Summary
Female-limited polymorphism is often attributed to selection to avoid excessive male mating attempts. It is encountered in various taxonomic groups, but is particularly common in damselflies, where one female morph (andromorph) typically resembles the conspecific male in colour pattern, while the other(s) (gynomorph(s)) do not. Two sets of theories have been proposed to explain the phenomenon in damselflies, which can be classified as the learned mate recognition (LMR) and male mimicry (MM) hypotheses. To test predictions of these hypotheses, we evaluated the rate of male sexual response towards female morphs and conspecific males in the damselfly Nehalennia irene. The LMR hypothesis predicts that males should respond sexually to andromorphs at greater rates in populations containing a higher relative frequency of andromorphs. The MM hypothesis predicts that males respond more often sexually to both andromorphs and males as the ratio of andromorphs to males increases. While LMR predicts that the rate of mating attempts towards gynomorphs should vary, the MM predicts that it should be relatively fixed. On experimentally presenting live specimens to focal males in five different populations with extreme variation in female morph frequencies, we observed that as the andromorph frequency and ratio of andromorphs to males increased, the proportion of male mating attempts increased on both andromorphs and males, whereas it decreased on gynomorphs. While the simplest form of the MM hypothesis is rejected, the results support specific predictions of both hypotheses and suggest that future studies should not treat these hypotheses as mutually exclusive.
Methodology
Study species and populations
The damselfly N. irene is widely distributed across Canada, from Newfoundland in the east to British Columbia in the west, but it is not present in the far north of the country (Walker 1953). It is a small, non-territorial damselfly (body length 25–28 mm; Cannings and Stuart 1977) that inhabits marshy or boggy areas with relatively dense vegetation. The species mostly stays close to the water’s edge and rarely ventures out over open water (Walker 1953; Cannings and Stuart 1977).
N. irene exhibits a clear colour polymorphism restricted to the female sex, with morphs classified into andromorphs and gynomorphs based on their body coloration. Andromorphs resemble conspecific males as they have a similar blue-green body colour and black patterning on the dorsum of abdominal segment nine. In contrast, gynomorphs (also sometimes referred to as ‘heteromorphs’; Johnson 1975) are different from males since their body colour is green-yellow and they lack the distinctive triangular patch which andromorphs and males show on segment eight (see Forbes et al. 1995 for more detailed description). The genetic basis of the colour polymorphism has yet to be established for N. irene; however, the polymorphism has been confirmed not to be age related (Forbes et al. 1995).
Experimental design
Focal male mating behaviour was examined within 5 m of the water’s edge by presenting mature live specimen types (andromorphs, gynomorphs and males) to a randomly selected mature focal male (for similar methodology, see Forbes et al. 1997; Van Gossum et al. 2007b; Fincke et al. 2007) during the reproductive season of 2007 (mid-June to late July). All experiments were conducted between 9 am and 4 pm local time, on days with suitable weather conditions (sunny without precipitation) when N. irene was reproductively active. The specimen types were obtained by gluing a live mature individual (andromorph, gynomorph or male) with the ventral side of the thorax and its mouth parts to the end of a long grass stem (~45 cm long) with UHU® Power Glue (see Fig. 2). Subsequently, specimen types were presented to sexually mature focal males by holding the grass stem at a 45–65° angle and slowly moving the specimen lateral towards (<1 body length from) the male. The specimens were slowly presented to the focal males to avoid eliciting a startle response. These focal males were found undisturbed, and they were typically perched freely in the surrounding vegetation. To avoid any form of observer bias, the same human observer always presented the three different specimen types on any given occasion.