88 VERTEBRATE REPRODUCTIVE CYCLES 



of these experiments the synthetic testosterone propion- 

 ate was used because it is biologically more active than 

 the naturally occurring testosterone. When it is injected 

 into animals which are immature or in anoestrus it 

 induces a rapid development of male behaviour. Under 

 its influence the non-breeding tree frog gives the typical 

 breeding croak and shows male copulatory behaviour^^; 

 the castrated male lizard, Anolis carolinensisy develops 

 sexual activity 1*^' i*^; month-old chicks of the black- 

 crowned night heron develop a guttural voice and 

 indulge in territory defence, nest building, all male 

 courtship ceremonies, copulation, and broodingi*^; and 

 a variety of mammals, immature, anoestrous, or cast- 

 rated, show similar behaviour changes. 



It is interesting to discover too that these powerful and 

 drastic results are produced in female as well as in male 

 animals. A striking example of this is described by Shoe- 

 maker^'® who injected female canaries with testosterone 

 propionate, and found that they suffered a complete 

 suppression of female behaviour. After a few days they 

 were singing loud and long in the manner of males, and 

 were indulging in normal male courtship behaviour. 



One effect of the male hormone that deserves special 

 mention is its stimulus to personal aggressiveness which 

 in turn leads to social dominance in a group. This has 

 been studied extensively, the original observations 

 being those of Schjelderup-Ebbe^^^ who concluded 

 'that there exists among birds a definite order of 

 precedence or social distinction'. What he found in so 

 many bird flocks has since been noted in groups of fishes, 

 reptiles, and mammals. In each case one individual in a 

 group has precedence over all the others so that it may 

 bully and drive them away without retaliation, while the 

 others can be numbered according to their positions on a 

 declining scale. Particularly in birds this scale has come 

 to be known as the peck order. 



In attempting to explain this hierarchical system, 

 Schjelderup-Ebbe suggested that such factors as 



