630 APES xxiv. 3 



a sectorial blade. The molars carry grinding tubercles and often show 

 a crenation of the enamel, which is characteristic of apes, though found 

 as an abnormality in monkeys and man. The trigon is still present in 

 the upper molar, the hypocone being small. The lower molars have a 

 hypoconulid, making five cusps, as contrasted with the four of monkeys. 

 All the apes are mainly vegetarians, but they may eat meat occasion- 

 ally. The use of the teeth for grinding is associated with powerful 

 masticatory muscles and the development of temporal and occipital 

 crests. The supraorbital ridges are also large. The canines are used for 

 attack and defence. In the digestive system apes and man differ from 

 other primates in the presence of a vermiform appendix. 



The brain is much larger than in cercopithecids, and shows a 

 pattern of convolutions similar to that of man, though more simple. 

 The behaviour provides many signs of efficient memory, leading to 

 the attitudes that we characterize as imitation and association of ideas. 

 There are extensive powers of manipulation and for obtaining ends by 

 indirect means. 



The communication system is highly developed. The young chim- 

 panzee is said to be able to make at least thirty-two distinct sounds. 

 The facial musculature is more highly differentiated than in monkeys 

 and produces a wide range of expressions such as of rage, surprise, 

 pleasure, and laughter. 



Social organization is always well developed. The gibbons are mono- 

 gamous, the family consisting of a pair and the young of current and 

 previous years. Chimpanzees, and so far as is known gorillas, live in 

 bands, led by a dominant male. The individuals of a band cooperate 

 in helping each other and the groups show differing social traditions. 

 The apes are diurnal animals, eating in the day. They make platforms 

 on which they rest at night (except the gibbons). 



Reproduction is based on a menstrual cycle of thirty-five days in 

 chimpanzees, with a great development of the sexual skin at mid-cycle. 

 Gestation is long, as is the growth period, seven to nine years in the 

 gibbon, ten to twelve in the chimpanzee. The life span is also long, 

 reaching forty years in chimpanzees, perhaps fifty in gorillas. 



The gibbons are fully arboreal, swinging rapidly with their ex- 

 tremely long arms (Fig. 404). This extreme brachiation may be a quite 

 recent specialization. They eat mainly fruit and leaves, also insects or 

 eggs. Gibbons are numerous throughout south Asia and the type is 

 certainly the most successful among modern great apes. The charac- 

 teristic cries are part of the defence of the territory that is occupied by 

 each group. The orang-utan of Borneo and Sumatra is larger and also 



