2 VERTEBRATE REPRODUCTIVE CYCLES 



provided by the polychaete Eunice viridis, also known as 

 the palolo worm, which lives in the seas around 

 Samoa. 13^ This worm spends the whole year hidden 

 in crevices in the rocks, but in each of the months of 

 October and November it spawns at dawn on the day 

 before and the day on which the moon enters its last 

 quarter. So striking is this act of spawning, and so 

 precise its timing, that the natives of Fiji have included 

 it in their calendar. 



These yearly breeding cycles affect both sexes equally, 

 although there may be slight differences in timing as 

 when in birds the male commonly attains sexual 

 maturity some weeks before the female. During a breed- 

 ing season a male is usually able to mate at any time, but 

 in a female still lesser cycles are sometimes evident and 

 the urge to mate is periodic. These cycles appear to be 

 rare among the fishes, amphibians and reptiles. As 

 regards the birds, reference may be made to the fact that 

 a female usually lays only one egg a day and often 

 produces two or more clutches at intervals of about six 

 weeks. However, the phenomenon of the lesser cycle is 

 most clearly seen in non-pregnant female mammals, 

 which, during a breeding season, commonly come into 

 heat at regular intervals. Thus an unmated female mouse 

 experiences a period of heat, when she is ready to mate 

 and to become pregnant, about once every five days. 

 Such a lesser cycle is called an oestrous cycle, the period 

 of heat being known as oestrus, and it is a common 

 phenomenon in both marsupial and eutherian mammals. 

 In a sheep the cycle has a duration of about i6 days, and 

 in the pig, cow, and horse about 21 days.^ In the case 

 of the great apes it is perhaps not strictly correct to 

 speak of heat in the usual sense, but nevertheless oestrous 

 cycles do occur with a duration of about a month, 

 whence they are also commonly called menstrual cycles. 



In this book the seasonal and oestrous reproductive 

 cycles of the vertebrates are examined, and an attempt 

 is made to analyse those factors which are now thought 



