Precocial Dinosaurs [long]

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

     In _Science_, May 3rd., a paper by Geist and Jones claims
all dinosaur embryos so far discovered were probably precocial! 
It has set everyone to squabling about what this means for the
bird/dino link (about which it says nothing).  As you can
imagine, I was delighted to hear of the study because its results
agree with an important prediction of the nonstealthy-nest theory
of dinosaur extinction: if the open-field nest was insecure,
dinosaur hatchlings ought to be precocial.  But this should come
as no surprise to anyone familiar with avian nesting habits. 
After all, most ground-nesting birds are also precocial.  This
leads to at least two questions: 1. If dinosaurs became extinct
due to overpredation of their eggs and juveniles, why then
haven't ground-nesting _birds_ become extinct?  Answer: Because
mortality rates accrue from _all_ life stages of organisms.  A
bird-juvenile, because it can fly away, perhaps suffers less
predation than the earth-bound dinosaur juvenile.  Also,
dinosaurs being large (because they couldn't be small) were above
the threshold of discovery, relative to birds at least.  You
might not know where the quail's nest is but you had no problem
pin-pointing the hadrosaur's (They're over there. By that big
sign which says: EGGS SERVED DAILY);  2. Why would any
organism be altricial?  Possible answers: Enhancement of parental
bond (see my later "mammal competency" post) resulting in
learning more complex behaviors.  Prevention of falling out of
the nest.  Additional time for more complex developmental genetic
programs, eg., neuronal "wiring".

     What must it have been like to lay your eggs out in the
open-field?  Think of an egg-laying gazelle, giraffe, lion--an
egg-laying anything on the open field must suffer.  Because of
its immobility, an egg is much more expensive to guard.  An
egg->viable adult takes more time and energy to protect than a
live-born mammal->to viable adult.  Indeed, in the open-field of
today many newborns are instantly viable and can at least feed
themselves!  Placentalism _and_ the marsupial mode of
reproduction obviated the need for laying your offspring prone to
all who would eat them!
     
     At certain times of the year dinosaurs could not roam freely
across their habitat.  For large herbivorous dinosaurs this must
have been a problem.  In breeding season it may have been that
patch-to-nest distances were great.  Modern open-field animals
wander through large areas to forage.  Keeping on the move they
have access to a greater number of patches.  This abundance also
enables them to stay together in large, secure herds.  A large
herd of dinosaurs may well eat down the available resources
quickly.  Starting near their (communal?) egg beds, and working
outward, dinosaurs may have had to travel long distances from
patch to nest.  This is expensive in two ways: hungry guards may
stray in search of their own food if they are not provisioned
often enough, thus leaving eggs unprotected; and energy expended
in travel is lost to the offspring.  Perhaps, if angiosperm
radiation or climate changing forces caused a problem for them,
inasmuch as dinosaurs would have to range further to eat, they
would be selectively targeted.

     Should causation in this instance be ascribed to the drop
in resource availability or the need to keep returning to the
nest?  It's a philosophical question, but one the mammals didn't
have to answer.  But at least, unlike other theories, it has
explaining power.      

The rest of this post is a response to Martin? which I wrote a
while ago (but got side tracked).  But it is relevant to dino
reproduction.  Please forgive some reepitions.  He said there is
no advantage in being placental relative to birds because: _Both_
parents can guard and forage for the clutch.  Care in mammals, he
said, is usually restricted to the mother (i.e., inter-uterine
care).  And this additional parental care makes egg babies just
as healthy as placental babies.

     But I would argue that all this moving too and from the nest
alerts predators which, consequently, lowers reproductive rates
(it also puts a premium on stealthiness!).  Also, while the baby
was in the womb, mother did not have to waste time moving from
patch to nest and back again.  This is good!  All other things
being equal, the delivery-to-viable-offspring-time in mammals is
shorter than the laying-to-viable-offspring-time in birds.  This
is reproductive security.  And then less time having to tend
babies can be turned into more time having other babies!  That
is, while she is nurturing one brood to maturity, she may be
bringing another to term.  Also, this "free" time can be used to 
train babies, to allow for more complex neuronal pathways to
develop, and to accrue other putative advantages of altricial
development.

Here I should add that gestational times of 6 + months are an
outrageous luxury when compared with the rapid development
necessitated by egg constraints.

     Martin says building a nest in a tree is not advantageous
because it is like a "tussock in the air" and is silhouetted
against the sky.  He also disputes my contention that a tree
provides more surface area for hiding places than a patch of
ground.

     But I disagree and note the recent occurrence at the
Agricultural Research Service in Beltsville, MD. (I don't have a
reference, sorry).  Eating down of the understory due to a deer
population explosion caused over predation of nests of local
birds.  This infers that the predators could _not_ see the nest
under normal conditions.  Leaf cover is very effective and, in
summer at least, provides no such thing as a silhouette against
the sky.  Instead, the bird predator is confused by
the dappling and flickering of many specks of light, the constant
shifting of thousands of leaves, the maze of branches on one tree
merging with those of its neighbours.  I know this to be true
because I recently lost my son's hot-pink aerobee in a tree. 
Since the leaves came in I have not been able to see it!  Also,
robins, who lay three clutches, lay the first one on the ground. 
Then, when the spring leaves come in they nest in trees.  Plovers
which lay eggs on the ground suffer more predation than those who
lay in trees.  And, as previously noted, most ground-laying birds
are precocial--meaning, they must vacate the non-stealthy
premises pronto!  
     As far as 3 dimensional hiding places are concerned,
consider this: an oak tree has a volume of say 20m x 20m x
20m.  That is, would you, the hungry egg predator, rather comb
400 sq m of ground for eggs, albeit they are hidden in tussocks,
or 8000 cubic meters of a leafy maze.  For starters, as Martin
admits, you may be the kind of predator who can't even climb
trees.  
     But isn't this the crux of the issue.  Birds lay their eggs
in places that things cannot reach them.  Tree-nesting is not
important to the nonstealthy-nest theory.  Whether or not they
fly to offshore islands, high cliffs, to the Arctic Circle, or
nest in trees--THEY ARE STEALTHY EGG LAYERS.  Surely, the
diversity of strategies for avoiding egg predation in birds is
convincing evidence of the egg liability.  Unfortunately for
dinos, they could not resort to any these strategies.  

     Yet, as the Geist and Jones paper infers, they suffered the
same kind of constraints which follow from laying eggs as do
modern egg layers.  Indeed, inasmuch as there were more and
bigger eggs back then, they may have had to deal with more egg
predators...which leads me to"mammal competency" post...