Re: Armadillos at the K/T! (long)

[Graydon <graydon@dsl.ca>]

On Fri, Oct 05, 2001 at 01:06:05PM -0400, John Bois scripsit:
> Because colonies are only valuable against similarly-sized
> predators. 

Not true at all.

If there is *any* risk to a predator from the adults, a colony functions
to make the risk of getting to a particular nest much higher,
irrespective of the specific risk.  

So a small predator with .0001 chance of being stepped on per meter
doesn't want to go any distance into the colony.

Consider: 10 meters -- five meters in, and five meters out -- is about
right for *one* nest on the edge of the colony.  Say that each meter of
travel inside the colony runs a 1 in 10,000 chance of getting squashed
for the predator in question.  If it needs to make 10 such trips to feed
itself for long enough to breed and raise offspring -- it's eating other
things besides eggs -- we get, as odds of survival:

((.9999)^10)^10 for 10 10 meter trips; this comes out to .99, one chance
in a hundred of being squashed per breeding cycle.

These are pretty good odds, if the principle risk to the predator
is being stepped on while foraging for eggs.

One more nest in is 20 meters;
(.9999)^20 is .998; do that ten times for .9801, one chance in fifty.

Three nests in is 30 meters
(.9999)^30, 10 times, is .9704, 3% chance of getting squashed or about 1
chance in 30.

20 nests in -- which is only 100 m, not a big distance based on the
infered size of the Maiasauria nesting grounds -- is: (.9999)^(20*5),
.99, so 10 trips will be about .9, one chance in ten of squashing.

That's going to exert a fairly strong selective pressure on the
predators to stay near the edges, since the ones that stay near the
edges will have ten times the reproductive success of the ones which
engage in deep raids on the colony.

So the colony works even if the affects on small predators are primiarly
accidental.  (one in ten thousand per meter supposes really bad reflexes
on the part of the adults, or a complete failure to make the connection
between the small mammal and the egg risk.)

-- 
                           graydon@dsl.ca
               To maintain the end is to uphold the means.