6 VERTEBRATE REPRODUCTIVE CYCLES 



selection can be seen at work in the odd cases of out-of- 

 season breeding that have been described. In the British 

 starling many such reported cases have been reviewed, ^* 

 and it is obvious that the eggs and young have only 

 the slightest chances of survival so that the number of 

 these misfits is kept at a minimum. However, if for any 

 reason it ever became an advantage for the starling to 

 breed in autumn or winter instead of in spring, there is 

 material here from which a new population with new 

 habits could be built. 



Spring breeding is an almost universal habit among the 

 birds of temperate regions. These are all rapidly growing 

 species that can pass from a single cell to adult size 

 and structure in the extraordinary short space of 5 or 

 6 weeks, although large species like ducks and geese 

 may take some weeks longer. Clearly it is greatly to their 

 advantage -that all this violent growth and differentiation 

 should occur at a season when food is most abundant 

 and most succulent. 



Among mammals the situation is complicated by the 

 habit of allowing the eggs to develop in the uterus for a 

 longer or shorter period of pregnancy. Thus it is not the 

 act of mating but the act of birth which must be timed 

 to take place in a season that provides optimum con- 

 ditions for the survival and growth of the young. In 

 mammals with only a short period of pregnancy, both 

 breeding and birth may take place in the spring. Among 

 small species this is a general rule, and is seen, for 

 instance, in the wild rabbit, which, with a duration of 

 pregnancy of about 30 days, produces a series of litters 

 between midwinter and midsummer.*^ In larger 

 mammals pregnancy is prolonged so that the breeding 

 season becomes separated from the season of birth. 

 This is shown by ruminants like the deer and sheep, 

 which have an autumn breeding season, a period of 

 pregnancy of about 150 days, and a season of birth in 

 late winter and early spring.^ With still longer periods of 

 pregnancy the breeding season may move still earlier 



