Ileret Fossils and Primate Sperm Competition

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…
Literature Cited
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

5 Responses

  1. Monogamous? I thought we were more closely related to the Bonobos…..
    Human history would imply that monogamy is only a public virtue.
    I mean if human history wasn’t full of husbands murdering wives, husbands murdering wive’s lovers, wives murdering husbands, wives and lovers murdering husbands and so forth and so on, there would almost be no history at all…..

  2. Well, yeah, but if we didn’t have people we identified as husbands or wives we wouldn’t have anyone to murder, would we?
    It’s too bad we’re not more like bonobos.

  3. What about temporal placement? When did each specimen die?

  4. Oldfart – Chimps and Bonobos went their separate ways around 2-3 MYA. As an afterthought to you comment on human history, playwrites and poets wouldn’t have much to write about either…
    Alan – The time factor, IMHO, isn’t relevant to the point I was trying to make. Namely that the size dimorphism in Homo erectus, illustrated by the Ileret H. erectus specimen, implies certain things about human social groups. The sperm competition article reinforces those implications.

  5. Sperm competition is not important unless copulation with different males is closely spaced in time.

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