[Odonata-l] Sexual dimorphism question?

[Adolfo Cordero Rivera]

About this:
"Indeed, the frequency explanation may not be the sole reason behind the
coexistence of several colourmorphs, but untill now, this seems the most
plausible. However, other hypothesis must certainly not be excluded. Maybe
someone on the list can mention the most important ones..."

The discussion about mechanisms that maintain different colour morphs is
still a hot one after more than 20 years. I really would not be so convinced
that frequency-dependent mechanisms are the resposible for this, at least
not alone, and clearly not in many species. 
We will shortly show in a paper in press in Anim. Behav. why this is not the
case for I. elegans. Below is the abstract.

Adolfo Cordero

Male-like females of a damselfly are not preferred by males even if they are
the majority morph

Adolfo Cordero Rivera and Rosa Ana Sánchez Guillén 
Grupo de Ecoloxía Evolutiva e da Conservación, Departamento de Ecoloxía e
Bioloxía Animal, Universidade de Vigo, EUET Forestal, Campus Universitario,
36005 Pontevedra, Spain. 



Summary
Animals searching for prey and males searching for mates share similar
problems of detection if their targets are diverse in colour or physical
appearance. There is good evidence for predators switching their preferences
for prey in a frequency-dependent way; predators focus on the most common
form, and the decreased predation on rarer forms allows multiple forms to
survive. This mechanism has also been proposed to explain the maintenance of
several female colour morphs in damselflies. However, the fact that one of
the female morphs is male-coloured (androchrome) and behaves similarly to
males, suggests the phenomenon of male-mimicry in this system, an
alternative explanation for the polymorphism. Here we compared androchrome
frequencies in populations and mating pairs in Ischnura elegans, over a
range of androchrome frequencies (8-90%). We found that in 22 out of 23
samples androchromes mated less often than expected (significantly in 13
samples). We found no evidence for males switching their preferences in a
frequency-dependent way. A test of male preference for female morphs in a
population with 85% androchromes indicated that males behave
indiscriminately, and do not prefer the commonest (male-like) morph. Our
results support androchrome male-mimicry rather than male learned mate
recognition (a purely frequency-dependent model) as the main mechanism
behind the maintenance of this sex-limited colour polymorphism.