Re: Body Temperature

From: Tim Tyler (tim_at_tt1lock.org)
Date: 07/07/04 

Date: Wed, 7 Jul 2004 22:27:43 GMT

In sci.life-extension Steve Harris sbharris@ROMAN9.netcom.com <sbharris@ix.netcom.com> wrote or quoted:
> Tim Tyler <tim@tt1lock.org> wrote in message news:<I0H86x.v3@bath.ac.uk>...
> > Steve Harris wrote or quoted:

> > Re: Good specific metabolic rate * lifespan = constant article:
> >
> > > Big exceptions to the rule are primates like capuchins and humans, and
> > > we both have very large brain/body wt ratios. So evidentally large
> > > brains are such a good evolutionary trick against predation that it's
> > > worth it for evolution to spend time repairing us, and thus we age
> > > more slowly metabolically and get 3 billion heartbeats in a lifetime,
> > > instead of the standard billion for mice, cats, cows, etc.
> >
> > There are other explanations for an effect of large brains
> > on lifespan that don't mention predatiton - so the effect
> > of brain power on lifespan via reduced predation is not
> > all that evident.
> >
> > The more usual arguments about the effect of large brains on
> > lifespan invoke a mixture of developmental plasticity and
> > neotony to explain the effect.
> >
> > Large brains are there to help cope with a complex variable environment.
> > Large-brained organisms are less hard-wired up at birth, and more
> > programmable by environmental influcences. Environmental programming
> > takes its time to occur - so brainy creatures tend to have long
> > childhoods - and typically are not sufficiently competent to
> > reproduce for some time after being born.
> >
> > The whole large-brain, long childhood thing can be produced by
> > twidlling some developmental rates downwards - and this tends to
> > retard both development and aging.
> >
> > Longer childhoods and later puberty tend to stretch out the whole
> > developmental program - including its final stage: senescence.
>
> COMMENT:
>
> Interesting thought, but it hardly explains why humans get about the
> same metabolic time as Capuchins, which mature in a far, far lower
> fraction of their total life span than do humans.

It's mostly a theory about the cause of extended lifespans of humans.

Other organisms might have extended lifespans for a wide range of
other reasons. For instance, organisms that are rare - and have
difficulty finding mates - can have extended lifespans for that reason.

> Capuchins are sexually mature at 2, but may live 40 years. If humans
> were ready to go out on their own and breed at 6, your theory might
> hold water.
>
> It's also an overly complicated hypothesis, since it posits one reason
> for delayed aging in primates (neoteny for purposes of programming),
> and at least one other for mammals and birds with wings, and reptiles
> with shells.

Unfortunately, nature is complicated - and more than just predation
frequency affects aging.

> But if the anti-preditor effect has an effect on aging in winged and
> shelled animals, it would be difficult indeed to see why it shouldn't
> have an effect for animals with brains, too, as it surely must in
> humans.

That wasn't actally my suggestion - I was attempting to point out that
an effect based on large brains reducing predation and thus extending
lifespan might not be the primary explanation for our relatively long
lifespans.

Other factors are also at work - for example, as well as the
developmental theory I mentioned, large brains might also decrease
mortality via reducing accidents, via increasing competence at
finding food - and via better evading parasites.

How heavily predated was our LCA with chimps? If it was already competent
at escaping predators by using social signals and climbing into trees,
then any life extension attributable to changes in relative brain size
among our cousins is likely to have been due to other mechanisms. 

-- 
__________
 |im |yler  http://timtyler.org/  tim@tt1lock.org  Remove lock to reply.

Relevant Pages

  • Re: Maybe thinking meat is a good idea
    ... Sure, our big brains are costly, yet our lifespans are rather long for mammals ... I suppose longevity was selected for independently ...
    (rec.arts.sf.written)
  • Re: Body Temperature
    ... > brains are such a good evolutionary trick against predation that it's ... There are other explanations for an effect of large brains ... all that evident. ...
    (sci.med)
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