THE " SEXUAL SEASON OF MAMMALS. 43 



But whenever gestation occurs it encroaches upon, if it 

 does not entirely absorb, the ancestrum ; that is to say^ it re- 

 duces the period during which the generative orgaUvS would 

 lie fallow if the sexual season were a barren one. Thus in 

 the case of the mare, a barren sexual season may consist of 

 a series of dioestrous cycles lasting for as long as six months, 

 in which case the ancestrum lasts six months also, after which 

 another sexual season again begins. 



A reproductive sexual season, however, results in a period 

 of eleven months' gestation; interfering not only with the 

 dioestrous cycles which would have recurred if conception 

 had not taken place, but also absorbing practically the whole 

 of the ancestrum ; for, nine days after parturition, the ma- 

 jority of mares again experience oestrus. 



Nursing. — Nursing also may or may not interfere with 

 the recurrence of the sexual season and of oestrus. The 

 rat suckles her new-born litter of young while pregnant 

 with another litter; so also does the domestic rabbit and 

 guinea-pig, and probably many rodents. The mare also, 

 as a rule, readily becomes pregnant while suckling her 

 newly born foal. Here, however, there is some evidence of 

 variation, for I am informed, by a breeder oF large shire 

 horses in the west of England, that many of the mares in his 

 stud become pregnant only once every two years ; the drain 

 on the system, in consequence of gestation and nursing, in 

 these large animals being, apparently, too great to admit of 

 the immediate recurrence of another sexual season. Another 

 breeder of shire horses, however, assures me that he gets a 

 foal each year from his mares. 



On the whole there is some reason to believe that, unless 

 these large mai"es are exceptionally carefully tended, they 

 are liable to miss bearing annually, from time to time. 



A few instances may be given here of animals in the wild 

 state which do not bear young every year. The grizzly bear 

 in British Columbia bears young only every second year 

 (Somerset, 1895). The wild yak in the Tibetan desert only 

 produces a calf every second year (Prejevalsky, 1876), and 



