GO 



SEXUAL DIMORPHISM 



The mandrill and drill, which have their callosities especially 

 ornamented, display it, even whilst quite young, more fre- 

 quently and more ostentatiously than do the other kinds. Next 

 in order comes Cynocephalus hamadryas, whilst the other 

 species act more seldom in this manner. It deserves especial 

 attention that Von Fischer never saw any specimen of a 

 species in which the buttocks were not coloured make this 

 gesture: for example, it was never observed in Macacus 

 cynomolguSy Cercocebus radiatus, three species of Cercopitliecus, 

 and several American monkeys. 



With respect to the origin of the habit (continues Darwin) 

 Von Fischer remarks that his monkeys like to have their 

 naked hinder ends patted or stroked, and that they then 

 grunt with pleasure. The habit of the adults is connected 

 to a certain extent with sexual feelings. Von Fischer 

 watched a female Cynopithecus niger through a glass door, 

 and saw her turn her posterior, which is strongly reddened, to 

 the male, uttering a gurgling sound the while; the male 

 evidently became sexually excited and likewise made similar 

 sounds with his voice. It would appear from the description 

 that the male was in a separate cage, or separated by a 

 grating from the female. 



Von Fischer suggested that the coloration served to render 

 one sex conspicuous at a distance to the other, while Darwin 

 thought that the bright colours served as a sexual ornament 

 or attraction. It seems to me that while the appearance is 

 doubtless associated in the mind of the monkeys in question 

 with sexual desire, the origin of the condition is sufficiently 

 explained by the habit of the monkeys of scratching the parts 

 in one another. It is evident that the desire for the sensation 

 produced by the scratching is due to the pleasure which the 

 sensation gives, and that the sensation is connected with 

 excitement of the sexual organs, but with regard to the 

 condition of the parts of the body concerned, as compared 



