Re: Back-evolution of limbs.

[Ronald Orenstein <ornstn@inforamp.net>]

At 08:43 PM 27/06/98 -0400, you wrote:
>I don't know whether the following has been discussed before, but:
>
>Why is it that birds--and only flying birds at that--are the only
>creatures to return to their former mode of locomotion, i.e., they may
>become secondarily flightless.
>
>I haven't heard of a bat or flying squirrel doing such a thing.

One answer is that birds, unlike other flying or gliding tetrapods (I assume
you know that flying squirrels don't fly!) have not involved their hind limbs
in the wing or gliding surface, leaving them unhampered for other uses.  Thus
even many flying birds may fly seldom, moving about instead by running,
swimming etc.  Under such conditions flightlessness is a relatively small
evolutionary step.  It is not "returning to a former mode of locomotion",
merely giving up one form of locomotion while retaining another that is
already
well developed and used.

Another answer is that the mystacinid bats of New Zealand, though not
flightless, may well be heading that way, and that there is a "flying
squirrel"
(Zenkerella) with no flight membrane, though whether this is a secondarily
derived condition or not I do not know.  It is actually not a true squirrel,
but one of the African scaly-tailed squirrels (Anomaluridae), all of the
others
of which have gliding membranes.

>And I don't know of any swimming mammal or bird which has returned to a
>terrestrial existence.  Why, for example, has no seal colonized some
>competition and predator-free island and redeveloped structures for
>walking?

Once again, this is a MUCH bigger environmental reversal than flightlessness. 
Remember that in birds the difference between flightlessness and flying
ability
sometimes occurs at the subspecific level (eg the White-throated rail
Dryolimnas cuvieri, whose Aldabra race is flightless) - the change is very
slight. Another corollary of this, which contradicts some of your points, is
that flightlessness can apparently evolve very quickly in birds under the
right
conditions.

Also - what would a seal gain by doing so?  What would it eat?  In most areas
where seals occur the sea is far more productive of animal food than the land,
and seals aren't just swimmers - they are modified to eat seafood.  Why
reverse
this for a much sparser food supply?  For birds there is no such shift - I
know
of no flightless bird (with the possible exception of the extinct Stephens
Island Wren Xenicus lyalli of New Zealand) that evolved from aerial-foraging
ancestors, and most eat much the same foods as their flighted cousins.

>I wonder if this apparent unidirectionality to limb development (if it
>exists) has any ramifications for the current dino-bird debate.  That is,
>something that once flew should not, in the presence of competition and in
>the absence of a _very_ long time, should not redevelop fully functional
>limbs!

Well, it seems that the phorusrhacids did just that!  But the real question
for
dino-birds is, how much functionality for activities other than flight was
given up when flight developed?  If the limb still retained a reasonable role
in climbing, object-handling etc the transformation could well have been
fairly
rapid - assuming it occurred at all, of course.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net