Contemporary theory predicts that the degree of mimetic similarity of mimics towards their model should increase as the mimic/model ratio increases. Thus, when the mimic/model ratio is high, then the mimic has to resemble the model very closely to still gain protection from the signal receiver. To date, empirical evidence of this effect is limited to a single example where mimicry occurs between species. Here, for the first time, we test whether mimetic fidelity varies with mimic/model ratios in an intraspecific mimicry system, in which signal receivers are the same species as the mimics and models. To this end, we studied a polymorphic damselfly with a single male phenotype and two female morphs, in which one morph resembles the male phenotype while the other does not. Phenotypic similarity of males to both female morphs was quantified using morphometric data for multiple populations with varying mimic/model ratios repeated over a 3 year period. Our results demonstrate that male-like females were overall closer in size to males than the other female morph. Furthermore, the extent of morphological similarity between male-like females and males, measured as Mahalanobis distances, was frequency-dependent in the direction predicted. Hence, this study provides direct quantitative support for the prediction that the mimetic similarity of mimics to their models increases as the mimic/model ratio increases. We suggest that the phenomenon may be widespread in a range of mimicry systems.
The sedge sprite, N. irene, is a small damselfly (Zygoptera; Odonata) that inhabits marshy or boggy waters and is common throughout most of Canada and the northern parts of the United States . The reproductive season starts in early June and lasts until mid-August, with population numbers peaking from mid-June to mid-July. Nehalennia irene exhibits a clear polymorphism restricted to the female sex, with morphs being easily classified into andromorphs and gynomorphs based on their body coloration  (electronic supplementary material, appendix S1). Adult andromorph females resemble the conspecific male's blue body coloration and pattern by having a triangular patch of pale blue on the anteriodorsal part of segment eight and large paired dark spots on the dorsum of segment nine . Gynomorph females, in contrast, are more yellowish in body coloration and lack the triangular patch and paired spots (for a more detailed description, see ; for colour drawings, see ). Although the genetic basis of the polymorphism has yet to be evaluated in N. irene, laboratory crossing experiments in several related coenagrionid damselfly species are consistent with the polymorphism being determined at a single autosomal locus with female-limited expression (reviewed in ).
Study sites and sampling procedures
Past research with N. irene documented considerable spatial variation in proportion of andromorph females [33,35]. Based on these previous findings, six populations were selected that differed largely in relative abundance of andromorphs to study variation in mimetic similarity (table 1). Previous experimental work in these populations has shown that male harassment rates (i.e. male-mating attempts) towards andromorphs increase with the mimic/model ratio .
To account for possible seasonal variation in body size , each of the selected populations was sampled during the reproductive season for 3 subsequent years from 2007 to 2009. Before collecting individuals, site-specific abundance of andromorphs was determined in a standardized manner as described by Van Gossum et al. . This method is based on counts of adult individuals captured with an insect net while walking slowly in a standardized manner through the reproductive area, sweeping ‘figures of eight’. Numbers of adult males and females of N. irene were recorded and marked prior to their release to exclude multiple counts. In doing so, andromorph frequency (proportion of andromorph females) and the mimic/model ratio (andromorph/male ratio) can be calculated for each site and year (table 1). Both frequency estimates are strongly correlated (Spearman correlation: r = 0.92; n = 18; p < 0.0001). Analyses using andromorph frequency gave similar results as analyses using the mimic/model ratio, such that we only report the latter in what follows. For morphological measurements, we aimed to collect 25 individuals of each adult type (male, andromorph, gynomorph) in each population for each year. All individuals were stored for further measurements in 95 per cent ethanol immediately after capture. A total of 1296 individuals were measured, comprising 422 males, 438 andromorphs and 436 gynomorphs.