THE CESTROUS CYCLE IN THE MAMMALIA 41 



to believe that Heape has overstated the duration of the sexual 

 season in these animals in a state of nature. 



The breeding season in the wild rabbit (Lepus cuniculus) 

 in this country generally lasts from about February to May, 

 but may be continued for longer. In the domesticated breeds 

 it sometimes lasts nearly the whole year if the circumstances be 

 favourable in regard to warmth and food supply. Heape says 

 that five or six months only is the usual duration of the period 

 during which dioestrous cycles recur in the domestic rabbit, and 

 that if oestrus is experienced in winter it may occur inde- 

 pendently of the possibility of pregnancy. 



The duration of the dioestrous cycle varies considerably. 

 " While some individuals exhibit oestrus every three weeks 

 fairly regularly, others do so every ten days ; on the whole, I 

 think ten to fifteen days is the usual length of their dioestrous 

 cycle/' l In Lepus variabilis recurrent dicestrous cycles are 

 probably continued for about two months. 



The squirrel (Sciurus vulgaris) in Britain, according to Heape, 

 is probably moncestrous ; but this animal, in Southern Europe 

 and Algiers, according to Lataste, is apparently polyoestrous. 

 In Britain squirrels breed early in the year, and sometimes have 

 a second litter in August. 



It is difficult to determine the length of the procestrum and 

 oestrus in Rodents, since the external changes which characterise 

 these conditions are comparatively slight. Heape says that the 

 prooestrum in the rabbit lasts, probably, from one to four days. 

 At this time the vulva tends to become swollen and purple in 

 colour, but there is no external bleeding. The same may be 

 said of the rat and the guinea-pig ; but, in the experience of 

 the writer, it is generally impossible to detect the procestrous 

 condition in either of these animals with absolute certainty. 

 (Estrus probably lasts for about a day. Lataste 2 states 

 that external bleeding occurs during the " heat " periods 

 of Pachyuromys duprasi, Dipodillus simoni, and Meriones 

 shawi. 



The guinea-pig (Cavia porcellus) in captivity can become 

 pregnant at any season, but more frequently in the summer 

 than in the winter. " Heat " rapidly succeeds parturition, as 



1 Heape, loc. cit. 2 Lataste, loc. cit. 



