Re: AAT - energetics

Alex Duncan (
15 Jul 1995 19:21:58 GMT

In article <> Elaine Morgan, writes:

>You say :"Is there a functional or adaptive reason why gibbons are
>largely bipedal when on the ground? I've never heard one suggested".
>You then go on to give a perfectly good one, which I believe to be
> accurate.i.e. because their arms have grown too long for them to able to
>walk efficiently on two legs. Why do you say that is not an adaptation?
>What do you mean by adaptation? What I mean by it is a change in either
>behaviour or physical structure in response to a change in the
>realtionship between an animal and its environment. The gibbon responds
>to the demands of brachiation by growing longer and longer arms. That
>is an adaptation. It then has to respond to the vicissitudes of
>occasional groundwalking by abandoning the quadrupedal gait which its
>ancestors, however distant, at one time employed. That is another

As you well know, and constantly remind us, there are better ways of
moving on the ground than bipedalism. In gibbons, bipedalism is an
epiphenomenon of thier extreme arboreal adaptation. They don't move
bipedally on the ground because its a good way to do things, but because
its the only method available to them due to their other specializations.
You are confusing "what its for" with "why its there". Your speculation
that everything has an adaptive purpose is baseless, and leads me to
think that you know very little about evolutionary biology. I strongly
recommend reading SJ Gould & ES Vrba's article about exaptation.

>Do you think our ancestors bcame more over specialised for arborealism than the gorillas
>and chimps so that they were driven down this path?

Do you think gorillas and chimps haven't evolved in 5 - 10 Myr? Your
question makes it apparent that you think we evolved from creatures
identical to modern gorillas or chimps. We do not have a fossil record
for gorillas and chimps, and thus are denied much information from those
branches of the hominoid evolutionary tree. Simply because chimps are
likely our closest relative does not mean we evolved from a chimp-like
animal. The fragmentation of the Miocene forests probably had dramatic
effects on all African hominoids, and a lot of the paleoanthropologists
I know suspect that African hominoids (including humans) have all added
more frequent terrestrialism to their positional "kit bag" as a response.
The implication of this is that all African hominoids had ancestors that
spent more time in the trees than their extant descendants.

Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086