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Re: book reviews, parsimony and some loose ends



Stan Friesen wrote:

>One point/possibility I came up with while recuperating from surgery is
>that there are essentially *two* types of extant arboreal animals (other
>than flyers and gliders).
>
>One type is the typical arboreal quadruped, exemplified by squirrels,
>lorises, and even most monkeys.
>
>The second type is the long-armed brachiator/leaper. This type has
>evolved at least three times that I know of (all in primates, to be
>sure): apes (especially gibbons), spider monkeys (new world), and
>the indri of Madasgascar.

I wish you luck on your recovery, but I can't agree with your
classification.  First of all, it only fits mammals - I'm not sure how
arboreal reptiles would fit into this (snakes, for example), just to stick
to tetrapods.  And what about sloths, just to name a mammalian oddball?

>Now it is interesting that these long armed arboreal forms tend to
>use a bipedal walking stance when on the ground (invariably so
>for gibbons).

Chimps and gorillas are usually quadrupedal on the ground, though their
long forelimbs give them a fairly upright stance.  As for the Indri, it is
really more of a vertical clinger and leaper (cf Mittermeier et al, Lemurs
of Madagascar (1994)) than a brachiator.  It occasionally comes to the
ground to eat earth, but I do not know how it gets around when there.

>However, in a brachiator any airfoil would tend to be concentrated on
>the forelimbs, which will also already be strongly muscled and mobile.
>Is it possible that this might overcome the difficulty in transitioning
>from gliding to powered flight?

I think this is flawed by the attempt to use a mammalian body plan to
arrive at a bird.  I would be astonished to hear of a reptilian brachiator;
I suspect that bird ancestors, if they indeed were dinos, were bipedal
BEFORE becoming arboreal and used their hands as climbing assists rather as
the hoatzin chick does today.  Certainly I find it hard to imagine a
brachiator evolving towards loss of grasping function in the hands!  I
suspect that the prime locomotor apparatus in bird ancestors must have been
the hind limbs, not the forelimbs, as the latter would have had to be
somewhat free to evolve for other functions.

Of course bats get around quadrupedally, too.....