[Odonata-l] Sexual dimorphism question?

[Carlo Utzeri]

Rob,
the point, as you make it, is somewhat complicated, because female 
polychromatism was so far reported in just some zygopterans. But the 
frequency explanation advantage you recall might not be the only valid 
explanation. For example, in Ischnura elegans in central Italy, while the 
androchrome females are much more abundant than the gynochromes (154 vs 
104), in experimental insectaries males show a strong preference for the 
latter (87% of total matings). However in this species the male is 
relatively inconspicuous and I would not quote it as a good example of 
sexual colour-based dimorphism, as far as this must be based on bright vs 
dull coloration. Besides, the Zygoptera usually perch among vegetation, 
which makes them even less visible.
My previous answer concerned the libellulids in general, in which commonly, 
males are more or less strongly aposematic while females show a 
greysh-yallowish coloration that appears relatively uniform across the 
entire family. So the question may be: why only males evolved specific 
bright coloration? It does not seem that females need to recognise their 
males by their colours, because almost never the males exhibit them to 
females prior to copulation. Thus possibly the male coloration did not 
evolve in the context of sexual selection. The unprofitable prey model 
hypothesis (my previous answer) is just an alternative explanation, which 
still needs strong experimental support. However I realise that the problem 
is not easy to solve and probably the evolution of dragonfly coloration has 
followed several different pathways.
Greetings
Carlo Utzeri

----- Original Message ----- 
From: "Rob van Bemmelen" <ixobrychus at hotmail.com>
To: <mipilon at nrcan.gc.ca>; <odonata-l at listhost.ups.edu>
Sent: Monday, February 05, 2007 11:26 AM
Subject: Re: [Odonata-l] Sexual dimorphism question?


>
> I have a little guess:
> Cryptic colored females may be less easily detected by males and thus less
> often harassed.
>
> Female-based colour-polymorphism (or polychromatism), in which there is
> always one male-like female, has been explained by frequency-dependent
> advantage, as males only get to learn to recognize male-like females once
> they become more abundant. This shows that the rare colourmorph is always 
> in
> advantage of the other. Thus, both a cryptic or a male-like appearance 
> have
> advantage(s).
>
> However, this does not explain the bright coloration of males......
>
> I hope somebody else has a better answer...
>
> Cheers, Rob
>
>
>
>
>>From: "Pilon, Michel" <mipilon at nrcan.gc.ca>
>>To: <odonata-l at listhost.ups.edu>
>>Subject: [Odonata-l] Sexual dimorphism question?
>>Date: Sun, 4 Feb 2007 16:10:23 -0500
>>
>>Hello again,
>>
>>I hope I don't annoy all of you with my naive questions...
>>
>>After reading the answers about my id's request and considering that in
>>both case the sexual dimorphism is important, I would like to ask you the
>>following question which came to my mind:
>>
>>What justify that sexual dimorphism amongst dragonflies...?
>>
>>With birds we know that the female is attracted by the colorful male and
>>that she has to be duller because she has to lay over the eggs and should
>>not be seen by predators...
>>
>>Is it the same thing amongst Odonata? Does the female attracted by the 
>>male
>>colors? And why usually are they duller than the male? (I'm thinking also
>>of Sympetrum obtrusum (white-faced Meadowhawk))
>>
>>Excuse-me again for my maybe so naive question...
>>
>>Michel Pilon
>>Sherbrooke
>>Quebec Canada
>>
>>Mes Parcours Nature:
>>http://parcours.pilonm.org
>>
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