[Odonata-l] Sexual dimorphism question?

[Carlo Utzeri]

Thank you, Michel,
with dragonflies, the question of differential dispersal vs mortality is a very aged one, and it was generally maintained that the lower recapture rates of female samples was probably due to the latter's higher dispersal. Since the 1970s I noted that recapture rates of some (non dimorphic) Lestes virens and L. barbarus samples were not different between males and females, while the contrary occurred in the dimorphic Crocothemis erythraea. Since neither of these species display their colours in the sexual context, I discarded the idea that the male coloration in Crocothemis could be selected for by sexual selection. Hence the unprofitable prey hypothesis of male brighter coloration. However, the outcome of this should be that males are more protected than females, i.e. preyed upon at a lower rate. In fact, during hundreds hours in the field never I spotted birds preying upon dragonflies, although some specialists, as the bee-eater or some small falcons are reported to catch a number. If this is true, the lower recapture of females should be caused by a higher mortality, not dispersal. But so far I did not get the opportunity to test this. In my place, both Lestes and Crocothemis mature in woods, so it is impossible to keep under control the marked samples and check if the female and male ones decrease in a different way. However if even possible, this would not be a conclusive observation. The ideal situation would be a pond in a very large grass field, say a small mountain lake, and a dimorphic species, with a relatively short maturation period, emerging from it. Here probably you could be able to follow day after day the marked samples, that would probably mature in the (open) grass field, record all predation events upon your marked specimens and draw, so to say, day by day the survival curve of your samples. If my hypothesis is correct, I expect that in the first period predation should be evenly distributed upon male and female samples, and then, after completed maturation, it should be biased towards females (or males). This could constitute a first step. However I suspect that I should have to build such a place and the dragonfly as well!
Beg you pardon for the somewhat longer comment.
To the next
Carlo

Hello Carlo,

Excellent hypothesis! Better than the sexual one!!! I really adopt it! Do we know if male are more often eaten as prey than female? Male being eaten more often is of less consequence than female being eaten for the dragonfly prosperity! Maybe it can be another reason why male should be more visible to predators... 

Thank you very much for this very clever hypothesis!!!

Michel
Quebec Canada



-----Message d'origine-----
De: odonata-l-bounces at listhost.ups.edu
A: Lista_Odonata_commenti
Date: 5/02/2007 5:04
Objet: Re: [Odonata-l] Sexual dimorphism question?

Hello Michael,
my hypothesis is that the sexual coloration dimorphism, at least in 
Libellulids, has evolved in the predator-prey context and could be
explained 
by the unprofitable prey model: by their colorful apparence, dragonfly
males 
signal to (potential) (bird?) predators that it is no convenience to
attempt 
to capture them. Dragonflies are in fact very good fliers and are quite
able 
to escape capture by most birds. They are active during day hours in
open 
habitats, often in full sunlight, and their coloration probably evolved 
bright (aposematic) because it is easier for birds to learn a non-prey
item 
by its bright coloration, thus promptly realising that it is not a good
job 
of theirs to insist chasing such prey. On the other hand, a dull
coloration 
makes females more protected (cryptic) in the shade of woods, where they

spend most of their time between oviposition bouts.
Greetings
Carlo Utzeri
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