[Odonata-l] Sexual dimorphism question?

[Dennis Paulson]

I would advise all who are interested in this subject to read a  
recent paper:

Fincke, O. M., R. Jödicke, D. R. Paulson and T. D. Schultz. 2005. The  
evolution and frequency of female color morphs in Holarctic Odonata:  
why are male-like females typically the minority? International  
Journal of Odonatology 8: 183-212.

This paper documents the occurrence of female polymorphism in  
northern-hemisphere odonates, and this probably represents most of  
the known female polymorphism in the order, as there is relatively  
little in the tropics.

Anyone with a serious interest in Odonata should consider becoming a  
member of the Worldwide Dragonfly Association, which publishes this  
journal.

Although I'm one of the authors of the article, which argues against  
male mimicry and for the learned mate recognition hypothesis (which  
involves frequency-dependent selection), I am still very open to the  
hypothesis that male mimicry plays a part in the evolution of female  
polymorphism, as Adolfo Cordero has mentioned in his post. I think  
there is little doubt that it occurs in the genus Aeshna, in which  
there are many species with polymorphic females. And I think the  
variation in how different the two morphs look from one another and  
from males might indicate that more than one selective pressure is at  
work, possibly learned mate recognition AND male mimicry.

I don't recall if territoriality has been put forth as a significant  
hypothesis, but many libellulid dragonflies are highly territorial,  
and a long-held hypothesis is that their bright coloration evolved to  
enhance the species recognition that seems important for efficient  
territorial behavior. In my studies of libellulids, I have seen that  
male aggression is often directed to males of other species when they  
look much alike. This certainly can't explain the situation in the  
many non-territorial species, but one aspect of sexual dichromatism  
that hasn't been mentioned is the evolution of bright coloration in  
males just so they can be recognized as males and not harassed by  
other males. This was nicely outlined in this paper:

Sherratt, T. N., and M. R. Forbes. 2001. Sexual differences in  
coloration of coenagrionid damselflies (Odonata): a case of  
intraspecific aposematism? Animal Behaviour 62: 653-660.

This is compelling to me, as I had the same thought 30 years ago when  
studying sex and species recognition in coenagrionid damselflies. All  
I had to do was put some blue paint anywhere on a female, and many of  
the males tested didn't want to have anything to do with her. Other  
females with much of the black markings on their abdomen covered by  
brown paint were just as attractive as the unpainted controls, and  
even females with red paint were more likely to be taken in tandem  
than those with blue paint. When you think of how quick males should  
be to respond to incoming females, when the sex ratio may be 50:1 at  
a pond swarming with male Enallagma, you can see that males colored  
like females might be harassed constantly.

Many more odonates exhibit sexual dichromatism than monochromatism,  
so it is important to try to explain the latter as well!
-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net




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