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Egg Layers etc



 John Bois wrote:

>
> 1. Mammals carried their young from place to place inside their
> body.  They therefore had fewer restrictions on their foraging
> time.  The extra resources thus gained resulted in healthier

Many birds _share_ the nest guarding and foraging, so that the nest is never
unattended and the parents feed. Indeed, in some species (hornbills,
seabirds) one of the pair brings back food for the one standing guard. You
could say there are three aspects for egg hatching:
- guarding,
- feeding the guard
- feeding the offspring only when hatched.

These can all be performed by _both_ parents, at all stages.

It seems perfectly reasonable to suppose dinos could have evolved similar
strategies.

For placentals two of the three processes can only be performed by one parent
(feeding the guard could be either parent, but this seems to happen only in
carnivores):

- guarding (inter-uterine)
- feeding the growing embryos

Only when born (and weaned) can both parents become involved. So the tasks can
only be performed by both parents for some stages.

So I do not see any advantage in being a placental here.


> offspring _and_ healthier adults.  This latter benefit could be
> cashed in on more future offspring.  When the baby mammals were

I do  not see why this follows......


> the Cretaceous, they had higher reproductive rates than their
> non-stealthy egg-laying competitors.  While the dinosaurs dealt

speculating a bit....

> For
> example, searching for an open-field egg, or even a hidden-under-
> a-bush egg, is easier than hunting for eggs in trees.  This is
> because a tree, occupying three dimensional space, has a greater
> surface-area for hiding places than a two dimensional patch of
> ground.

I disagree. A nest is essentially a "tussock in the air" for containing eggs.
Identifying nest silhouettes against a background of sky is a lot easier than
against the ground. Eggs in trees can only be in one of two places: a nest or a
hole. Seems easy to me; a tree represents a fixed place to look. The tree is
_not_ safer because of its 3 dimensional surface area. Eggs may only be safer
because only a limited number of predators can get at them.



> many nests hatch at the same time.  Predators take only a small
> percentage of the broods and most escape to the succour of water.

Not so. For leatherbacks, for example, the predation is horrendous. Predators
(birds, cats, pigs, man, fish, sharks) seem to know when hatching occurs, and
line the beaches and offshore for the annual feast. The reproductive strategy
here is many laid, few survive.

> These animals have one traditional strategy: immunity from
> predation.  Armoured bodies, even armoured egg-shells in some

Young turtles are soft-bodied. And any animal tacking a crab can tackle a baby
turtle.

> most dinosaurs were _large_.  Trying to hide the egg was absolutely
> pointless.  They must try other defences.  But this is a very clumsy
> proposition.

And one of those strategies could be the collective parenting of eggs as
described above (and perhaps handed on to avaians).

> don't know.  But I know that ostriches have a terrible time getting
> by.  They have many clutch failures--I don't know if they suffer egg
> predation or not.

"Terrible time getting by" doesn't seem to stop them from exiting though,  does
it? Thye're not going extinct to my knowledeg> I think you also need to do a
check on reproductive rates of mammals. Remember that live young are only the
"successful" clutches. You don't see the unfertilised pregnancies, aborted
foetuses, stillbirths, unviable offspring (humans, too!), infantile deaths.
Being a mammal is no guarantee of successful reproduction.

> Look, the scarcity of ratites vs. stealthy egg layers is so stark, this
> has to tell you something about the effectiveness of the comparitive
> reproductive strategies.  Stealthy egg-layers win, 8600 to 4.

It tells me that if I grow longs legs I'm going to exploit a niche not used by
8596 other egg layers.


martin