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Re: Extinction once again (crocs)



From: "John Bois" <jbois@umd5.umd.edu>
Sent: Monday, February 19, 2001 3:57 PM


> I agree, and this is a very curious fact.  What is so important about
> proximity to water?  It's not for the sake of the eggs--the eggs can be
> placed in sand away from water (presumably).  Anyway, alligators building
> nest mounds of vegetation could do it away from water if they wanted to.

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

1. egg physiology: croc eggs require high levels of moisture during
incubation, so the surrounding substrate must be moist or the eggs will
dehydrate and the embryo inside will die. Obviously moist substrate is
common close to water.
2. adult physiology: adults require access to freshwater during the
incubation period. They might not feed for 2.5 months while they guard the
nest, but they definitely drink to avoid dehydration. They also need to
thermoregulate, which is most efficient when water is available - if the
nest is too far from the water, the female's extended absence may invite
predators
3. hatchling physiology: newly-hatched crocs require access to freshwater,
not only for their physiological requirements, but also for shelter and
defence at their most vulnerable life-stage. Hatchling crocs are basically
water's-edge animals (edge = shelter, and easy access to terrestrial
invertebrate prey)

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.


> Well, there may be competing factors here.  Nest defense is a big deal for
> Nile crocs.  So, proximity to water may also be favored so that parent
> unit can be close at hand.

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.


> How is it that this obligatory nest defense of easily
> discoverable eggs works for only one taxon?  Why hasn't it evolved in
> other species?  A reasonable hypothesis is that the crocs enjoy a home
> field advantage over would-be nest robbers (I realize they suffer heavy
> predation, anyway).  That is, predators are relatively less likely to prey
> on a croc nest than (for example) a hadrosaur nest, because they are more
> likely to become prey themselves.  The crocs can be waiting in ambush, can
> drag unsuspecting predator into the water, shred it to bits in seconds,

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! However, I agree with your overall hypothesis - if
you're no threat to a potential nest predator, you'd better make sure you
and your eggs can't be found. Conversely, crocs (and birds!) are very good
defenders of their nests and the strategy works well, and really it's only
the sneaky predators (small varanids raiding crocs nests, snakes raiding
bird nests, for example) which succeed in raiding nests most often when the
defending female poses the greatest threat. Large mammals (eg. big wild
boar) can often put up a good fight against a female croc, however, and many
bird nests fall prey to suitably belligerant predators. Interestingly, here
in Australia it's speculated that Johnston's freshwater crocs flee more
readily from their nests because of 20,000 years of nest-raiding by
Aboriginals. In that case, the predator is quite capable of skewering a
defending freshwater croc with a spear, and hence vigorous defence is not
selected for as a nesting strategy. Sounds reasonable if nothing else.

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.


> Any other ideas on the value of semi-aquatic niche for this
> all-but-defunct strategy, i.e., obligatory nest defense?

I wouldn't call nest defence an "all-but-defunct" strategy, if you consider
the birds (including those in semi-aquatic niches).


re: survival of crocs at K-T...

> 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?

Best wishes,

Adam