l6 VERTEBRATE REPRODUCTIVE CYCLES 



the interruption in laying between clutches is due to 

 some appreciation by the female that the nest already 

 contains sufficient eggs or young. If a clutch is kept small 

 by the daily removal of an egg, then many females will 

 continue to lay steadily for a considerable time; in this 

 way a flicker has been induced to lay 71 eggs in 93 days. 

 So while the habit of laying an egg each day may be 

 due to the operation of an internal cyclic mechanism, as 

 in the true oestrous cycle, the habit of laying successive 

 clutches is controlled by external circumstances. 



However, the most clearly defined oestrous cycles 

 are those of the mammals. Indeed, it may be better to 

 reserve the term oestrus for use in this class only, and to 

 find pther terms to describe the cyclic phenomena of the 

 lower groups. By far the best known and most closely 

 studied oestrous cycle is that of the laboratory mouse, 

 which breeds at all times of the year. An unmated female 

 ovulates repeatedly about once every 5 days from the 

 age of about 3 months to that of about 12 months when 

 old age begins. Such an animal, which experiences more 

 than one oestrous period in a year, is polyoestrous. Of 

 course in a wild mammal, in the absence of pregnancy, 

 the polyoestrous condition lasts for only a limited season, 

 and alternates with a non-sexual season of anoestrus. 



A few mammals, whether they become pregnant or 

 not, have only one period of heat per year. Such animals 

 are monoestrous, and this term is sometimes also loosely 

 applied to males and to many of the lower vertebrates. 

 An example is the English fox which ovulates spontan- 

 eously in January, and which remains in anoestrus for 

 the rest of the year. 



There remains to be considered the special question 

 of the primates (the monkeys, apes and man), which are 

 characterized by the unusual habit of breeding through- 

 out the whole year. Most of them appear to experience 

 a smooth and uninterrupted sexual life extending from 

 puberty to old age.201 



The females of the lower primates show links with 



