Re: An alternative to ST and AAT

Rohinton Collins (
17 Nov 1996 20:22:43 GMT

Paul Crowley <> wrote in article
> I've partly answered this in a reply to Gerrit. I suggest that
> reasons for dimorphism should be looked for in the wider ecology.
> Chimp males demonstrate a high degree of competition, but show
> little dimorphism; this is probably because if they got much
> heavier they would not be able to sleep, or generally function,
> in the trees.

Sorry Paul, but this is abosulute rubbish. Orang-utan males are twice the
size of females, and lead an arboreal existence. The reason for little
sexual dimorphism with respect to body size in the chimpanzee is because
the social system is multi-male polygyny. The male chimpanzees DO NOT
directly compete for females, but they do compete indirectly - the size of
the chimpazee testes are very large with respect to body size in comparison
to other hominoids.

> What would happen if a group of chimps forged an
> existence away from trees? The major restraint on the dimorphism
> would be lifted and I'd suggest that we would see something like
> A.afarensis.

Based on a false assumption, I'm afraid.

> Large dimorphism certainly does not necessarily
> imply a single-male harem type of social structure, and the
> assumption that it does is far too facile. For example, highly
> dimorphic baboons do not have one.

Incorrect again I'm afraid Paul. In fact a large degree in sexual
dimorphism with respect to body size correlates very highly with a unimale
polygynous social structure. Baboons do actually have an dominant male in a
group. The only way other males get to mate (without having confronted the
dominant male) is by deception (mating out of sight of the dominant male).

> The real question is "Why is Lucy so small?". I'd suggest a
> niche where a small female could forage just as well as a big
> one; where large distances did not need to be covered; where
> heat elimination was a problem during the day; where it was
> warm at night; and where predation was minimal.
> (Does anyone want me to suggest a location?)

You are not really saying anything here. If all other things were equal
(for both sexes) then there would be an optimum body size.

> Arm length (or leg length) is just the sort of thing that would
> rapidly attain optimum size. Natural populations of mammals have
> wide variations in limb length (glance around at some H.s.s) and
> over a few thousand generations, there is no question that the
> "right" one is always reached. If Lucy had a long forearm, it was
> because she needed one. The question is: "What did she need it
> for?". If she had short legs and big feet, it was because she
> needed them. But what for?

Agreed. Lucy was well adapted for her particular lifestyle which was ...

> Primitive retentions are of a quite different nature; they are
> essentially non-functional; selective pressures have little
> effect on them.

Agreed, unless they are specifically selected against.

> No scenario of hominid evolution claims that our ancestors had
> a high proportion of animal fat in their diet. That, I hope, is
> the main reason I feel it's unhealthy. (I'm sure I'm also affected
> by the large number of people I known who've died suddenly of heart
> attacks or who have had bad hearts.)

This is largely irrelevant though, since early hominids would have been
lucky to have lived over forty, at a guess. Anyone have any references for
the life expectancy of the australopithecines?

> Think of the reproductive unit - the
> mother/infant dyad. Ask how a _mother_ with a large infant on
> her belly (or on her back) could decide to walk or run bipedally.

Why, why oh why? (Don't answer that Paul).