gorilla, totaling 668 capacities in all, includes nearly twice as many males as 

 females (414 males, 254 females); while the combined-sex sample for chim- 

 panzee has a majority of females. High sexual dimorphism, together with 

 marked inequality in the number of each of the 2 sexes in the combined 

 sample, may well be responsible in large measure for the extremely high 

 variability of the combined-sex sample of gorilla capacities. 



However, not all the high variability of the gorilla sample is to be 

 laid at the door of sexual dimorphism and differing sexual composition of 

 samples. In an all-male gorilla sample, variability is smaller than in the com- 

 bined-sex sample, V being 10.67 P er cent U1 tne former and 13.06 per cent in 

 the latter. Nevertheless, even in the all-male sample of gorilla, V is definitely 

 higher than in any of the large all-male samples of other apes. This con- 

 firms that cranial capacity in the gorilla remains very variable, even when 

 allowance is made for the high degree of sexual dimorphism. 



In contrast, the hylobatines show a much smaller degree of sexual 

 dimorphism than the large-brained pongids. In H. lar, the mean capacity 

 for adult females is 97.0 per cent of that for adult males, and in the siamang, 

 S. syndactylus, the figure is 97.6 per cent. Thus, sexual dimorphism might be 

 expected to contribute but little, if any, additional variance to a combined- 

 sex sample of hylobatines as compared with separate male and female sam- 

 ples. This inference has been tested for Schultz's samples of H. lar and is 

 supported by the following figures: 



Males (n = 95) V = 7.22 per cent 



Females (n = 85) V = 7.69 per cent 



Combined males and females (n = 180) V = 7.62 per cent 



Irrespective of the sexual composition of the sample, the cranial capacity 

 of H. lar has a moderate variability. 



If we may generalize from the foregoing, it seems permissible to infer 

 that for species with a high degree of sexual dimorphism the combining 

 of data for both sexes does increase the variability appreciably: this is not 

 so, however, when male and female data for species with slight sexual di- 

 morphism are combined. 



Variability in relation to mean capacity. It has already been men- 

 tioned that, in general, V seems to be higher the bigger is the mean cranial 

 capacity. Table 1 1 summarizes relevant data for adult male specimens 

 (data for the siamang are omitted as n is only 23). 



47 K 



