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Re: Subject: Bird hibernation/torpor
On Sat, 22 Mar 1997, Ronald Orenstein wrote:
> Their ability to excrete uric acid allows them to conserve water quite
> efficiently - but I would hardly call the desert an avian "refugium" but an
> area they have successfully invaded.
Who can adjudicate the distinction you have made? One would have to be
present at the moment of desert colonization. Were mitochondria
originally engulfed by cells and became immune to their enzymes, _or_ were
they immune invaders to which cells in turn became immune? When
vertebrates first made it onto land did they enjoy some freedom of
predation from the fishy seas or were they trying to exploit a Garden of
Eden? Probably both, right? Again, the complex interplay of evolutionary
"forces" is close to beyond the power of language to describe.
But how about this: ostriches, with their enhanced osmoregulatory
abilities were able to range and reproduce in habitats unavailable to
many of their predators. This being true, desert ostriches enjoyed
greater reproductive success than tropical rain forest ostriches.
> This is so dependent on behaviour I wouldn't care to say. Wre they diurnal
> or nocturnal, for example? Who knows? Modern desert animals include
> birds, mammals and reptiles, so who can say what is "best"?
So, in this welter of selective forces, what is there that has
specific relevance to this list? My non-stealthy nest idea was
not a "preconceived notion". It derived from the relatively
simple observation that nearly all of today's egg layers are
stealthy. Then came the thought: perhaps this apparent fact had
relevance for who were the winners and losers at the K/T.
Challenges to this idea have been serious. One of the more
serious is this: How can you say that large non-stealthy egg
layers were doomed in the cretaceous when we have a history of
rampantly successful creatures such as phorusrhacoids, and we
have today quite successful non-stealthy egg layers such as
ostriches and emus.
Now I have come up with a parsimonious idea which can well
account for the "terror birds" viz.., they could hide their nests
in the tall grasses of the savannahs. These hiding places were
not available to dinosaurs because there were no grasses at the
K/T. This idea was jarred lose by the thread on hibernation:
perhaps the big birds of 40 mya lay low for extended periods of
time just as the emu does! In any case, they did have an
enhanced stealth capability and it was afforded by grass.
This still leaves ostriches. And I think an explanation
might again be found in the grasses. I am going to argue that
deserts were not as available to dinosaurs as they are to
ostriches; and that this meant they could not exploit or hide in
areas as far away from those who would eat them, their eggs, or
their offspring. I base this hypothesis on the idea that because
of grasses the margins of the desert are much more productive
than they once were. Ostriches depend on grass and the things
that live in it. In cretaceous times these marginal areas were
probably covered a lot more sparsely than today. Drought
tolerant forms existed, low shrubs and such, but there was
nothing like the grasses in their productivity. The upshot of
this was that although some dinosaurs may have had the osmo and
thermoregulatory abilities of ostriches, they could not extend
their range as deeply into the desert as can ostriches because
there was nothing for them to eat. They were therefore closer to
those who would eat them.