From the Ileret article by Spoor et al:
The intraspecific variation of vault size in H. erectus, including KNM-ER 42700, is larger than in extant humans and chimpanzees, but smaller than in gorillas … This degree of variation may well imply that H. erectus showed marked sexual dimorphism, rather than the reduced levels that characterize the derived condition in H. sapiens …
One correlate of body size dimorphism is group composition. In species that live in single male groups there is a large amount of size dimorphism. In monogamous or polyandrous species there is little size dimorphism, while in multimale groups the amount of dimorphism is somewhat intermediate. Testes size varies in a somewhat similar fashion (except that testes size in polyandrous groups with testes size in multimale groups).
I mention this because of an interesting article on Science Daily concerning sperm competition. According to Science Daily:
“Rapidly swimming sperm cells would be evolutionarily favored when the mating pattern is polygamous and that is consistent with our measurements of chimp and rhesus macaque sperm,” said Nascimento.
The researchers found significantly lower swimming forces and slower swimming speeds with human sperm, and the slowest of all belonged to gorillas. “Dominant silverbacks are known to effectively discourage other males from mating with the females in their harems, so faster sperm wouldn’t seem to be an advantage to them,” Nascimento said.
However the scientists were surprised that the speed and force of human sperm fell in between the gorillas and the chimps. “Maybe humans haven’t always been as monogamous as we had thought,” Berns said.[emphasis mine – afarensis]
I mention it because it reinforces the point Spoor et al were trying to make…
Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya
F. Spoor, M. G. Leakey, P. N. Gathogo, F. H. Brown, S. C. Antón, I. McDougall, C. Kiarie, F. K. Manthi & L. N. Leakey
Nature 448, 688-691 (9 August 2007) | doi:10.1038/nature05986