Re: Why Large Gap Between Species...?

Paul Crowley (
Tue, 10 Dec 96 12:48:44 GMT

In article <01bbe602$51959060$LocalHost@dan-pc> "Rohinton Collins" writes:

> What about the orang-utan? It is almost totally arboreal, but the male is
> twice the size of the female. Your argument of an increase in sexual
> dimorphism in body size (SDIBS) being curtailed by arboreality is simply
> not borne out by the evidence.

Do you seriously maintain that a large ape can get around
in the trees as easily as a small one? Do you really need
evidence to help you judge the issue? You must be aware
that the bigger you are, the harder you fall.

Then take a *careful* look at the evidence: Competition
between male chimps involves much chasing, display and
fighting in trees. They operate in *multi-male* groups.
OTOH male orangs rarely see each other. They generally go
about quite slowly, announcing their presence with very loud
calls, whereupon subordinate males make themselves scarce.
Dominance is established in rare fights. But these are on
the ground (although few have been witnessed in the field);
however, dominance goes with size and some male orangs are
so large that they can hardly climb at all.

> The extant apes show a very good correlation between
> SDIBS and social organisation.

You mean "dimorphism=polygamy". (Why use acronyms and big
words?) Firstly, chimps are a special case. Multi-male
polygamous groupings are too rare to make or fit any rule
about dimorphism. That leaves gorillas, orangs and gibbons.
Each of these has its own distinct lifestyle. It is not
sensible to base general rules on so few and on such
heterogenous cases and extrapolate them to hominids. The
crude application of such rules is sloppy, lazy thinking.

What makes it worse is that australo's either had
multi-male promiscuous groups or were probably recently
descended from an LCA with them. We can apply no rules
to this type of grouping, since we have so few examples.
We do not know whether such a grouping was likely to
have large dimorphism or not -- even *if* we felt it
was appropriate to apply such a rule.

> Why do you insist on pitting yourself against the entire
> palaeoanthropological community Paul?

Because it's permeated with sloppy, lazy thinking.