Lifetime fitness components in female colour morphs of a damselfly: density- or frequency-dependent selection?

Different stress from parasites and mate choice in two female morphs of the blue-tailed damselfly

In Odonates, female colour polymorphism is common and implies the presence of two or more female types with different colours and behaviours. To explain this phenomenon, several hypotheses have been proposed that consider morph frequency, population density, the presence of parasites, and mating behaviour. We studied the blue-tailed damselfly Ischnura elegans, a species with a blue androchrome morph and two gynochrome morphs (the common green infuscans, and the rare orange rufescens-obsoleta). The size of adult males and females, the presence of parasites, and pairing behaviour between males and the three female morphs was assessed in field conditions throughout the reproductive season in NW Italy. Moreover, growth and emergence success of larvae produced by the different morphs was analyzed in standardized conditions. In the field, males showed a preference for the gynochrome infuscans females, despite a similar frequency of androchrome females. In test conditions, male preference for the infuscans females was also observed. Paired males and paired androchrome females were larger than unpaired individuals, while there were no differences in size between paired and unpaired infuscans females. Males and androchrome females were more parasitized than infuscans females. The survival and emergence success of larvae produced by androchrome females was higher than those of offspring produced by the infuscans females. Our results suggest that a higher survival of progeny at the larval stage could counterbalance the higher parasitism and the lower pairing success of andromorph adult females and highlight the importance of considering the whole life-cycle in polymorphism studies.

Socially mediated trade-offs between aggression and parental effort in competing color morphs

Individuals often face trade-offs between investment in parental care and alternative investments of time, energy, and resources into other life-history components, such as dominance, attractiveness, and health. Selection is thought to promote the optimal balance between the costs and the benefits of these conflicting activities by favoring individuals that adopt different tactics to maximize their overall evolutionary fitness in different environments. To test this, we experimentally manipulate both aggression (i.e., competitive environment) and parental effort (i.e., brood size) in red and black morphs of the Gouldian finch (Erythrura gouldiae). Although aggressive red males provide parental effort comparable to that of black males in environments where competition is low, irrespective of their relative brood size, they severely reduce or abandon parental investment in highly competitive environments. In contrast, nonaggressive black males are largely unaffected by the competitive environment and instead adaptively adjust their provisioning effort to the relative demands of their brood. Consequently, in highly competitive environments, although dominant red males defend higher-quality nest sites, they produce fewer and lower-quality offspring (in terms of mass and immunocompetence) than black males do. These opposing effects of frequency-dependent competitive environments on red and black males underlie their differential trade-offs between the costs and the benefits of aggression and parental effort.