RE: Possums was Anhingas (A Challenge)

[don ohmes <d_ohmes@yahoo.com>]

Trying to combine replies....
> Don Ohmes 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.

Tim wrote: 
> The same applies to rats, AFAIK.  Last I heard, the
> brown rat _Rattus 
> norwegicus_ is doing quite well (alas).

Don sez:
On this point I have to outright disagree.
Unfortunately I have little but personal observations
to back my opinion. That said, IMO, the brown rat and
more to the point, its wild cousins (like the pack
rat), are both relatively and absolutely athletically
superior to the possum. Rats can jump (several body
lengths, in my estimation), run, dodge and fight like
hell. Possums cannot jump, dodge or fight AT ALL
(pers. comm., Willie the bulldog, 1992). I suspect
that possums actually cannot run, in the scientific
sense of the word. I'd bet a dollar that no one has
checked...

Mike Keesy wrote:
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.)

Don replies:
IMO, possums are in a class by themselves, relative to
mammals that find/mates food by walking around, in NA.
Locomotion in possums could be vastly improved w/out
any re-allocation of relative body mass, and IMO, with
vast improvements in efficiency. Not longer legs, just
better. Possums even wobble when they walk. I have
seen apparently healthy, not-in-a-hurry possums fall
down trying to cross a dirt road. Observe an inbred
German Shepard with moderately severe hip problems, it
is the best analogy I know of.

While I agree stabilizing selective tradeoffs are the
default explanation for the apparent 'possum
locomotive stasis', I see no specific pressures that
result in stabilization at a point of seeming gross
locomotive incompetence. This forces me into
speculative fantasies of genetic bottlenecks, low
variances, and the possibility of 'plesiomorphic
gridlock'. In other words, possums are extremely
prolific maximal generalists because they are stuck
with a sorryass skeleton that has lost the ability to
evolve.

Did I mention I have no data to back any of this up?
And it is all personal obs/IMO?

See ya.

Don