The miniscule samples of P. paniscus suggest a mean capacity about 10 

 per cent lower than that of P. troglodytes for each sex. 



Orangutans. Table 6 gives data for 5 main series of each sex, as well as 

 pooled and combined-sex data. Once again, there is an impressive range of 

 sample means, that for males extending from 395 c.c. (Oppenheim 1911— 

 1912) to 455 c.c. (Selenka 1898, 1899), whilst that for females stretches from 

 338 c.c. (Schultz 1965) to 390 c.c. (Selenka 1898, 1899; Gaul 1933). The size 

 of the extreme range of sample means is thus 50 to 60 c.c. That there are 

 population differences in mean capacity within Pongo pygmaeus is suggested 

 by the work of Selenka and of Gaul: this factor may account for the appre- 

 ciably greater range of sample means in the orangutan than in the chim- 

 panzee, although their overall mean capacities are close. Sexual dimorphism 

 is greater among the orangs than among the chimpanzees. Thus, while the 

 means of the total pooled male series are 434.4 c.c. for orang and 398.5 c.c. 

 for chimpanzee, the female means are practically the same in the 2 species, 

 being 374.5 c.c. in orang and 371.1 c.c. in chimpanzee, for comparably 

 large samples. The sex ratio for the pooled samples of orang is as low as 

 86.2 per cent, as compared with 93.1 per cent in chimpanzee. However, 

 there are wide fluctuations among the results of various workers: for indi- 

 vidual series, the sex ratios are 90.5 (Oppenheim), 89.8 (Gaul), 89.2 (Ash- 

 ton and Spence), 85.7 (Selenka), and 81.3 (Schultz). 



Gorillas. In Table 7 are given available data on gorillas. For 8 male 

 series the sample means range from 497 to 551 c.c: the size of the extreme 

 range of sample means (54 c.c.) is thus rather less than that for male orang- 

 utan (60 ex.). For female gorilla the sample means cluster more closely, be- 

 tween 442 and 478 c.c, an extreme range of only 36 c.c, whereas the range 

 for female orangutan is 52 c.c. It should be noted, however, that 4 out of the 

 8 female gorilla samples are extremely small (5 to 12 crania). If we confine 

 our attention to the remaining 4 samples, those of Selenka (1899), Randall 

 (1943-1944), Ashton and Spence (1958), and Schultz (1965), our sample 

 means range only from 443 to 461 c.c. Thus, both for male and female 

 gorilla the range of sample means is absolutely smaller than the range for 

 orangutan, despite the 80 to 100 c.c. by which gorilla means exceed orang 

 means. This might suggest that, if technical and sampling errors are not to 

 blame, intraspecific diversification of mean endocranial capacity is less 

 among gorilla than among orangutan. 



37 U 



