding, in which case the periods tend 

 to recur irregularly or even cease altogether. 6 It has been 

 observed also that the recurrence of the sexual season tends to 



1 Guldberg and Nansen, " On the Structure and Development of the 

 Whale," Bergen, 1904. 



2 Millais, loc. cit. 3 Ibid. 

 * Marshall and Jolly, loc. cit. 



5 Stonehenge, The Dog in Health and Disease, 4th Edition, London, 

 1887. ' Heape, loc. cit. 



