Re: Why Large Gap Between Species...?

Paul Crowley (
Sun, 08 Dec 96 20:39:34 GMT

In article <> "Gerrit Hanenburg" writes:

> (Paul Crowley) wrote:
> >However, this cycle puts chimpanzees into an evolutionary rut.
> >Success is penalised. The breaking of this cycle by hominids
> >must have been *the* major step. Learning how to manage stable
> >groups of more than 14 males must have been exceedingly difficult
> >and required great ability in diplomacy and made language and
> >other social skills a high priority. Monogamy would probably
> >have replaced promiscuity at an early stage.
> Body size dimorphism in A.afarensis expressed as the ratio male/female
> bodyweight is estimated to be 1.52 (human formulae) or 1.78 (ape
> formulae) (McHenry 1991). Both figures are well above that of Pan
> troglodytes (1.37) and make a monogamous mating system unlikely.

This is yet another example of the unthinking application
of a general rule. And it's a particulary obnoxious and
pervasive case.

There is NO reason why the "great dimorphism = polygamy" rule
must necessarily be applied. It is one indication -- and in
this case very probably a false one.

Dimorphism in mammals is likely to arise when males do little
for their young except pass on genes and when they compete
for access to females. It will be restrained by factors
arising from the habitat and lifestyle. But dimorphism does
not *prove* that males fight (or fought) over females. It
can arise simply from differences in role. H.s.s. dimorphism
has causes. It is IMO unlikely to have arisen from physical
competition between males; differences in role are a much
more probable cause. If such differences had required that
males had to be twice the size of females, I doubt if we
would find that at all strange.

Suppose H.s.s. males were twice the size of females. What
conceivable difference would it make to the relationships
between the sexes? (Those who support the rigid application
of this rule, must maintain that we would be more likely to
be polygamous.)

It is commonly accepted that the australo's had multi-male
groups; firstly, because they had to be able to resist
diurnal predators and had no natural weapons; secondly,
they had a common ancestor with chimps which possess this
system and its development (with female exogamy) is
unlikely to have happened independently.

The principal restraint on dimorphism in chimps is probably
their partial aboreality -- while there are advantages in
extra size for male chimps, the disadvantages of extra size
and weight in the trees are manifest. If chimps were to
adapt to a 100% terrestrial existence, then substantial extra
dimorphism would almost certainly result. This is probably
close to what actually happened about 4.6 mya.

Multi-male groups cannot readily operate with polygamous
systems. So they must be promiscuous or monogamous.
Promiscuity is only viable in small groups. Large groups
developed at some point in the hominid line. The australos
may have kept small promiscuous groups going for some time,
but I doubt it. Fierce inter-group conflict exists in
chimps and H.s.s. so it's reasonable to assume it in the
australos. They probably used clubs and stones. Larger
groups would have overwhelming advantages in such conflicts.