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On 12/3/2011 6:27 PM, David Marjanovic wrote:
Bears do, however, have plenty of "adaptations" for climbing. It just so
happens that, in terms of their evolutionary history, they aren't
adaptations; some are mammalian plesiomorphies (like the fairly short
but not too short limbs and their wide ranges of movement), others are
probably exaptations. Many of these character states do not occur in the
theropods we're talking about.
Why are bear-like traits _necessary_?
That would in turn highlight a potential advantage accruing to
pre-wings, feathers and tails in an animal that climbs up a tree,
sleeps in it and then jumps/climbs/falls down -- slowing the
Why, then, don't birds look more like *Ptychozoon*?
Actually, Microraptor kinda does look like that cute little thing,
Human vertical claw-climbers use prosthetic devices to *reduce*
Yes, because we're descended from brachiators. Our flexibility isn't on
the same planet as the ballpark you can find *Microraptor* or
I don't take your point here, I guess.
BTW -- they also make it a point to keep their CG *away* from the
That makes sense if you're so big you can grasp the trunk _from behind_.
No. It is a matter of traction.
Watch a cat -- forelimbs are splayed to give lateral stability and good
claw-hold in vertical bark fissures, which necessarily pulls the
head/chest into the tree-trunk, but the back feet are kept close
together and legs nearly as extended as on the ground.
Or watch a pole-climbing contest -- note the limited planes of motion by
Note also the position of the claw on the ankle. Not a perfect analogy
to the partially retroverted hallux though...
Brian Choo's painting of *Microraptor* hunting birds shows it high up in
the crown of a tree, where the branches are thin. How did it get there
in the first place?
Does not seem likely to me -- thin branches are tough hunting.
But maybe a nocturnal bird that roosts by day might be vulnerable. You
climb up the tree, shake the limbs, the whole flock bails out, one flies
into a tree-trunk (blinded by sunlight) and is stunned...
That could work, I guess. :D
Adopting as a working hypothesis that it did in reality both climb
and sleep in trees, what evolutionary mechanism would change the pes
beyond that seen in the fossil?
Natural selection for better trunk-climbing, of course.
Why? This seems non-rigorous from the evolutionary perspective.
It could happen, of course -- assuming there is a drift in lifestyle
toward tree-foraging, or assuming the tree is frequently used intra-day
as refuge during attack -- but the selective box imposed on the pes by
terrestrial foraging in the absence of flight-powers seems quite rigid,
given the relatively high number of critical events encountered daily in
Restated -- given probabilities x and y (wherein x = the probability of
surviving a critical event during terrestrial foraging, and y = the
probability of surviving a critical event during trunk-climbing) -- the
number x^b/y^c increases aggressively as b becomes larger than c.
Assuming a critical event is equally likely to occur at any time during
a 10 hour day, and a 5 minute climb-time at day's-end -- an attack is
120 times as likely to occur at some time _other_ than trunk-climbing --
meaning that the exponent b = 120 and the exponent c = 1.
This implies that even a large advantage in trunk-climbing in a given
sub-population is reproductively swamped over time by a small advantage
in terrestrial foraging in the remaining population.
So your task is to show that there is no functional conflict between a
climbing pes and a running pes. Restated -- it seems to me that you must
show that an incremental change in the optimized basal running pes can
improve climbing talents significantly w/out diluting running talents
even a little.
I see Tim's argument that turkeys (American-style) demonstrate a
no-conflict compromise solution as weak -- temporarily flight-less
turkeys can be caught by even a normal human, and the ostrich pes shows
little or no sign of it's perching past.
Note also that a flightless trunk-climber has a built-in rapid escape
route in the relatively unlikely event that a predator attacks during
the narrow once-a-day trunk-climbing time window -- let go and scamper
off into the nearest shrubbery. Not so when chasing bugs on the ground
-- you have to outrun the bugs _and_ whatever wants to eat you.
That just makes it that much tougher to conserve whatever small climbing
advantage might happen to occur...
BTW -- the above reasoning assumes that all that is happening in the
tree is sleeping. No reproductive activities or eating in bed allowed...
There are no functional conflicts in this arbitrarily sequential
suite of feather functions: insulation --> sun shield --> camouflage
--> display --> brooding --> air resistance...
Nor is there only one valid sequence and/or combination of
Exactly. The trick is to find out which of them has actually happened.
Well, all those functions currently exist -- I am guessing even in the
same animal, in cases.
Oh, you mean which sequence happened. True...
Probably not really gonna find that out, though.
That is why I favor ranked speculative scenarios, although a truly
objective means of ranking them is not entirely clear to me.