More egg-laying v. live-bearing [kinda long]

[djsmith@ccnet.com (Derek Smith)]

On May 26, John Bois wrote:

>Then let's compare the strategies of an open-field egg layer
>(dinosaurs) with those of a modern day open-field animal (i.e., big,
>live-bearing mammals).
>
>1. Dinosaurian nest defenders must return to the nest every night to
>provision the guard.  Mammals do not have to do this.  This may be
>important in maintaining a big secure herd.  Mammals can range all
>day, sleep, and move on the next day to ever-fresh patches.
>Dinosaurs, having to start from the same place every day (at least in
>breeding season), must not be able to support as big (secure!!!) herds
>because they tend to over-graze the range around the nest site.  In
>this regard, any environmental stress would selectively target the
>dinosaurs!

This argument ONLY applies to herding mammals, you realize.
Non-herding mammals, like, say, gophers, still must venture forth from
the nest to find food, and return to it later.  In the meantime, if
the young are discovered, they're most likely going to get eaten.  As
in modern mammals, not all dinosaurs were of the herding variety.
Sure some, perhaps many, of the herbivores were, but that is because
safety-in-numbers is a viable strategy for survival - one which
herbivorous mammals utilize as well.  How, then, does your argument
apply to carnivorous mammals, which usually establish territories from
which they do not stray?  These animals DO have to go out every day to
find food, and return to the "nest" at night!
Also, there is a big difference between a herd of 20 large,
herbivorous dinosaurs and a herd of 2,000 large, herbivorous
dinosaurs, in terms of environmental impact.  General question for the
dino-experts out there: What info do we have about the "average" herd
size for hadrosaurs, ceratop(s)ians, etc.?
If the herds were kept small, then the environmental devastation that
John refers to may be nothing more than another just-so story.  After
all, these dinosaurs DID herd together, they DID nest together, and
they DID survive the breeding season without lethal over-grazing for
millions of years!  Thus, I don't agree that any environmental stress
will selectively target "dinosaurs" (what John really meant to say, I
am sure, is that such environmental stresses would selectively target
HERDING dinosaurs, but even that I disagree with).  Consider a group
of large tree-browsers, like a sauropod herd.  Now, suppose that some
blight struck the forest in which they lived, during the brooding
season, just after most individuals had layed their eggs (assuming
that's what they did, of course), so that most of the trees died off.
What are these poor sauropods to do?  According to John, they die,
because they have to stay and protect the nests.  I say that they
would probably just move, be it breeding season or no.  Most animals
have self-preservation instincts (humans tending to be a noteable
counter-example), and would probably sacrifice this year's brood than
stay and starve (probably resulting in the death of the brood as well,
despite their efforts!).  After all, there's always next year.

>2. Guarding a nest is less secure than carrying babies within an
>organism.  This is true for at least three reasons, a) A parent
>guarding a nets has two choices: stay and fight, risking injury and
>loss of nest, ; or abandon the nest to the predator.  On the other
>hand, an open-field live-bearer is just not a target for an offspring
>predator.  If they themselves are attacked they simply run away with
>their selves and their babies intact

And carrying babies around with you seems to me to be a liability as
well!  If a pregnant female is eaten, the carnivore essentially gets a
2-for-1 deal - mother AND unborn young!  That female can no longer
reproduce, for obvious reasons.  An egg-layer will simply deposit
eggs, without much of a gestation period.  If the eggs are threatened,
the parents can actively defend the nest, using whatever methods the
particular species concerned uses.  This assumes, of course, that the
parents ARE, in fact, nest-defenders. They could just as well lay
several eggs and let the young fend for themselves.  And, John, I'm
afraid open-field live-bearers ARE targets for predation.  Remember,
the easiest catches are the old and the very young.  If everyone runs
away, then the young are virtually assured of being eaten, because
they simply cannot run fast enough to keep up with the adults of the
herd.  Thus, they a) have no protection, and b) become targets by
default, because they are easier to catch than the adults.  This is
true whether the young were born live or just hatched or whatever.  I
don't see how this supports your arguement in any way.

>b) Egg guards must risk injury
>from intraspecific competition for nest sites.  I have a video
>(National Geog. _Amphibians and Reptiles_) of 2 Galapagos marine
>iguanas fighting over a nest site while a sea bird eats the defender's
>eggs!  Live bearers in the open-field avoid these risks altogether.

And what of the baby seals who get crushed by dueling bull seals?  ANY
guard must risk injury, whether they are guarding eggs or live young,
or fighting over nest sites!  That's just common sense.  The young are
the easiest targets, so that's what a predator will go for first.  If
the predator is a particularly nasty one, like a lioness, then any
animal which chooses to defend its young had better be at least a
match for any potential predator; otherwise, the predator gets another
two-fer.
Any animals which congregate together in great numbers pose a threat
to the young, simply because the chance of getting stepped on,
crushed, or whatever, increases with the number of individuals
involved.

>c) Time from laying to viability is _much_ longer than time from
>bearing to viabiltity.  This gives live bearers the luxury of
>developing much more complex behavioral patterns.  It also gives
>offspring predators _much less_ time to find the babies!!!, d)
>Agressive defence _and_ constant provisioning visits to the nest
>announces to the would-be predators the location of their next meal.

However, the time from fertilization to viable offspring probably
isn't much different, and that is what really counts.
It seems to me that the decreased time from bearing to viability
almost necessitates, rather than permits, the development of complex
social behaviors.  Animals with low birth-to-viablity times tend to
have long gestation times.  During this time, the mother is at a
greater risk than when she is not pregnant.  Without adequate
protection, such a female, and its subsequent offspring, are at
greater risk of predation.  Thus, herbivores tend to gather in numbers
for safety, and most carnivores, having teeth and claws and such, can
usually defend themselves, or have no need to as they have no
predators anyway.  Of course, herbivores can also just get really big,
so that no carnivore in its right mind would even try to attack it, or
its offspring.
Egg-layers, on the other hand, suffer no such period (or at least, a
much reduced one) of heightened vulnerability, except that associated
with protecting the young (which, as you recall, is something that
even live- bearer's have to deal with).  While the eggs are certainly
more vulnerable than new-born herd animals if left alone, we have much
evidence that many dinosaurs attended their nests.  While the herd may
not have been as mobile as a mammalian herd, it would be far from
defenseless.
 
>       All of these factors must add up, in the open-field at least,
>to a big payoff in relative fitness for live bearers.  Perhaps this is
>why practically the only vetebrates in this habitat are live bearers,
>or stealthy egg layers.  Once upon a time there were _only_ nest
>guarding species.  Is this just a monstrous coincidence _or_ does it
>tell us something about the relative success of these alternate
>strategies?

Or perhaps it has nothing to do with relative fitness (egg-laying vs.
live-bearing).  Perhaps it has more to do with the fact that the
animals which previously occupied the open-field niche (herding
dinosaurs, which happened to be egg-layers) died off, for whatever
reason (probably not due to egg predation), and certain mammals (which
just happened to be live-bearers) moved in and occupied the now-vacant
niche in a classic example of population succession.

>If it does, perhaps dinosaur extinction is the _fait accompli_ of the
>development of a better strategy--live bearing!

And who can define what is a "better" strategy?  
Quick, which group is more successful, killer whales or sharks?  Both
occupy a top-slot in the marine carnivore niche.  By John's defintion
of "success," the killer whales, having a "superior" development
strategy, are more successful.  Yet, sharks are not extinct.  Sharks
have been around since the Devonian.  Whales, since the Paleocene.
John said himself that "Strategies suitable for one kind of organism
are not for another," and "Many complex strategies have evolved, each
presumably adaptive for its organism."  And yet, he argues that
live-birth is superior to egg-laying in an open-field environment.
Because live-birth was perhaps not suitable for dinosaurs in general
does not make egg-laying, in this niche or any other, inferior.  The
best strategy for an organism is the one that works.  That the
dinosaurs died off and mammals then occupied their niches is a far cry
from proof that mammalian developmental strategies are superior.
Why, then, did the great mammalian radiations take place AFTER the
death of the dinosaurs, instead of during their reign?  If mammals did
indeed have something "superior" going for them, wouldn't we expect to
see a species-by-species replacement, with mammals nudging the dinos
out of their niche and into extinction?  At any rate, I would expect
such an extinction event (or rather, series of events) to be far more
gradual than what appears actually happened.
It seems to me that this arguement rests more on a mammal-ocentric
bias than it does any real evidence for it.

Derek Smith
djsmith@ccnet.com
DSmith0531@AOL.com