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,^ 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.^ 



Humpbacked whales and other Cetacea have been described 

 as indulging in amorous antics at the breeding time, rubbing 

 against one another and patting one another with their long 

 fins.* 



Caenivora 



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

 oestrous 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 monoestrous. 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.* On the other 

 hand, in Great Danes the duration of the oestrous 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 htters of 

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

 that there is much individual variabiUty 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 

 pecuHar 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.' It has been 

 observed also that the recurrence of the sexual season tends to 



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

 Whale," Bergen, 1904. 



^ Millais, loc. cit. ' Ibid. 



* Marshall and Jolly, loc. cit. 



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

 1887. . " Heape, loc. cit. 



