Re: [Re: [Endotherms and island life (was: follow-up on sauropods)]]

[John Bois <jbois@umd5.umd.edu>]

On Sun, 7 Jun 1998 archosaur@usa.net wrote:

> jbois@umd5.umd.edu wrote:

> I kinda follow and yet I kinda don't. Reptiles and 
> birds play just as an important role as mammals when it
> comes to egg thievery. 

Varanids probably have a hard time getting into ostrich-sized eggs.  As
far as I know they don't prey on large ratite eggs.  Birds do.  The famous
case of Egyptian vulture throwing stones at ostrich egg, for example; and
birds are a serious threat to rhea and ostrich chicks, so
your question is a good one.  
Here are my guesses.  Islands are less likely able to
support top carnivores.  Vultures, for example, cover great distances to
scavenge a full meal.  I know New Zealand has hawks: I wonder if a
mammal-less island allows moa to exploit close cover and that this
protects it from the hawk. Finally, all ovipositing species suffer a
variety of predators.  I believe the inclusion of mammals in this guild
makes relatively non-concealable nests (it is _a priori_ more difficult to
hide a bigger parent at a nest) less supportable where those mammals
exist.  Yes, rhea, ostrich and emu do well on the mainland.  but they 
exploit vast semi-arid grasslands, a biome not present on islands as far 
as I know.

> Varanids are notorious egg eaters
> and are the major predator of reptile and bird eggs in Australia 
> and I think Africa too. One form of snake took to a completely egg
> eating existence. 

These snakes swallow the egg whole and digest it internally.  This would
seem unlikely for large ratite eggs!

> Also, the largest reptiles today, the crocodilians all are oviparious
> and are just plain huge. So I don't think a release from egg eaters
> would have done it.

If my idea that big oviparous species are supressed on the mainland by
mammals is right, I need to explain the existence of crocs.  I do it this
way: crocs enjoy a habitat specific advantage, i.e., they are dominant
in (yet dependent on) the semi-aquatic niche.  For example, it has been
found that racoons will avoid preying on a species of crane (I think) eggs
because alligators are nearby.  Such avoidance advantage no doubt accrues
to themselves as well.  Imagine a nest predator choosing between a croc
and a similarly-sized dry land parent.  They may well avoid the
home-field advantage enjoyed by Mother croc what with her ability to drag
you into the water and rotate you until your legs and arms are sheared off
and the blo...getting carried away here.  And then there is the crocs
ambush ability.  On dry land you can see mummy coming from afar.  Next to
the water you never know when she will drop in.

> I guess competition wouldn't either. I know what
> would though. unfilled niches. Whenever there is a niche that is not
> filled something comes along to fill it. Little monitors  going to
> Komodo, found all kinds of large animals that had no predators and they
> took advantage of it by taking over that niche. 

> Same thing for turtles and iguanas on the Galapagos. Maybe that is what
> makes small animals big?

But the iguanas on the Galapagos eat seaweed.  And they are not very
big. The tortoises are vegetarians too, I believe.
This was also true of the Elephant bird, moa.  Perhaps the only scenario
which agrees with your idea is the Komodo dragon on pygmy elephants.