RE: Herbivore protection

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

On Fri, 30 May 1997, Nathan Myhrvold wrote:
> So, the total population of predators in an area depends on the leanest
> time of year - i.e. a period of time longer than they can go without
> eating.   This more or less sets the base number of predators.  

     For the sake of argument let's keep this as simple as
possible.  I would just like to talk about the interval between
when hadros start laying and when their eggs hatch.  Leave the
nestlings out of it for now.  Hadrosaurs (some, anyway, and not
just maiasaurs, right?) were colonial nesters.  Why?  If there
were T. rex and hadrosaurs in a given range, and the hadros want
to swamp the predators, they don't need to gather together in
colonies to do that.  Surely they can just stay and lay wherever
they may be.  Now, I can understand how individual hadros gain
protection from predator satiation if they all lay at the same
_time_.  But I don't see advantage from nesting in the same
_place_, unless of course they employ mutual defence or that
place has lower predator density.  Indeed, optimal foraging
theory predicts predators would prefer to eat at a place with
high prey density.  And the usually cited benefit of herding
animals--increased vigilance--is irrelevant for a fixed-site
colony.  I'll put my head on the chopping block again (perhaps
Ron Orenstein is still satiated from the last time I did
that--no, come to think of it, since he's in Madagascar this is,
for a brief time, a place of lower predator density) and
hypothesize the following: all extant, colonial, vertebrate
egg-layers are gregarious for one or more of three reasons, 1. nest
defence, or mutual aid of some kind (crows, for example), 2. they
are gathering on scarce low predator-density real estate (such as
off-shore islands), or 3. scarce suitable nesting substrate
(crocs, for example).  Substrate was almost certainly crucial for dinos. 
But I believe nest defence was too.  I am arguing that the reason
they came together was to avoid predators, not satiate them.  I
also argue that hadrosaurs did not have any particularly huge
skill differential (such as swimming and temp. tolerance in
penguins vs. their predators); I have difficulty imagining
predator-free real estate that they and not their predators could
reach.  Can anyone here cite an unequivocal case of a relatively
large nesting species which sites its nests in order to satiate
predators rather than to avoid them?

     Another reason I believe predator satiation was not the
prime adaptive value of colonial nesting in dinos is this: On the
range (or wherever their regular seasonal foraging grounds were),
hadrosaurs most likely had their "base number" of predators.  But
at nesting time it is probable the predator pool was bigger. 
Predators who, on the foraging grounds, could not exploit
hadrosaurs (perhaps they couldn't find them as well as some
others, or because they were marginally slower than some other
predators), now could help themselves to easy nesting hadrosaur
pickens.  Big, visible egg-layers in a hospitable place would
likely have a surplus of predators, over and above the base
number.

     Grunion, cited by Stan, and salmon spawning in the Chilkoot
River, cited in a private post to me, are not analogous to
colonial dinosaurs.  In both cases the species appear for a brief
time.  And in both cases they are seeking to spawn in areas of
low predator density, the high tide sand and the fresh water
upper-reaches of the river respectively.  Again, the critical
issue here is in discovering how hadrosaurs could find any area
of low predator density.

     Now, when the nestlings come, the issue becomes enormously
more complex.  We know that some nestlings were precocial, others
probably were not.  So onto Nathan's next point. 

> Dinosaurs had such a high fecundity rate that it is very likely that
> they relied on predator satiation to some degree.  There is no reason to
> have that many young otherwise.

     But clutch size exists to increase Darwinian fitness, not to
satiate predators.  It can depend on factors other than
predation: food availability, nutrient quality, amount of
parental investment, intraspecific competition for resources,
nestling mortality from accident and disease.  And predation on
dinosaur offspring must have involved attrition too.  Dinosaurs
may well have had to grow up through a veritable gauntlet of
predators, each specializing on a specific cohort.  Selection on
clutch-size must be influenced by hatchling->juvenile->through
adult predation.  I think predator satiation needs justification before it
can be claimed for adaptive value for colonial hadrosaur nests.