Re: Bipedalism in different substrates

From: J Moore (anthrosciguy_at_yahoo.com)
Date: 06/22/04 

Date: Tue, 22 Jun 2004 21:11:17 GMT

Pauline M Ross <pmross@ross-software.co.uk> wrote in message
news:i2ifd0p88bkod6cr81t8ied4r652bfj9rd@4ax.com...
> On Mon, 21 Jun 2004 22:09:34 GMT, "J Moore" <anthrosciguy@yahoo.com>
> wrote:
>
> >There's this new, highly radical idea in evolution called "phylogeny". :)
> >Why are there animals with different shapes? Why indeed; why aren't they
> >all the same, each and every one -- why is there NOT just one species of
> >animal in all of time, in all the world? That's what you're asking me?
(It
> >is actually what you're asking me, although I'm betting you didn't intend
> >that question when you wrote the post.) There have been all sorts of
sizes
> >and shapes of animals that have evolved; obviously there is a great range
of
> >sizes and shapes that can work quite well for animals.
>
> No, that isn't at all what I'm asking - quite the reverse. I'm saying
> that every species has the body shape and size which best suits it for
> its particular way of life, taking into account diet, climate and what
> else lives in the neighbourhood. Naturally they vary widely. But
> species with a similar way of life tend to have a similar body shape.
> The interesting question is why some are different.
>
> The standard-issue browser/grazer has a moderate-sized body (dependent
> on the gut needed for its diet), moderate-sized legs, a moderate-sized
> head and a neck long enough to reach whatever it eats. That body plan
> scales up or down as needed, and the details can be varied somewhat,
> but there isn't a huge difference in shape between (say) a muntjac
> deer and a bison. The size varies, but the proportions remain the
> same.
>
> Then you get elephants and rhinos (let's leave hippos for the moment;
> let's stick to dry land). They are larger than almost all other
> terrestrial mammals, they have barrel-shaped bodies and the rhino has
> those short legs. Why? What is the problem to which a rhino-shape is
> the solution?
>
> >[Snip] One big (no pun etc.) advantage: once you're grown, you have
> >little problem with predators (at least since the larger sabertooths died
> >out).
>
> This is true, but how do you get to that point? How does an average,
> predator-prone herbivore ever get the chance to grow to the point
> where predators are not a problem? If there's a chance of being eaten,
> you have to maintain the defences - be slim enough and fast enough to
> run away, be small enough to hide, be agile enough to nip up a tree,
> be populous enough to stay in a large herd, and this constrains the
> size and shape.
>
> >[Snip] Lastly, is losing fur really a
> >sacrifice? I mean intrinsically. Isn't it more something which seems
> >useful to some and not for others, which in fact seem to get along fine
> >without it. Not a sacrifice to lose; not a sacrifice to have -- unless
you
> >need it and don't have it, or have it and don't need it.
>
> Well, virtually every other mammal species seems to find it useful :-)
> Apart from some aquatic, armour-plated or burrowing species, only four
> groups of terrestrial species have abandoned it to any degree -
> elephants, rhinos, pigs and great apes. It seems to work well in every
> environment, from desert to forest to the polar regions.
>
> And even if we say that elephants and rhinos lost their fur because of
> the heat-dumping resulting from their size, that still leaves the
> problem of pigs and great apes (especially humans); why do you think
> they lost (or partially lost) their fur?
>
> This is not a trick question: I genuinely would like to know how you
> (or anyone else who wants to jump in) account for fur loss in humans.
> We hear enough about aquatic explanations; I want to know what the
> non-aquatic explanations are.
>
> --
> Pauline Ross

First, on pigs: remember that one, and only one, wild pig has little body
hair (and it's one of two wild pig species that takes to water a lot, the
other being very hairy, so the "it's aquatic" argument doesn't hold up
well). Domestic pigs are not a good example of a "hairless" mammal, any
more than other domestic animals are, such as some chihuahuas or hairless
cats -- they were bred for that feature.

You've got a couple other questions there, and I doubt I'll get a really
good, thorough post on them, but here goes with a few points. I don't have
a good, offhand response to the question of how an animal gets to be really
big through its evolutionary history. About being predator-prone, though.
One point often forgotten is that defense against predators often, with many
animals, involves offensive behavior, even with relatively small potential
prey versus larger predators. This is also how herding can help with
animals which are somewhat big if not really large. In fact, even with
solitary animals, this is done; counterattack is often enough an effective
strategy (although almost completely ineffective against certain types of
predators, such as crocodiles). Giantism has been in the past something
which is/was not uncommon at least (many presentday species had much larger
relatives in the ancient past -- sloths, bison, bears, crocodiles).
Interestingly enough I think (could be wrong) that presentday African
elephants are the largest elephants ever; if not they're really close to it.
So that's really a non-answer I'm afraid, but some points that might be
helpful in thinking about the question.

I think the business about apes having less hair than other primates is
interesting and rarely addressed. Now there's a good reason why this sort
of thing is rarely addressed, and that's because we haven't been able to see
any means of proving a reason, so we end up with essentially cocktail party
conversation instead of science. It's possible that things like the recent
idea of looking at species of lice and their genetic history might help
solve some of the questions, but it's still pretty early to tell if that
really provides even the "when" answer the researchers hope it does/will.
"How" and "why" answers are even more elusive. In apes we definitely see wh
at seems to be a progression with lessened body hair, and this is one of the
things that lead us in the past to make those ladders of evolutionary
progress with us at the pinnacle, since the progression of that and other
features happens to make a sort of pyramid with us at the top (where,
naturally, we belong, according to most of our old philosophising, and that
certainly helped that idea along). But I don't think anyone has a good idea
why apes show this progression -- it could be size, as the apes tend to be
on the large end of the primate size scale.

As for humans specifically, we do see rather obvious signs that our body
hair features indicate a sexually selected feature. There is the
differences between people in different areas, differences between the
sexes, differences during our lifespan, with the most dramatic differences
happening between the sexes at puberty and continuing through the fertile
stages of our lives. This screams out sexual selection (by contrast, a
feature due to natural selection from a lifestyle that all members of the
species shared would not tend to have such dramatic sex and lifespan
differences). Coupled with this is the soemwhat related features of our
sebaeceous glands. There is also a large difference between male and
female humans in the number of their sebaeceous glands. Males have a lot
more of them -- as they do apocrine glands, which are also used as a sexual
"lure" -- and complimenting this, females have better scent receptors. And
these sweat glands also become most active at puberty and after. It's also
related to hair, since these sebaeceous glands tend to be most prevalent in
those spots where body hair is present, where the scent can be more
effectively spread because the hair helps -- this is also something seen in
other animals, including primates, with scent glands.

There may well be some other reasons for the differences between apes and
monkeys, and between humans and our ape relatives, but I don't see, at
present, a means of testing these, so they aren't really very effective as
science. It's possible that we can, at some point, figure out how to test
these ideas (figuring out when the changes actually occurred might help) but
until then they remain interesting speculations. But we can see sexual
selection written all over our presentday condition when it comes to our
hair.

JMoore
__
For a scientific critique of the aquatic ape theory, go to
www.aquaticape.org

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