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Re: Upper size limits in dinosaurs and whales
>I wonder if anyone out there can help me. I am putting the final touches on
>a children's question-and-answer book on nature, which has involved me
>checking some text I wrote over a year ago, and (with a publisher's deadline
>breathing down my neck) I find I have written myself into a bit of a corner.
>The question I posed was, simply, "How can whales grow so large?" and my
>answer was that the buoyancy of water permits a much larger body size than
>would be possible on land. I even said that a hundred-ton animal on land my
>be crushed by its own weight.
>Well, of course, along comes David Gillette's book on Seismosaurus, with
>estimates in the hundred-ton range for both Seismosaurus and Ultrasaurus.
>So much for my point.
>What I would like to know, though, is: are these weight estimates generally
>accepted? Has anyone analysed the mechanics of such a beast? And is the
>reason a large whale dies on the beach independent of size (as it might be
>because even dolphins die during strandings, I suppose).
>Anyway, I would appreciate any guidance on this. Here is my original text:
I read your message on the dinosaur listserver today, and since I
need a short break from working (procrastination!) I thought I'd send you a
few thoughts on your question. You might have a look at a very good book
on engineering in large animals called 'The Dynamics of Dinosaurs and Other
Extinct Giants' by R. MacNeil Alexander (I think I got the title correct -
I can't seem to find my copy).
The problem of why some animals evolve to be very large is one that
probably does not have a simple answer. Or at least, not a very satisfying
answer because in part I think that it is safe to say that animals become
very large because they can. Gravity is certainly a big problem, but the
dinosaurs prove natural selection can sometimes find engineering solutions
to overcome gravity. For some animals such as insects, large size may be
unattainable because of engineering constraints. If you think for a
minute, there are no really large insects. Even the giant dragonflies of
the carboniferous were really not much larger than your average bird, and
probably weighed a lot less. Insects are limited in size by their own
physiology - the chitinous exoskeleton becomes unable to support even its
own weight when scaled up to lengths of a foot or more. Also a problem for
insects is their method of respiration. The system of holes and tubes that
provide oxygen to insect tissue becomes very inefficient at large sizes.
So, giant grasshoppers climbing skyscrapers in Chicago is something that we
are never likely to see, except in the movies. Along these lines, it is
interesting to note that the largest arthropods are marine arthropods who
have both the support of bouyancy to take the load off of the exoskeleton
as well as gills to increase the efficiency of respiration.
Back to whales. Whales probably did not become large because
bouyancy permitted them to. Certainly it helps in that the skeleton of
whales is freed somewhat from the problems of support and can be modified
in other ways to enhance the efficiency of an aquatic existence. Natural
selection is not all-powerful in its ability to solve engineering problems
- it does require that an organism's structure contain the seeds or the
potential for particular adaptations. However, regarding size in
vertebrates, I think that we need to restate the question. Instead of
asking, 'what prevents many animals from becoming large?', let's ask, 'What
encourages some animals to become large?'. The history of vertebrate
evolution seems to be full of lineages that progressively increase in size.
So, what is so good about being large? I can think of a few
possibilities, perhaps others can think of more.
1. Escape from predation. The bigger you are the harder and more
dangerous you are to kill and eat. Certainly this applies to whales?!
Many researchers think that it applied to sauropod dinosaurs.
2. Energy efficiency and the old surface to volume ratio problem. If your
tissue generates heat (via true endothermy, muscular activity, or whatever)
then the bigger you are the less easy it will be for this heat to escape
out across your surface (provided you are some vague approximation of
rounded - large flat animals need not apply). This explains why arctic
animals such as polar bears tend to be more massive relative to their
temperate cousins - you have to eat less to maintain the same degree of
3. Sexual selection. In cases where there is intrasex competition for
mating opportunities, large size is often an advantage because, a. it is a
sign of fitness that can be recognized by the sex doing the selecting, and
b. it increases your chances of winning in a contest over mating
privledges. If larger animals preferentially mate, then this should drive
selection to favor genes for large size.
4. Survival in general. In many animals large size may be selected for
because large animals are often simply more robust - more difficult to
injure, more difficult to starve, etc.
Of course, the above is only a start at half the answer, because there is
still the other half of the question: why is it that large animals often
seem more prone to extinction? Comments anyone?