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RE: gigantism in mammals vs. dinos. (was: crocs eating hadrosaurs
On Wed, 8 Jan 1997, bruce thompson wrote:
> Forgive me if I sound thick, but I don't follow you here. Are you
> speaking only of _large_ mammals? When you say that mammals only have to
> have one offspring to maintain populations, I flash onto
> bottom-of-the-food-chain mammals like mice and voles. These of course
> breed like mad to stay ahead of predation.
Thanks for the chance to clarify.
Yes, we were talking about large mammals only. Nathan
Myhrvold used the word "limited" referring to the constraints of
placentalism on mammal size. This is undoubtedly true: for a
really big animal it is a lot easier to drop a dozen or so eggs
than to carry a monster baby within.
But then I put in my two cents saying that the use of the
word "limited" should not connote any sense of inferiority to the
mammal state of relative smallness (I didn't mean Nathan was
suggesting it did). I believe it was the dinosaurs whose size
was actually limited and limiting. At the small end they were
squeezed out of the close-cover niche by low-to-the-ground,
stealthy quadrupeds (lizards, snakes, mammals). By the end of
the cretaceous they could no longer be really small.
Generally speaking, dinosaurs were forced into a limiting
strategy of increasing size. This trend was caused by an
evolutionary arms race which centered around their mode of
reproduction. Notwithstanding the many possible strategies to
reduce the value of each egg and therefore the effect of egg-
predation on overall fecundity--these could include laying many
small eggs, double, triple clutching, etc.--dinosaurs were more
likely to employ strategies of defence over abandonment. For any
egg layer whose nest is discovered, there must always exist the
choice of whether to abandon or defend. The response to a
particular threat should follow some genetic optimal strategy
rules. These would vary from animal to animal. However,
generally speaking, the bigger the animal, since its new nest is
more likely to be discovered, the more likely it is to choose
defence over abandonment. If this modest claim is true it MUST
set up a selection pressure to become bigger than those who would
eat your eggs and/or offspring.
It is absurd to suggest that a significant number of
dinosaur species readily abandoned their nests. Such a species
would be providing a kind of moveable feast where egg and offspring
predators would dine at one abandoned nest after another. There is
also evidence of site fidelity in some dinos (colonies and such).
I admit that predation out of breeding season might also
fuel increasing size. But if dinos didn't have to nest they
could have resorted to speed to escape predators. Mammals have
the luxury of doing just that! Most avoid predation simply by
running away. And, of course, they can use the same strategy
during pregnancy. Predation out of breeding season, therefore,
doesn't force gigantism, egg predation on big animals, by big
animals, does. This is a crucial distinction!
Three quick closing points:
1. I realize there appears to be some circularity to this
argument, i.e., if you are big you will be forced to be big. I
would counter this by arguing that at some point there is a
threshold of discovery--perhaps the point at which your second
clutch is more likely to be discovered than not--and that animal
size influences this threshold (perhaps _determines_ it, though
behaviors such as constant nest attention, nest siting, etc.,
must be important too). Pressured from below by _small_ enemies,
as a clade, non-avian dinos had an intrinsic tendency to grow
toward that threshold. Once above it, the runaway evolutionary
arms race could begin in earnest.
2. There are economies of scale for being big (less energy
required/kg). But these must be balanced against such things as
more total food mass required, mechanical stresses on bones and
muscles, blood pressure considerations, etc. I believe dinosaur
size was ultimately a pathological condition _forced_ by the
constraints of their reproductive mode. In this sense mammals
are not "limited" to non-gigantism.
3. I take as evidence for my arguments the success of the
placental strategy (in the relevant niches) today. Others have
said, no, mammals are successful because of their energetics and
their dentition. I don't know how one could ever prove this one
way or another. Perhaps it is only possible to convince by force
of argument. The above arguments are my best shot!