THE (ESTROUS CYCLE LN THE MAMMALIA 55 



period of lactation is very variable in duration, and may extend 

 until the commencement of the next procestrum. 



The wild dog of South America (Canis azarce), according to 

 Rengger, 1 breeds only in winter, and therefore but once a 

 year. The same is said to be the case with the wolf and the 

 fox in their wild state ; but these animals, in the Zoological 

 Gardens in London, experience two annual " heat " periods 

 like the dog. 2 The wolves in the Dublin Gardens, how- 

 ever, are stated to have only one annual sexual season 

 when permitted to breed ; otherwise they come " in heat " 

 more frequently, but are always monoestrous. 3 The period of 

 gestation in the wolf and fox is approximately the same as in 

 the dog, i.e. about two calendar months. 



Bischoff 4 refers to the fact that the sexual season of the 

 fox is affected by the nature of the country which it inhabits, 

 foxes which live at high altitudes breeding later than those 

 residing on the plains. Millais 5 says that fox-cubs in most 

 parts of Britain are not born until the end of March or beginning 

 of April, whereas, in the south of England, they are often pro- 

 duced as early as January. 



The Cape hunting-dog (Lycaon pictus) has been known to 

 have bred in captivity on several occasions, and notably in the 

 Gardens at Dublin, where six litters were produced from a 

 single pair between January 1896 and January 1900. The 

 first three litters were born in January, the fourth in November 

 1898, the fifth in May 1899, and the sixth in January 1900. 

 Cunningham writes : " It is not easy to offer a satisfactory 

 explanation of the irregularity of the fourth and fifth litters. 

 1^ am inclined to believe, however, in the absence of definite 

 information on this point obtained from the animals in a state 

 of nature, that the lycaon breeds only once a year, and that 

 the irregularity noticeable in the fourth and fifth litters is due 



1 Rengger, Naturgeschichte d. Saugethiere von Paraguay, Basel, 1830. 



2 Heape, loc. cit. 



3 For the information regarding the breeding of the animals in the Royal 

 Zoological Society's Gardens, Dublin, I am indebted to the late Professor D. J. 

 Cunningham and Dr. R. F. Scharf. (See Marshall and Jolly, loc. cit.) 



4 Bischoff, " Ueber die Rauhzeit des Fuchses und die erste Entwicklung 

 seines Eies " Sitz. der Math.-phys., Wien, Clause vom 13 Jmii, vol. ii., 1863. 



5 Millais, loc. cit. 



