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Sexual dimorphism comments from Chris Brochu
Posted for Chris Brochu, who is in transit to new fields in Iowa.
Ladies and Gents,
I took a look at the dino list archives, and the subject of sexual
dimorphism was discussed a bit. Here are my thoughts on the subject:
1. The chevron criterion.
This is problematic for three reasons:
- It doesn't really work for crocs in the first place, because the
position of the first chevron is not truly dimorphic - there are three
or four, not two, possible placements of this element. As far as I
know, modern croc populations only show two sexes. Skeletal museum
specimens rarely include sex information, but from what I can see, the
position of the first chevron does not necessarily correlate with sex.
- It relies on negative evidence. If the chevrons are not in
articulation (and the anteriormost chevrons were not articulated in
Sue), how do you know that the first chevron in your collected specimen
was the first chevron in the living animal? Morphology does not always
help you out here - it can, but not always.
- We would have to assume that the derived genital morphology of crocs
reflects that of nonavian theropods. Modern avian theropods have a very
different genital morphology.
2. The size and shape criterion.
Also problematic, for a variety of reasons:
- It is true, as is claimed, that reverse sexual dimorphism (RSD, where
the females are bigger) is common in nonavian reptiles. But this
statement, by itself, means nothing in the absence of a phylogenetic
context. Yes, lots of herps have RSD, but they're almost all turtles
and snakes. The minimum number of evolutionary transformations (a much
more meaningful number) is not many thousands, but two. And the males
are bigger in crocs, which are phylogenetically much closer to dinosaurs
(living and extinct).
- It is also true that RSD is found in several bird groups. But
optimizing for the ancestral condition (regular SD versus RSD) in
crown-group Aves is difficult; the basalmost neognaths (chickens, ducks,
things like that) rarely show RSD, and the pattern of RSD versus SD in
paleognaths is very confusing, with some groups showing one and others
the other. On phylogenetic grounds, we would have to regard the pattern
shown by any nonavian dinosaur as unknown.
- It is true that RSD is common in "birds of prey" (falconiforms and
owls), a point made by several authors. But if one looks more closely
at modern birds, some very interesting patterns emerge:
a. RSD is best developed in birds that fly after other birds -
accipiters, falcons, etc.
b. RSD is least developed in birds that feed or hunt on the ground. In
vultures (which feed on the ground), the sexes are roughly the same
size. In secretary birds (which hunt on the ground), the males are
larger. Among owls, RSD is least developed in burrowing owls.
c. Similar patterns are seen in predatory birds outside the falconiform
or owl groups. Birds flying after other birds (skuas, jaegers,
frigatebirds) have extreme RSD, birds hunting on the ground
(roadrunners, ceriemas) show regular SD.
d. Sue was supposedly the "robust" morph of T. rex. (More about that
below). But although it is true that female falconiforms are usually
bigger, this is based on measures of wingspan or overall body mass.
This is an expression of size, but "robust vs. gracile" is an expression
of shape, not size. So I measured a bunch of bird bones. Female birds
of prey may be larger, but they're more gracile, NOT more robust.
All of this assumes that there is shape dimorphism in T. rex. This, I
think, is unsubstantiated. We're dealing with a handful of specimens
ranging from Saskatchewan to Texas, covering 2 million years of time,
and we haven't ruled out ontogenetic, regional, stratigraphic,
preservational, or plain old individual variation. I am not saying that
T. rex was not dimorphic; but we need a better sample to demonstrate
The chevron stuff will be in the Sue description, which should be in
review now. The shape vs. size issue is not treated there; that will be
discussed in another venue. But the bottom line is this: we cannot at
present determine gender in any nonavian theropod.