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Differential reproductive failure of dinosaurs



Jeffrey Martz says: "I've also heard the argument stated that
mammalian reproductive strategy beat out the dinosaurs.  (But
saying) they were in direct competition with...dinosaurs is
absurd."

And Nick Pharris is also unimpressed with mammal-reproductive
developments: "Forgive me if I see no great leap forward (in the
development of placental mammals)."

     Forget for the moment that there was any "direct
competition" in the way of egg predation between placental
mammals and dinosaurs, marsupials and dinosaurs, birds and
dinosaurs, crocodiles and dinosaurs, lizards and dinosaurs, and,
finally, dinosaurs and dinosaurs.  Simply compare the strategies
available to each of these groups with regard to their
reproduction.  

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
offspring _and_ healthier adults.  This latter benefit could be
cashed in on more future offspring.  When the baby mammals were
born they could stay put or, if discovered, cling, walk, or run
to safety. Ultimately this mode of reproduction was liberating,
not because it made mammals winners in face to face combat with
dinosaurs, but because, immune from the ambient egg predation of
the Cretaceous, they had higher reproductive rates than their
non-stealthy egg-laying competitors.  While the dinosaurs dealt
expensively with the hazards that must attend depositing a rich,
the mammals got busy exploiting their niche (a new dimension of
which may have been dinosaur egg predation).

2) Birds were able to lay their eggs in out-of-the-way places. 
Already selected for smallness for its advantage in aerial
agility, birds could hide their eggs in trees, in ground
thickets, prairie tussocks, marsh grass, on cliff faces, or on
remote islands.  Granted, all of these places were accessible by
egg predators (be they mammals, other birds, dinosaurs, or
snakes), but they afforded lower rates of discovery.  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.  Optimal foraging theory predicts that egg predators
would preferentially target easier-to-find eggs.  Because,
perhaps, dinosaur eggs were preferred, and because they were
impossible to hide, birds, too, enjoyed the reproductive
advantages of relatively milder egg predation. 
Again, relative to dinosaurs, they were winners in the battle,
not of tooth and claw, but of differential reproductive success. 

3) Snakes were also winners because they could easily hide their
eggs.  This was due to their own body plan.  A snakes body is
undoubtedly a response to selection for stealth.  Able to slither
down smaller holes than other animals of the same weight, they
enjoyed not only milder predation, but less intense egg predation
as well.

4) Crocodiles dominated the semi-aquatic environment.  Extant
crocodiles typically approach their prey stealthily, surprise it
and drag it into the water for the kill.  During the Cretaceous
there was one crocodile which had a head six feet long.  Any
dinosaur, mammal, or bird foraging in the crocodile's habitat
must do battle on its terms.  Egg predators working on
crocodile's nests in the Cretaceous must suffer the same
disadvantage they do today--they frequently get eaten themselves.

Indeed, given the choice of a terrestrial or semi-aquatic escape
route, a terrestrial egg predator must prefer the former. 
Crocodiles, niche sovereigns who feared serious egg predation
perhaps only from other crocodiles, enjoyed differential
reproductive success relative to terrestrial egg layers.  Still,
this niche-dominance restricted them to their marginal habitats
just as it does today.

5) Turtles and Tortoises appear to have changed little.  They
probably continue to live just as they did in the Cretaceous. 
Their general evolutionary posture is one of defence.  Not
dynamic like the dinosaurs, and not needing much in the way of
parental investment, they lay their eggs and forget them.  Turtle
nests are resistant to discovery because they do not need
constant attention.  Indeed, some turtles swim off to remote
islands, lay their eggs, and swim away.  Many hatchlings from
many nests hatch at the same time.  Predators take only a small
percentage of the broods and most escape to the succour of water.

These animals have one traditional strategy: immunity from
predation.  Armoured bodies, even armoured egg-shells in some
species, remote and stealthy nests unvisited by predator-alerting
parents, turtles and tortoises also secured their marginal piece
of the post-Cretaceous landscape.

Jeffrey Martz correctly notes that..."egg-eaters munching too
many animals or eggs during poor reproductive seasons could be a
cause of extinction."
Nick Pharris cannot see a difficulty dinosaur reproduction (I
should say a difficulty which could cause extinction).  At
different places he says the following things:

1. "...small dinosaurs...(probably) looked for leaf cover when
they were picking a nest site."

Yes, but they were above the threshhold of discovery.  They were
observed coming and going to and from their nest.  They couldn't
be small (for reasons I suggested in another post--reasons which
are yet to be challenged, unless I missed something)!  They could
try to hide their eggs, but ultimately, as the skill of the
predators evolved, they could not be successful because they were
stuck with severe constraints--they could not respond to
predation pressures in a way which reduced it enough.
     You've heard of Dianne Fossey living with the gorillas,
you've heard of Jane Goodall living with the chimps.  Well now,
here is Joe Hutto living with the turkeys!  "The odds of a wild
turkey reaching maturity are small.  Approx 50% of all nests are
destroyed or abandoned...Of the young turkeys who hatch 70% will
not last two weeks, and the number of survivors will continue to
be diminished.  The attrition rate of wild turkeys who have
survived into maturity can be 70% per year...Prey species
typically compensate for these (predation) pressures by producing
large numbers of offspring."  
     Here's my point.  Today there are few successful ground egg-
layers and so there are few predators who prey on them.  So they
are able to scrape by and keep their numbers barely above the
point of extinction (as Hutto puts it: "Any poult that survives
to maturity is primarily a lucky turkey.").  Before the K/T, the
place was lousy with ground egg-layers, and probably lousy--and
getting lousier by the millenium--with egg-predators.  Omnivores-
-most things which eat eggs today _are_ omnivorous--could help
themselves to eggs and not be "concerned" about wiping out their
food supply.  They could simply switch to other foods.  Who knows
if a turkey could have survived the predators of the Cretaceous,
I doubt it.  In any case, a turkey-sized dinosaur would probably
be subjected to more predation than a turkey.  Why?  Because, as
an adult it could not fly away from predators.  The cumulative
effect of predation at its various stages was enough to push it
into evolutionary oblivion.

That is in the small animal-niche.  But by the end of the Cretaceous
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 WITH A FEW EXCEPTIONS (exceptions which prove the
rule - see Ratites Post) OPEN FIELD EGG-LAYING BY LARGE ANIMALS IS A
DEFUNCT STRATEGY--if, of course, animals exist to exploit it.  There
was a critical mass of those animals in the late Cretaceous and it did
in those big egg-layers.