Re: Possums was Anhingas (A Challenge)

["T. Michael Keesey" <keesey@gmail.com>]

On 5/3/05, don ohmes <d_ohmes@yahoo.com> wrote:
> 
> They are prolific, omnivorous and adapt well to urban
> environments. That explains how they survive in their
> present form, but does it explain why they don't
> "improve", particularly relative to locomotion? They
> don't climb well, run well, or even walk well relative
> to other mammals. Granted, it is not possible for
> selection to change a trait w/ zero variance, and the
> difference between a fast and a slow possum may be
> effectively zero relative to predator speed. But do
> cast-iron stomachs and prolificity somehow mean
> locomotive competence is disadvantageous? Even possums
> have to compete for food.

Developing greater speed, agility, etc. requires more resources than
developing stubby little 'possum legs. An opossum with longer legs
might be able to evade predators or catch prey more successfully than
other opossums, but it would also need to eat somewhat more to
maintain those larger legs. Apparently for opossums (and many many
other species), this is not an advantageous trade-off. (See for
yourself--they're doing just fine.)
 
> That is very interesting. Thanks! If possums are a
> young species, that negates the basic premise ("little
> or no change since 'way back when") of the question
> right there.

Not necessarily. Even if _Didelphis virginianus_ has not been around
all that long (relatively speaking), you could still argue that the
closest common ancestor of _D. virginianus_ and most other therians
would have more resembled _D. virginianus_ in gross anatomy,
lifestyle, etc. than those other therians. That is, it's a recent
species, but a fairly conservative one (as far as I am aware--I'd be
delighted to hear any evidence to the contrary--it does seem odd they
they alone among marsupials have successfully invaded North America).

> Saying "oh possum" can get you run out of town where I
> come from...can't be high-hatting folks like that. :)

Well, then just say "'possum" instead of "possum"--the difference to
the untrained ear is indistinguishable! ;)

--Mike Keesey, who says "'raptor" instead of "raptor" when referring
to dromaeosaurids (and even that sparingly)