Re: Carnivore Energetics: Why Are Lions Not As Big As Elephants?

[Graydon <oak@uniserve.com>]

On Wed, Jan 17, 2007 at 01:20:32AM -0800, Jura scripsit:
[snip]
> That the largest cats today (tigers) are jungle cats,
> also suggests that grasslands might be a prime factor
> in size contraint. If more large prey lived in
> jungles, tigers might grow bigger.

Since there were Pleistocene cats larger than current tigers in savanna
and grassland environments, I don't think this holds as a constraint.

Predators -- with some funky oceanic exceptions -- wind up eating the
largest things they can, which means stuff almost as high as they are in
the food web, which constrains supply.  The rough biomass factor of ten
gets into this as well -- if you're a hundred pound solitary ambush
predator, and need hundred pound prey animals (the cougar-deer dynamic),
all the bunnies going don't help you if your environment hasn't got at
least a thousand pounds of deer.  It's the seasonal minimum of hundred
pound prey animals that's the constraint.

So maybe tyrannosaurs got large because there were an enormous number of
elephant sized herbivores for them to eat, and they got bigger to take
advantage of this.  (Which goes along pretty well with the fossil
record of general size increase in the tyrannosaur lineage.

Also, mammals are constrained by gestational limits in time and
offspring size; having larger numbers of smaller offspring and caring
for them socially is the standard work-around, but it's got limits,
mostly limits to do with food supply.  (the longer it takes the
offspring to mature, the longer you're betting their won't be a
drought or other cause of curtailed food supply.  So you can't start
_too_ small, and make it up with long term care.)
T. rexes in particular might have had a more reliable climate and thus
food supply than surviving megafauna do.

Dinosaurs have a different set of trade-offs -- egg production costs
versus time to mature versus offspring numbers versus survival chances.

-- Graydon