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Re: FLUFF AND FALLING (fwd)
On Fri, 8 Dec 1995, D.W.Naish wrote:
> > Drop a mouse and a human from a 30m cliff, and I'm sure the mouse has a
> > much
> > higher chance of surviving, even if the human were well-padded. Similarly,
> > drop a large theropod and a small one out of a tree, and the smaller one is
> > less likely to suffer broken bones and such.
> > Derek Smith.
> Arthur Clark has said something like: 'Drop a mouse down a 30 metre well, and
> it will survive; a man will break; a horse will splash'. Ewww!
It all has to do with volumes increasing exponentially faster
than areas. One of these areas, of course, is the skin- small
animals have more of it, so they probably don't reach the same speeds as
say, an elephant, when dropped out of a plane. Probably more important is
the cross-sectional area of the bones, which determines the strength.
What happens is if you double all dimensions in an animal (scaling it up
in perfect proportion) then volume will increase 8x and areas, 4x, and
length, of course, 2x. One
effect this has, of course, is less surface area for heat loss, another,
however, is that the load on the bone has just doubled (strength being
cross-sectional and hence an area. Areas increase as the square, volume
increases as a cube). So when animals get bigger, the bone will tend to
change proportions- it will become substantially thicker, in general, to
compensate for the inceased load.
So yes, small animals like squirrels are a lot less likely to
suffer broken bones. One reason arboreal animals often tend to be small.
Another way, of course, you can decrease broken bones or injuries in
general, upon hitting another branch or the ground, is to increase
surface area: probably one function of a squirrel's tail, probably a
reason why flying squirrels might have started evolving patagiums, and
possibly a good reason for an arboreal theropod to evolve some longer
feathers (not necessarily the vaned kind).
Somebody mentioned penguins outcompeting small seals, and I'll
venture a reason why. Water causes heat to be lost at 25x the rate it is
lost on land. So it just sucks up the heat, warm-bloods in water have to
be big animals to hold in the heat. That's why there are no mouse-size or
weasel-size sea-mammals. Sea-otters are giant weasels growing to about a
hundred pounds, small porpoises get maybe five foot long (maybe a little
smaller, I don't know) and small seals aren't really that small. They
have to be big so that their proportional surface areas and hence heat
losses are smaller. Why, then, are penguins smaller? I would suggest
because they have superior insulation. Seals and sea-lions have a layer
of insulatory blubber. Penguins have that AND their feathers. They can
reduce their heat losses further, and therefore, can reduce the amount of
insulation they need to get from size. This might also explain why
hummingbirds get smaller than bats or other mammals- better insulation,
as well. This might also explain why dinosaurs don't get very small-
most dinosaurs, to the best of our knowledge, did not have hair or
feathers or other insulation, and so might have had to use size to
control heat losses.