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Sexual dimorphism musings



Hi,

a few thoughts about _Confuciusornis_:

The initial reaction is to assume that the
streamer-tailed (and larger?) specimens are males.
Now, in Neornithes we have reversed sexual dimorphism
(females larger and prettier and taking an active role
in courtship) - apart from larger carnivores like
eagles and owls where it is apparently due to resource
partitioning - occurring with the highest frequency in
the more ancient lineages, namely:
* tinamous (all I think)
* ratites (many)
* Charadriiformes (widespread)

It is generally absent in Galliformes and
Anseriformes, but these are highly polygynous or
strictly monogamous and therefore this comes as no
surprise.

So an increased likelihood for smallish birds that
mate monogamously for a breeding season but are liable
to change partners from year to year to evolve - for
whatever reason - reversed sexual dimorphism appears
plesiomorphic among Neornithes.

What I have no idea of is if anything is being known
or suspected about sexual dimorphism in Mesozoic
theropods. Apart from the _T. rex_ debate, but that
wouldn't apply here (large predator).

It may never be possible to say anything of substance
on the pattern of streamers vs lack thereof in
_Confuciusornis_. But the question whether the theory
that the streamerless animals were female can be based
on robust facts at all is possibly answerable.

(I do not consider the fact that the usual sexual
dimorphism is widespread in small birds today "robust"
here. Phylogenetically, most bird linages are rather
monomorphic, and a large number of species that are
not fall into the highly derived Passeroidea.)


Eike


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