40 THE PHYSIOLOGY OF REPRODUCTION 



RODENTIA 



There can be little doubt that the great majority of Rodents 

 are polycestrous. Most of them, so far as is known, have 

 one annual breeding season, which may, under favourable 

 conditions, extend over several months. Thus the rat (Miis 

 decumanus) and mouse {M. musculus) are known to experience 

 ■a recurrence of the dioestrous cycle in the absence of the male, 

 while, if pregnancy occurs, a new " heat " period very rapidly 

 succeeds parturition. In a state of semi-domestication M. rattus 

 and M. decumanus have, in my experience, a fairly regular breed- 

 ing season from about the end of January until the end of May. 

 During this period the majority of mature females are either 

 pregnant or suckHng their young (that is, of course, among 

 females which have been allowed to run freely with males). 

 Pregnancy may occur at other times of the year, but is not 

 nearly so common. The duration of the dioestrous cycle in the 

 rat is said to be about ten days ; ^ the period of gestation is 

 approximately three weeks. Heape states that M. minutus and 

 M. sylvaticus are also probably polycestrous. The bank vole 

 {Arvicola glareolus) is almost certainly polycestrous, since it can 

 become pregnant immediately after parturition at certain times 

 of the year. The same condition no doubt exists in the field vole 

 (A. agrestis), which breeds in Britain from January to October.^ 

 According to Lataste,^ Eliomys quercinus, Gerbillus hertipes, 

 Dipodillus campestris, D. simoni, Meriones sham, and M. longi- 

 frons are also polycestrous. The length of the dioestrous cycle 

 in all these animals, as observed by the same investigator, is 

 usually about ten days, as in the common rat. 



In the wild condition in Britain, according to Heape, re- 

 cxirrent dioestrous cycles last " about three months, probably, in 

 Arvicola agrestis ; from four to six months, probably, in Mus 

 minutus ; about nine months in Mus rattus ; and even longer, 

 perhaps, in Mus musculus and M. decumanus." From my own 

 experience with the two latter species in captivity, I am disposed 



' Heape, loc. cit. 



2 Millais, British Mammals, vol. il., London, 1905. 



' Lataste, Recherches de Zoodthique sur les Mammifires de I'ordre des Rongeurs, 

 Bordeaux, 1887. 



