Re: gigantism as liability

[Graydon <oak@uniserve.com>]

On Sun, Nov 02, 2008 at 10:48:27AM -0500, john bois scripsit:
> The other main selective pressure invoked is predation, i.e.,
> dinosaurs grew big to avoid predators. But, unless sauropods
> vigorously protected their offspring, those dinosaur babies faced a
> gauntlet of fresh predators at each size class--growing from chicken
> size through adult! This should rather be seen as a predator
> opporunity rather than a predator-resistant strategy. 

Depends.

Consider sea-turtles; no parental care, young born very small and having
to spring down the beach to the water. Most don't make it, and must then
get through many years in the ocean to reach breeding size.  Yet, absent
humans, turtles do very well.

Perhaps the essential thing was staying alive as an adult long enough to
lay enough eggs.  That would make predator defence through size a
component of the r-strategy.

> So it might be argued that mammals have fewer advantages to being
> large than sauropods, that selective pressures for large size are not
> as great for them. If not, why not?

Because being really big means really strong reproductive constraints if
you're a mammal.  One offspring every five years is not robust over
evolutionary time.

Sauropod nest defence has to explain how it could happen; 2 kg verses 20
tons is four orders of magnitude size difference.

There's _three_ orders of magnitude between the adult and the probable
predators; how well would you expect rhinos to do, guarding against
foxes?

-- Graydon