Re: An alternative to ST and AAT

Paul Crowley (
Mon, 18 Nov 96 11:29:39 GMT

In article <01bbd4c4$ce230260$LocalHost@dan-pc> "Rohinton Collins" writes:

> 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.

You're not thinking. Orangs live in tropical rainforest in Borneo
and Sumatra. They have NO predators (other than H.s.s.). Even if
leopards were present they would find it hard to climb those trees.
Orangs' nests can be large platforms. OTOH chimps live in general
forests on mainland Africa and are constantly predated by leopards.
Males need to be near females when they are in estrus and the trees
in which they sleep would not support large nests.

I'm not saying that is the only reason. It's impossible to take
out one factor from a whole lifestyle. And intelligence, social
skills, speed and altheticism count highly in chimp conflicts;
the alpha is necessarily the largest.

> The reason for little
> sexual dimorphism with respect to body size in the chimpanzee is because
> the social system is multi-male polygyny.

This is an assumption. Evidence against it (proof?) would be a multi-
male group with large dimorphism; i.e. baboons.

> The male chimpanzees DO NOT
> directly compete for females, but they do compete indirectly - the size of

Chimp males do compete intensely for the alpha position, and the
alpha keeps females for himself at the height of their estrus.

> > 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).

This is not correct. Chimps are nearly identical as regards such
matings. All multi-male groups have a dominance system.

I abominate the mindless application of general rules. They were
not laid down by God. They are derived from general observations
and have no other validity. At best their application to a
particular case is an assumption. Why should multi-male
promiscuous mating rule out dimorphism? Size counts in dominance
conflicts and alpha's usually do most of the procreating.
Size counts in inter-group conflicts - invariably between males.
There is no reason IMHO to conclude that A.afarensis had a
polygamous system and numerous reasons for not doing so.

> > 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.

Excuse me, but I was saying a lot. I was spelling out criteria for
the niche. (Where've you seen that done before?) And things are
rarely equal between sexes; it's the female who is most likely to
get near to an "optimum" size. She is the basis of the reproductive
unit; it's males who are compelled by sexual competition to adopt
less than optimum shapes or sizes (e.g. peacocks' tails).