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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. 

        -nick L.