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Re: dinosaur copulation?

From: larrys@zk3.dec.com
 > >swf@elsegundoca.ncr.com wrote:
 > >What would the selective advantage be of growing to such large size
 > >after reproductive age?  Remember, natural selection deals in
 > >*reproduction* only.  An extreme adaptation can only evolve if it
 > >improves *reproduction*.  Thus, a non-reproductive gigantic post-
 > >adult would nt be selected for under most circumstaances.
 > And yet, somehow, humans have _evolved_ so that their individual lifetimes
 > _do_ exceed their reproductive lifetime, as least as far as the female half
 > goes

Yep, and please note how extremely social humans are, and the
extent to which grandparents and other adult relatives care for
the young.

Thus, in a human social unit (tribe, village, town ...) the non-
reproductive individuals help protect and care for the young of
their kin.

This is *exactly* the sort of exception I mentioned in the very
next paragraph.

 > The larger size of older dinosaurs
 > might have made them more formidable in defense of the herd, pride, or what-
 > ever, and so be selected for despite the early shutdown of the reproductive
 > system because they can protect the herd (and their progeny) better and 
 > longer
 > if relieved of the brooding instinct.

I doubt they had a properly organized social structure for this
to be workable.  Even though sauropods probably lived in herds
as adults, the evidence is that the extremely young did *not*
move with these herds, but lived on ther own until they were big
enough to keep up with the adults.

The evidence for delayed entry into the herd is even stronger
for Maiasaura, where it is virtually certain that young individuals
did not enter the herd until about 5 years of age, and about 2/3
of adult size.

In this sort of a social system post-reproductive adults would
have little imapct on the reproductive success of their younger

Interestingly, the group in which kin care might have been
significant was the ceratopsians, where single bull herds
of several adult females and young seems to have occured.
But in this type of a herd, it is usually the *pre*-reproductive
sub-adults that provide extra care for their kin.  Post-reproductive
males in such a system are quickly ousted by younger, virile males,
if they live that long.  A post-reprodcutve female might be of
some small benefit to her kin, but only a little as the younger
sisters would be more useful care givers.
 > Another possibility: dinosaurs never grew "up".  Maybe, like fish and
 > reptiles, they never reached an absolute upper limit in size, but just
 > continued to grow throughout their lives at a decreasing, but continuing
 > pace, until they either met a violent end, ...

There is actually some evidence that they didn't have determinate
growth - that is that they kept growing all their life. However,
growth rates *did* slow down significantly upon reaching adult
size.  Check out the growth data on Maiasaura as presented in
Jack Horner's book "Hunting Dinosaurs".

 > In
 > that scenario, "breeding" age might be the middle sizes, and become 
 > impossible
 > later either through the sheer mechanical impossibility or because of 
 > declining
 > hormones versus weight, or both.

Not so.  At the age they became "too large" to reproduce, they
would no longer have the benefit of prior selection for good
health, and so they would almost certainly exceed the capacity
of their support systems at about that size, and quickly die.

In short, old age generally sets in at about the age that an organism
stops having a positive reproductive effect, either through direct
reproduction or through kin-care.  This is true as much in beasties
with indeterminate growth as in those with determinate growth like
us.  (It is interesting that old age sets in in humans at about the
age that grandchildren are reaching adulthood, for instance).

This means that post-reproductive dinos that did not have a
substantial positive impact on the reproduction of kin would have
quickly aged and died.

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.