162 SEXUAL DIMORPHISM 



that the cocks, although they share in the duties of incuba- 

 tion, are pugnacious, and probably before eggs are laid 

 make the advances in courtship. Their habits are not, 

 therefore, inconsistent with their larger size and finer 

 plumage, and are in perfect harmony, according to the views 

 maintained by me, with the fact that they are less superior 

 to the females than the males of such polygamous species as 

 the pheasants. 



In the Common Cassowary, Casuarius galeatus, the femah 

 is larger, and the appendages and naked skin about her head 

 are more brightly coloured. The female, according to Mr. 

 T. W. Wood, is very pugnacious in the breeding season, and 

 then her wattles become enlarged and more brilliantly 

 coloured. This fact is evidence for the view that the 

 appendages and bright colour of the skin are the result 

 of the irritation produced by the blows of the beaks of the 

 birds when fighting, and are not necessarily related either 

 to a particular sex or to selection. The habit of fighting 

 with the beak also explains the presence of the bony crest 

 on the skull, although it is present in both sexes. It is 

 absent, however, in the young birds. 



In the Emeus, Dromams, similar differences of habit 

 correspond to a similar reversal of the usual differences in 

 the sexes. The male alone incubates the eggs and takes 

 care of the young ; he has even to defend them against the 

 attacks of their mother. She is considerably larger than 

 the male, and possesses a slight top-knot, but is otherwise 

 indistinguishable in plumage. 1 



During the breeding season, at least, the hen utters a 

 peculiar loud, booming sound, while the voice of the male, 

 according to Mr. Bennett, is much less powerful "a 

 suppressed hiss when angry, or a croak." The peculiar 

 voice of the female seems to owe its character partly to a 



i " A. W. Bennett, Land and Water, May, 186?. 





