Re: gigantism as liability

[Graydon <oak@uniserve.com>]

On Sat, Nov 08, 2008 at 07:05:19PM -0500, john bois scripsit:
> ----- Original Message ----- From: "Graydon" <oak@uniserve.com>
>> 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.
>
> But turtles have additional predator defences: a tough shell...meaning that
> increasing thickness of shell led to decreasing number of potential
> predators. Rather than enjoying such incremental predator-immunity, each  
> cohort of sauropods would be greeted by a new guild of predators. Turtles 
> have a defensive strategy; I'm only arguing that sauropods must have had 
> one too.

Newly hatched sea turtles are effectively defenceless.  (Swallowed whole
by gulls, etc. = effectively defenceless.)

By analogy, just getting bigger involves incremental predator immunity;
remember that there are many fewer large predators than small predators,
just on trophic web/food pyramid grounds; get too big for the 10 kg
predators and your risk drops by an order of magnitude, and this keeps
happening until you're eventually not worth the bother for the 2,000 kg
predators.

>> 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.
> Is there actually a constraint on producing 5 smaller babies?

Nothing big does.

Largest mammal with typical multiple births are suids, and they don't
get past 500 kg.  (Rare large individuals and domestic strains
notwithstanding.)

Whales, rhinos, horses, elephants, pretty much all ungulates, it's
single births with rare twins.  This is probably -- but I don't know
that it has been definitively demonstrated -- due to a cost/benefit case
with investment in offspring.  (Your best odds of success are dependent
on the ability of the offspring to keep up with the herd, so the
larger/fitter it is at birth, the better the odds are.)

>> Sauropod nest defence has to explain how it could happen; 2 kg verses
>> 20 tons is four orders of magnitude size difference.
> Not sure what you mean here...

The putative 20 ton parent is defending 2 kg eggs.  How -- what possible
mechanism -- does it do that, against the 5kg mammal digging away on top
of the nest?  Or the ~100 kg juvenile allosaur standing among the
hatchlings?

>> There's _three_ orders of magnitude between the adult and the
>> probable predators; how well would you expect rhinos to do, guarding
>> against foxes?
>
> There are plenty of examples of small predators picking on
> large...mice on albatross, armadillos on rhea, black-backed jackal on
> ostrich, monitor lizards on Nile crocs.

These are all at most a single order of magnitude typical difference.
(Maybe two in the case of the monitors and the crocs.)

It makes parent/nest predator interaction practical.

What you're proposing is that the ~10 kg and ~100 kg nest predators were
defended against by the ~20 tonne parents, defending the nest.  How?