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Re: Extinction once again (crocs)




On Mon, 19 Feb 2001, Adam Britton wrote:

> Water is very important for crocs (and some dinos?) for three principal
> reasons:

Agreed on all of these.  And thanks for exposition.

> In many croc species, it's clear that the adult female is selecting nesting
> sites for her own benefit, and not necessarily because they are suitable
> places to incubate eggs.

I don't know how this could be known.  Wouldn't you have to be a croc mind
reader?

> I agree - I'm not saying nests aren't close to water (see above), but
> they're not necessarily in "wetland" habitat. I'm arguing that crocs have
> some adaptability in their nesting strategies. The water area they need may
> be quite small and restricted, or part of a drying chain of pools, but at
> the very least there has to be a muddy pool and some moisture in the
> substrate.

Yes.

> Although your hypothesis makes perfect sense, aren't you forgetting birds?
> Nest-defence is a widely-adopted strategy in the avian group. If you've ever
> tried to approach a goose nest in the middle of a swamp, you'll wish you
> were being attacked by a crocodile instead of the tornado of beak, feathers
> and feet that greets you! 

I t may be that geese situate nests partly to reduce predator
density.  Certainly, reproductive collapse of (snow geese?) occurred when
predator-free islands were populated by arctic foxes.  So, I argue that
geese, unlike crocs, situate nests out of harms way and defend them when
necessary.  But, as you say below with regard to crocs and humans--nesting
species have a sense of when to fight and when to abandon the nest to lay
another day.  On this run/fight continuum, crocs are at the most radical
extreme of any species: rhea defend against caracara hawks but give uo to
hairy amadillos; ostriches defend against black-backed jackals in the day
time but give up the nest if attacked at night, and so on.

> Defending female crocs tend to use a variety of warning signals, and a quick
> bite if it comes to it, rather than rending predators limb from limb. People
> who've been attacked by nesting females usually escape with their lives -
> the croc isn't interested in feeding or killing, just repeated warnings.

Interesting.

> > As above, croc nests more secure than dinosaur nests (assuming active nest
> > defense was the prime strategy of at least the largest dinosaurs).
> 
> I don't follow you. If large, predatory dinos practiced nest defence like
> crocs, what makes them more susceptible in this context?

Crocs were more tied to water (for all the reasons you gave earlier) than
dinos.  But I argue that the freedom to exploit a greater diversity of
nest sites came at a price--security.  In terms of staging defense,
all other things being equal, dinos
were on an equal footing with other dinos trying to attack them.  Crocs,
semi-aquatic specialists, had a kind of home-field advantage.  I can think
of several factors which might come into play: foothold in moist sand,
ability to drag predators into the water and spin them around--I know you
said that most defense consists of threat display, but I would argue that
potential predators are aware that this is not an empty threat.