Interspecific aggression, not interspecific mating, drives character displacement in the wing coloration of male rubyspot damselflies (Hetaerina)

Abstract

Traits that mediate intraspecific social interactions may overlap in closely related sympatric species, resulting in costly between-species interactions. Such interactions have principally interested investigators studying the evolution of reproductive isolation via reproductive character displacement (RCD) or reinforcement, yet in addition to reproductive interference, interspecific trait overlap can lead to costly between-species aggression. Previous research on rubyspot damselflies (Hetaerina spp.) demonstrated that sympatric shifts in male wing colour patterns and competitor recognition reduce interspecific aggression, supporting the hypothesis that agonistic character displacement (ACD) drove trait shifts. However, a recent theoretical model shows that RCD overshadows ACD if the same male trait is used for both female mate recognition and male competitor recognition. To determine whether female mate recognition is based on male wing coloration in Hetaerina, we conducted a phenotype manipulation experiment. Compared to control males, male H. americana with wings manipulated to resemble a sympatric congener (H. titia) suffered no reduction in mating success. Thus, female mate recognition is not based on species differences in male wing coloration. Experimental males did, however, experience higher interspecific fighting rates and reduced survival compared to controls. These results greatly strengthen the case for ACD and highlight the mechanistic distinction between ACD and RCD.

Keywords: agonistic character displacement; competitor recognition; interspecific aggression; mate recognition; species recognition.

Figure 1.

Photographs of a representative (a) unmanipulated Hetaerina americana male, (b) a H. titia male and H. americana males with (c) clear ink and (d) black ink on their hindwings. All males shown here were photographed during the course of the experiment. (Online version in colour.)

Figure 2.

Lack of an effect of the experimental treatment on overall male mating success, measured either as (a) the proportion of successful tandems, (b) the mating rates of males, (c) the length of the copulatory wheel or (d) the probability of a female remating within 1 or 3 days. In panels (a–c), black dots indicate blackened males, grey dots indicate males with clear ink, empty circles indicate unmanipulated males and horizontal lines represent group means. In panel (d), black bars represent blackened males and empty bars represent control males.

Figure 3.

Effects of the experimental treatment on survival of males seen in close proximity to H. titia males. Kaplan–Meier plot, crosshatches indicate censored data points.