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One of the Larsons showed how one can sex a theropod by looking at the
chevrons- in the females the first is one vertebra further back and
smaller; in males it is further forward and robust; this is the pattern on
crocodiles, in which the males attach a penis retractor muscle to the
first chevron. So one would just assume that since T. rex appears to have
had the same pattern that it kept the penis too- at least I see no reason
they wouldn't have.
Probably the reason birds ditched the penis was that it was just one
more way to reduce weight. Don't laugh, birds have gone to some real
extremes- try seasonal atrophy of genitals for a starter. Teeth, claws,
and long tails are just the tip of the iceberg I guess.
Incidentally, using Larson's method it looks to me like the type of
Sinornithoides is a little she troodontid. And the female T. rex turns out
to be the big buff one.