THE CESTROUS CYCLE IN THE MAMMALIA 37 



EODENTIA 



There can be little doubt that the great majority of Rodents are 

 polyoestrous. 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 (J///.s iln-uniinm*) and mouse 

 (M. jini.x<:nlii*) are known to experience a recurrence of the dicestrous 

 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. ritftus and M. dccmnn ,ms have, in my experience, 

 a fairly regular breeding season from about the end of January until 

 the end of May. During this period the majority of mature females 

 are either pregnant or suckling 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 dicestrous cycle in the rat is said to 

 be about five days, 1 but observers seem to differ, some stating it to 

 be longer ; the period of gestation is approximately three weeks. 

 Heape states that M. iniimtn* and M. sylvaticus are also probably 

 polyo3strous. The bank vole (Arvicola glareolus) is almost certainly 

 polyoestrous, 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. 2 According to Lataste, 3 J/i<n/>ys quercinus, 

 Gcrbillus hertipex, Dipodillus rmnpestris, D. simo/ii, Mcrinm^ xhaici, 

 and M. lo/u/ifronx are also polyoastrous. The length of the dioestrcms 

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

 usually about ten days. 



In the wild condition in Britain, according to Heape, recurrent 

 dicestrous cycles last "about three months, probably, in Arriniln 

 //_y/v.s7/.s ; from four to six months, probably, in Mu* miiii'tax] about 

 nine months in Mus rattus; and even longer, perhaps, in J//'.s- 

 nnixniliix and M. decumanus." From my own experience with the 

 two species of rats in captivity, I am disposed 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 (/,</'*< /////</////*; 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 



1 Long and Evans, "The (Estrous Cycle in the Rat," Ant. //., vol. xviii., 

 1920. 



2 Millais, /irif </< M<imin<il$, vol. ii., London, 1!X).~>. 



3 Lataste, Iteclicrches de Zoovthiyii.e sin- lex Mammiflm </ Porcfa >/< /.' 

 Bordeaux, 1887. 



