94 



VERTEBRATE REPRODUCTIVE CYCLES 



thesis that migration is a form of sexual behaviour, but it 

 must be remembered that, even in the absence of any 

 other urge, the flock instinct may be sufficient to induce 

 the few experimental birds to accompany the mass of 

 their fellows.^^ This is an important point which has 

 usually been overlooked and which is liable to affect any 

 experiments in which attempts are made to inhibit 

 normal movements. 



So once again the psycnological or social factor must 

 be introduced into this question of sexual behaviour, 

 and once again the warning is necessary that, at least in 

 the higher vertebrates, the hormones alone cannot 

 furnish the full explanation of the problem. However, 

 while a theory of migration based entirely on hormonal 

 considerations may prove to be quite inadequate, such 

 a theory may at least be of value in providing some 

 foundation on which our future knowledge can be built. 



5. CONCLUSIONS 



The evidence summarized above emphasizes the 

 dependence of sexual behaviour on the presence of active 

 testicular tissue in the male and of active ovarian tissue 

 in the female. Thus cyclical changes in gonad activity 

 are usually accompanied by cyclical changes in overt 

 behaviour. 



It has been shown that the gonads exert their influence 

 through the intermediary of the hormones which they 

 secrete. Consequently the appearance of male behaviour 

 can be induced by injections of testosterone, while that 

 of female behaviour can be stimulated by oestradiol. 

 If the gonads are removed sexual activity diminishes 

 rapidly in the female and more slowly in the male, the 

 partial maintainance of , activity in the latter being 

 perhaps related to a compensatory oversecretion of the 

 male-like hormone of the adrenal cortex. In any conflict 

 between the male and female hormones, due to congenital 

 abnormalities or to experimental conditions, it is the 



