The evolution and frequency of female color morphs in Holarctic Odonata: why are male-like females typically the minority?

Abstract

We compiled data on the occurrence and frequency of distinct female variants among Holarctic Odonata and interpreted the data in light of harassment-based hypotheses. The major source of male confusion for male mimicry hypotheses is predicted to be signal similarity between andromorphs and male distractors; for the learned mate recognition hypothesis (LMR), it is predicted to be variation in female signals. Mapping morphism state onto molecular phylogenies of Ischnura and Enallagma failed to resolve the general ancestral female condition. However, it appeared that the andromorphic state may be ancestral in one case, and that blue structural colors were ancestral to orange and green pigmentations. Of the polymorphic species surveyed, 13% had more than two morphs, 4% had multiple heteromorphs but no andromorph, and 7% of ‘monomorphic’ congeners were functionally polymorphic because developmental variants mate. Such female signal variation lies beyond the scope of simple male mimicry, but nevertheless should exacerbate a male’s problem in searching for mates. Andromorphs were the majority morph in at least some populations of 17% of the species for which data were available. Andromorph frequencies of Enallagma species were generally higher than in Ischnura species, as expected if Ischnura andromorphs have higher signal apparency. Andromorph frequency varied significantly across habitats and species, as expected if per capita harassment and signal apparency vary among habitats. Quantification of signal apparency and per capita harassment across populations and among species is required to more rigorously test the extent to which variation in signal crypsis can explain observed variation in morph frequencies.

Keywords: Odonata, dragonfly, color polymorphism, andromorph frequencies, ancestral color, Holarctic

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