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Continental predators, etc.




On Wed, 29 Aug 2001, Tyrberg Tommy wrote:

>They
>can certainly kill dogs so I suppose they might well have been able to
>deter Thylacines, Marsupial Lions and Megalania.

Gad!  So much is not known--hard to say what relationships existed, what
predators were capable of with regard to locating a well-hidden cassowary,
chasing a cass, etc. They may have deterred some of these but not others.

> Generally speaking prospective prey will often be "aggressive" when
> flight is not practicable. This is often successful since predators
> will, generally speaking, avoid attacking any prey which is even
> moderately dangerous.

Yes, if they have a choice.  This is often not the case, however.  The
fossil record is loaded with evidence of "arms races".  Such escalation
could only occur if envelopes were being pushed.

>There is a basic asymmetry here since the prey
> risks its life while the predator only risks missing a meal and is
> therefore probably unwilling to take more than a small risk.  

In tough times, missing a meal means risking your life.

> However we must make a distinction between defending Your life and Your
> nest. In the latter case desperate measures would normally not be
> selected for, at least not in long-lived species that can lay many
> clutches during their lifetimes.

Yes and no.  I think this depends on the strategy employed.  A species
that makes no attempt to conceal its nest will be discovered every
time.  It must defend against all-comers--or it will never reproduce.  A
species that successfully hides its nest should abandon to lay another
day.  So, it depends.

> Generally speaking it is probably true that predation-pressure is
> greater on the "mainland continents" than in Australia. However when it
> comes to nest-predation differences between habitats are probably more
> important. Rain-forest is probably worst of all in this respect.

How about comparing rainforest on different continents? Grasslands on
different continents (I mean, most continents have all these biomes).  So,
we still reduce to the original statement, viz, that big continents exert
more pred. pressure.

> Rainforest birds go to extreme lengths to conceal or protect their nests
> but still suffer very heavy nest-predation (there is a good chapter on
> this in D. W. Snow's "The Web of Adaption" which I strongly recommend).

Thanks.  I'll see if I can find it.

> Of course not all strategies used by birds would have been available to
> non-volant dinosaurs (nesting in trees, inaccessible cliff-ledges or
> small islands for example, though most of these should  have worked
> quite well for pterosaurs) but many would have been quite practical. For
> example burying Your clutch close to a Tyrannosaur nest might be a good
> idea. Nesting in burrows would be quite practicable for small dinosaurs.
> Colonialism definitely did occur. Extand crocodiles use both concealment
> and/or direct defence of the nest. It should also be noted that Marine
> Turtles have used the simplest strategy of all (bury them at night and
> forget them) quite successfully for 65 million years, even on
> continental beaches and using traditional sites year after year.

Which continental beaches?  Usually, these are barrier islands, or some
kind of beach which is inaccessible to most predators.  Also, I think it
highly unlikelyn that dinos practiced lay-and-leave strategy.  I know this
is suggested for saurapods.  yet, being the biggest also means nesting
sites would be easiest to spot.  Traditional nesting grounds would be
traditional egg-eating grounds.

> Also if nest predation became really bad one might expect that some
> dinosaurs would have become viviparous.

Possibly.  But remember that they still had the small baby problem: i.e.,
although they were safe as adults, their babies were bite-sized.

> Ichtyosaurs definitely were
> viviparous and many extant snakes are ovoviviparous (the eggs hatch as
> they are laid). Admittedly no birds have ever been viviparous as far as
> known (though it has been suggested that Hesperornis may have been). The
> reasons for this is obscure, but the most plausible suggestion is that
> their highly specialized breathing system makes the abrupt changeover
> from a fluid-filled to a gas-filled system at birth impractical.

Interesting

> Also it seems to me that at least large dinosaurs would be able to
> tolerate a heavier degree of egg/young predation than just about any
> other tetrapods. They were almost certainly quite long-lived so they
> would have had time to lay many clutches. Also their eggs were small
> relative to their body size so clutches could be large without putting
> too much strain on their physiologies.

I have a counter-intuitive paper somewhere which says that large ovip
sp. spend a bigger proportion of daily energy expenditure on eggs, than
small sp.  Anyway, I agree with your point.  But I feel predators can keep
up with this production (it is their fuel, after all).  this still reduces
to the defensive or concealment performance of the prey vs. the abundance,
searching and eating performance of the predator.  So why didn't such
large egg layers "re-evolve"?