62 SEXUAL DIMORPHISM 



which have the habit of presenting their buttocks when 

 approached. But the two specimens to which I refer knew 

 the keeper very well, and I noticed that when the male ran 

 up at his call, he turned his buttocks, which were not red 

 beyond the callosities, towards the man, and showed pleasure 

 when the latter scratched them with his finger. I noticed 

 that at the same time an incipient erection of the penis took 

 place, showing that the gesture is essentially sexual. I asked 

 if the female behaved in the same way, and the keeper called 

 her and showed me that she did. 



I also saw a large female, Macacus inuus, the Gibraltar 

 species, in which the parts below the anus and about the 

 vagina were immensely swollen and protuberant. These 

 parts were naked and blue in colour. The keeper told me 

 that the animal was " in use," and that the swelling always 

 occurred during that condition. 



There was in the collection a full-grown male Cynocephalus 

 anubis, in which the buttocks w T ere red beyond the callosities, 

 but I did not see the gesture above considered practised by 

 him. He had a mane round the neck, thick but not long. 



Darwin mentions that in two other species of Cynocephalus 

 the adult males have a great development of hair on the nape 

 of the neck, forming a mane. In the C.porcarius of the Cape 

 of Good Hope the mane is longer and the canine teeth 

 larger than those of the female, and in C. hamadryas the 

 mane is absent in the young male and in the female. Darwin 

 was informed by the keepers in the Zoological Gardens that 

 monkeys did not attack each other by the nape of the neck 

 except in the case of C. porcarius. Doubtless this is also true 

 of the hamadryas. Darwin of course interprets the facts as 

 evidence that the mane is a protection. The Lamarckian 

 interpretation is that the irritation of the attack has stimulated 

 the hair to increased growth. It may be objected that in 

 the case of the mandrill the absence of hair on the cheeks 



