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Re: Sexual dimorphism in T. rex



    Last semester I took an upper-division ornithology class at a
local university.  Our professor (T. Balgooyen at San Jose
(California) State University) was rather a specialist in raptor (not
_Veloci_-!) ethology, and discussed at some length several hypotheses
addressing reverse sexual dimorphism (females larger) in birds of
prey.  A summary might be relevant to those interested in this
feature in other predatory dinosaurs.  So...
    1) Scant fossil and DNA evidence suggests that reverse sexual
dimorphism has evolved several times separately in birds of prey
(falcons, buteos/accipiters, shrikes).  (Shrikes are not
conventionally classed as raptors, but are birds of prey.)
    2) Degree of dimorphism varies with body weight in different ways
in different groups.  Thus in North American accipiters, the ratio of
female mass to male mass declines with increasing species mass,
whereas in North American falcons, the reverse obtains.
    3) Hypothesis A for explaining dimorphism: Males and females take
slightly different types of prey (thus more efficiently using
resources than if both took the same), and have evolved different
morphologies of anatomical features used in predation.  This one is
testable in a decent museum.  Balgooyen (as part of his doctoral
thesis, I think I remember), took dimensions of several features of
beak and claw for many species, and found no statistical differences
between the sexes.  That is, size varies, but weapons size does not,
between the sexes.
    4) Hypothesis B: Notwithstanding the same sizes of weaponry, males
and females do in fact take different sizes of prey -- possibly the
larger females take larger prey, et cetera.  Thus there is behavioral
difference even in the absence of differences in functional
morphology.  This one requires some field work of some kind or another
(scat, stomach contents, observation).  I don't remember the work that
had been done, and if memory serves, Balgooyen described results for
only a few species, but he cited only a little inconclusive data for
one species -- the American Kestrel (_Falco_sparverius_) in support of
this hypothesis.
    5) Hypothesis C: Male provisioning -- males are size-optimized for
the period when females spend most of their time sitting on eggs.  At
this time, males have to spend essentially twice as much time hunting,
since they must provide not only their own food, but also their
mates'.  Perhaps energetics favors a smaller male, whose lower total
basal metabolic rate while perched for a long time more than makes up
for the possibly slightly higher energy consumption during the much
briefer intervals of chase, capture and portage home.  Now it does
appear clear that many raptor species indeed do male provisioning, but
it would take a great deal of study of energetics, et cetera, or else
some very clever correlation of behavior with dimorphism, to refute
this hypothesis.  The work seemed not convincingly done yet, which
leaves this hypothesis in the embarrassing position of being not only
unrefuted but also untested.

    I am personally curious whether you paleo- folks have any other
interesting hypotheses about what might select for reverse sexual
dimorphism.

                                                --  Jay Freeman