From: Richard Sharpe
To: All Nov-16-93 07:05PM
Subject: Re: More Aquatic Ape Hypothesis
Organization: Digital Equipment Corporation
From: sharpe@nmesis (Richard Sharpe)
The proponents of the AAH have asked the following question:
"If bipedalism is such an easy (refering to it developing in the
Savannah environment, rather than the more arduous aquatic approach)
and such an advantageous adaptation, why do other savannah dwellers
not develop it, and why do other apes not develop it?"
From these questions, it seems that the proponents of the AAH cannot
conceive that there is variation that would allow for bipedalism to
develop on the savannah, or even of these is, that there is strong
selection against, but they do not say what it is.
I found it interesting to think about that question: What might prevent
an ape (other than hominids) from developing full bipedalism, or indeed
another savannah dweller from developing bipedalism?
Before doing so however, I think that it is important to point out that
there is selection pressure favouring bipedalism for some animals, eg
apes. Despite Pat Dooley's attempts to persuade us otherwise, there is
very good reason for non-predators to have their eyes as high off the ground
as possible: seeing predators as soon as possible. It will not have
escaped people's attention, I hope, that almost all herbivores on the
savannah have eyes set at least as high as those of h.s.s if not higher.
In addition, since apes have much poorer hearing and sense of smell than
herbivores, it is that much more important for apes to detect predators
with their good eye sight, as soon as they can. Standing bipedally,
while making you more visible, also allows you to see further. Since
predators are going to be using their sense of smell and their hearing
as well as their sight to detect prey, it is most likely that a predator
has detected you well before you see them. Being more visible, while
being slightly more detrimental to you, is vastly outweighed by being
able to detect predators sooner. Thus I contend, there is selection
pressure on apes to become more bipedal. Vervet monkeys, for example,
stand up erect and scan around when they hear a "snake" signal from
another member of their troop (see  for more details).
Now to why other apes or other savannah dwellers would not develop
bipedalism. Well, there are no surprises here, I contend that selection
opperates against it :-)
In the case of herbivores and predators, the question hardly needs to be
asked. Since they are both locked in arms wars with each other (in
regard to features they evolve to ensure survival against the other,
predators need to evolve strategies against the latest prey defences,
and vice versa) any variation that arises in either type that allows
them to head back towards an unspecialized animal, and thus towards
bipedalism, will be heavily selected against. A prey species would soon
become lunch if it cannot run fast enough, and a predator would soon
starve if it could not run fast enough or hunt well enough.
But with apes the case is seemingly not so clear cut. Unless that is,
you forget about estrus displays in both species of pan (revealed
ovulation). Both species of pan employ revealed ovulation to one extent
or another. It is used by both Pan Paniscus and Pan Troglodytes as it
allows them to minimise the amount of time spent on reproducing and
maximising the time that they spend elsewhere, like foraging, rearing
young, status striving, etc, all activities that have a direct impact on
the number of young a chimp can successfully rear!
A female chimp which is more bipedal is not going to be able to
effectivly display the fact that she is ovulating, and is going to be
less successful in producing offspring. Males that invest effort mating
with her (assuming that they can recognize that she is ovulating) are
also going to produce less offspring.
I contend that there is strong selection pressure against full
bipedalism among species of pan for exactly the reason above, and that
the current posture (knuckle walking etc) of chimps is a compromise
between the need to be more upright in order to see predators more
effectively, and the need to be less upright to make revealed ovulation
It is interesting (although perhaps not compelling) to note that there
seems to be a progression. We are told (by Herb Huston and others) that
Pan Paniscus has less well developed estrus displays, and is more
upright that Pan Trogledytes. H.s.s has no estrus displays, and is
This of course just changes the problem from why did hominids develop
bipedalism, to why did hominids develop concealed ovulation.
Well, perhaps not. Who says that revealed ovulation was the ancestral
condition that hominids evolved away from? I would like to see more data
on aboreal primates and hominoids, especially in regard to their
reproductive behaviour and strategies before deciding for myself.
Perhaps somewhat appropos this discussion, and t.o in general, I have
located a copy of _The AAH: Fact or Fiction_ and I will read it while on
my way to Darwin next week. Perhaps it will bew more persuasive than
many of its proponents have been so far on t.o :-)
 _How Monkeys see the World_, Seyfarth and Cheney, 1991,
Richard Sharpe, email@example.com, Ph: 61-8-235-7237, FAX: ...-7299