82 VERTEBRATE REPRODUCTIVE CYCLES 



rapidly that the drum may make some 4,000 revolutions 

 in the same period. 1^3 This is one of the quantitative 

 methods available for measuring sex drive, and indeed, 

 until simpler techniques became available, it was used 

 for determining the degree of maturation of the ovarian 

 follicles. 



As regards the seasonal reproductive cycle, vs^hich is 

 present in most vertebrates, the correlation between 

 gonad size and and overt behaviour is a close one, as 

 may be illustrated by the case of the British starling.^* 

 During July and August, when gonad size is at a mini- 

 mum, song is quiet, nest-hole visiting is rare, and a 

 communal life is the general rule. In September and 

 October gonad growth commences and immediately 

 song becomes loud and prolonged, nest-hole visiting 

 is common, and the birds become associated in pairs. 

 During autumn and winter the gradual growth of the 

 gonads is accompanied by increasing sexual activity, 

 which shows itself in increasing attachment to the nesting 

 site and to the mate, and in an increasing intolerance of 

 the social life. Thus about the end of December com- 

 munal roosting gives place to nest-hole roosting in pairs, 

 and in mid-February nest-building may begin. By 

 April, when the gonads are fully developed, the sexual 

 behaviour patterns are completely formed so that 

 successful breeding can occur, the eggs can be incubated, 

 and the young can be fed. When the gonads regress in 

 June all such activity ceases, the pairs break up, the 

 birds lose their attachment to their nesting sites, and the 

 communal life begins again. 



In contrast to this yearly cycle in the British starling 

 it is significant to notice that in the Continental starling 

 gonad growth does not begin until February and no 

 sexual behaviour is seen until early March. ^^o Then the 

 males begin to sing, the birds return to their nesting 

 places, and the social life is discontinued. 



Many other such cases could be quoted from all the 

 vertebrate classes to show the relationship between 



