58 



SKXCAL DIMORPHISM 



naked prominences may thus be regarded as inherited swell- 

 ings and scars. If this were true we should at any rate 

 understand why the peculiarity only develops in the adult, 

 since the males evidently fight only when they are sexually 

 mature, and, as in other cases, fight for the possession of the 

 females. As for the possible objection that a similar result 

 has not been produced in other cases, I would reply that 

 the gregariousness of the mandrill, and the violence of his 

 sexual excitement, would account for the unusual frequency 

 and fierceness of his combats, and that these peculiarities are 

 due to the conditions of his life. 



We have next to consider the peculiarities of the posterior 

 aspect of the animal. Here again the appearances are due 

 not to pigment but to the absence of hair and dilatation and 

 congestion of the superficial blood-vessels. These appearances 

 are similar to those which in the individual would be pro- 

 duced by frequent and energetic friction. Such friction 

 would be produced by the habit of squatting on the haunches 

 on the hard, bare rocks among which baboons live, and I 

 have no doubt that this habit among baboons and monkeys 

 in general is the cause of their naked ischial callosities. 

 But the callosities of the mandrill are exceptional in their 

 colour, and the bare skin thus brightly coloured is more exten- 

 sive than usual. This exceptional condition corresponds, or is 

 associated with, the peculiar habit of presenting the buttocks 

 to persons approaching the animal, a habit which is noticed 

 in the earliest description of the mandrill by Gesner. 



This gesture must have some meaning, and its probable 

 significance is sexual. It is possible that the libidinous 

 nature of the mandrill has developed some habitual artificial 

 irritation of the region of the generative organs. The animal 

 probably presents its buttocks from the desire of having them 

 rubbed, and possibly it is accustomed to have them rubbed 

 or scratched by the female. Some action of this kind would 



