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Sexual dimorphism in T. rex

I read in THE COMPLETE T. REX by Horner and Lessem that Tyrannosaurus rex
may come in two different forms -- larger females, smaller males.  This
theory came about when a croc researched discovered a way to sex a croc
from its skeleton.  The males seemed to have a penile retractor bone, the
first tail rib past the hips -- and the females didn't have this bone
because, of course, they didn't have penises.  They tested this procedure
on Tyrannosaurus rex, and it appeared that according to the procedure, the
rexes came in two sizes.  The ones thought to be females were larger,
while the ones thought to be males were smaller.  It may be that instead
of the two forms being separate species, they could be different sized
sexes of the same species, but Horner says we need more specimens to be
sure.  But in birds of prey, raptors, generally the females are larger
than the males.  And in a way, T. rex is a bird of prey.  According to
Bakker, "The eight-ton roadrunner from hell."

But why larger females?  Males in many species are larger because they
rival over the females.  But if the larger rexes ARE females, this may
tell us something about the mating habits of T. rex.  A larger body
capacity in female T-rexes may indicate that she needed more room to hold
her eggs.  Or maybe the males had no need to fight over the females --
could the females have selected their own mates?  But one interesting
thing is that if the females were many feet higher than the males, and
much larger, he may have had difficulty mating with her.  But as all
animals do, the male rexes would have found a way.

The T. rex females, if they were larger and more robust than the males,
may tell us some interesting stuff about T. rex life.  Maybe T. rexes were
social, and hunted in mating pairs.  To me, they seem like the kind of
predator that would work together and maintain a good mating-couple
relationship.  If the male was bad to the female, she could have easily
done him in.  

But why sexual dimorphism in T. rex??

Raptor RKC (Rachel Clark)