cues for male mate preference in an ... - Wiley Online Library

Ecological Entomology (2011), 36, 544–548

DOI: 10.1111/j.1365-2311.2011.01300.x

Show your true colour: cues for male mate preference in an intra-specific mimicry system A R N E I S E R B Y T and H A N S V A N G O S S U M

Evolutionary Ecology Group, Department of

Biology, Antwerp University, Antwerp, Belgium

Abstract. 1. Polymorphism limited to the female sex occurs in a variety of animal species and has been shown to be an attractive model system for examining general questions in signal detection theory. 2. When observed in damselflies, typically one female morph is an example of sexual dimorphism, whereas the other is considered as a functional malemimic that resembles the male’s phenotype in several traits. 3. While several studies focused on male harassment and subsequent cost/benefit trade-offs in female morphs, it remains understudied at the proximate level, which precise cues are relevant to mate-searching males for discriminating among potential mates. 4. In the present study, we scored male mate preference to natural and manipulated phenotypes in the polymorphic damselfly Nehalennia irene Hagen. 5. In contrast to expectation, male preference did not change when colour was manipulated and male preference remained consistently for andromorph > male > gynomorph across treatments. 6. This suggests that cues other than body coloration primarily affect male mate preference in the present study system. Key words. Female polymorphism, male preference, mimicry, Nehalennia irene,

phenotypic manipulation, sexual conflict, signal detection.

Introduction While male reproductive success is generally increased with rising mating rate, female optimal mating rates are typically lower compared with males (Arnqvist & Rowe, 2005). As a consequence, females may face more mating attention than desired, thereby suffering reduced fitness (e.g. McLain & Pratt, 1999; Schlupp et al., 2001; Sakurai & Kasuya, 2008). In this context, female polymorphism may have evolved as an evolutionary strategy to lower redundant male mating attention (e.g. Robertson, 1985; Hinnekint, 1987). Mate-searching males may then become challenged when confronted with multiple female morphs and in turn benefit from being selective, rather than randomly approaching females (Härdling & Bergsten, 2006). Specifically, adopting a search image for the more common female morph may allow faster access to a mate. Consequently, the uncommon female morph will receive relatively less male attention (Van Gossum et al., 2001a). Such female polymorphism is commonly observed in damselflies (Insecta: Odonata), with wide variation in female Correspondence: Arne Iserbyt, Evolutionary Ecology Group, Antwerp University, Groenenborgerlaan 171, Antwerp, B-2020 Belgium. E-mail: [email protected]

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morph frequencies across populations (e.g. Sánchez-Guillén et al., 2005; Iserbyt et al., 2009). Males are generally conspicuously coloured, which has been suggested to signal their unprofitability as a mate towards other males (antiharassment aposematism: Sherratt & Forbes, 2001). In this way, mate-searching males diminish their probability to mistakenly attempt mating conspecific males rather than females. Complexity rises when female polymorphism occurs, with typically one female morph (andromorph) resembling several aspects of the males’ phenotype [body coloration and melanin pattern (e.g. Forbes et al., 1995), behaviour (e.g. Van Gossum et al., 2001b), body size, and shape (e.g. Abbott & Gosden, 2009)], whereas the other female morph (gynomorph) does not. Although females need males to fulfil their reproductive needs, when beyond a females’ optimal mating rate, andromorphs are considered to be functional male-mimics that benefit from reduced levels of male harassment (e.g. Robertson, 1985; Hinnekint, 1987). While assuming that males use one, or a combination of traits as cues for mate discrimination, empirical work indeed confirms that andromorphs, compared with gynomorphs, experience lower yet frequency-dependent mating probabilities (Cordero Rivera & Sánchez-Guillén, 2007; Hammers & Van Gossum, 2008). Further, Ting et al. © 2011 The Authors Ecological Entomology © 2011 The Royal Entomological Society

Mate preference and intra-specific mimicry 545 (2009) showed that with rising proportions of andromorphs, the readiness of males to respond sexually towards andromorphs increased at an approximately similar rate as towards other males. This outcome can be understood from the perspective of signal detection theory, which is now widely applied to understand the evolution of mimicry (e.g. Holen & Johnstone, 2004). Specifically, if the number of profitable andromorphs, relative to unprofitable males increases, then males should become more prepared to attack male-like individuals, including both males and andromorphs. In this way the underlying probability of a correct classification rises. In the present study, we elaborate on previous research (Ting et al., 2009) and study male-mate preference for males and female morphs in populations where andromorphs are the majority female morph. As indicated, andromorphs resemble conspecific males in several traits, but at a proximate level it remains understudied which precise cues are most relevant for mate-searching males to discriminate between potential mates. Body coloration of andromorphs and males is very similar for human observers. Furthermore, odonates have well developed colour vision (Briscoe & Chittka, 2001). Hence, we manipulated body coloration across sexes and female morphs, and compared the degree of male attraction with unmanipulated individuals. Because we only manipulated colour, all other sex- and morph-specific traits, such as morphology and behaviour, remained unaffected. Based on previous findings, our prediction is that males will have a frequency-dependent preference for the most common female phenotype in the population (cf. Van Gossum et al., 2001a; Ting et al., 2009). Further, when changing body colours of males and andromorphs into gynomorphs and vice versa, we would predict males to switch their phenotype-specific preference in accordance to their preferred body coloration. Finally, when all phenotypes are uniformly coloured, we expect male-mate preference to be a random choice.

were glued (UHU® Power Glue) on a grass stem and were presented to sexually active males in an alternating manner at the ponds edge (cf. Fincke et al., 2007; Ting et al., 2009). Five experimental colour treatments were applied to the models: (i) ‘not manipulated’, i.e. natural body coloration, (ii) ‘control’, painted the lateral sides of the thorax and the last three abdominal segments in their own colour to determine the paint effect, (iii) ‘switched phenotype’, same as ‘control’ but painted andromorphs and males on thorax and abdomen as gynomorphs and vice versa, (iv) ‘pink’ and (v) ‘black’, painted body parts as in ‘control’ but in pink or black, respectively. Previous studies showed that painted phenotypes change male mating preference successfully (Bick & Bick, 1965; Cordero et al., 1995; Gorb, 1998; Miller & Fincke, 1999). We used a mixture of acrylic paints (Golden Fluids Acrylic™) and used human sight to match the paint as closely as possible to the natural body colorations. In total, 16 models of each type (andromorph, gynomorph and male) in each treatment (v) were presented to five sexual active males, i.e. in total 1200 presentations to different focal males (16 × 3 × 5 × 5). The behavioural response of the focal male towards the model were categorised as follows: (i) ‘no response’, the male did not respond within a presentation time of 120 s, (ii) ‘fly away’ when the male showed no interest and flew away (sometimes after inspecting but without touching the model), (iii) ‘contact’, the male approached and briefly made a physical contact, (iv) ‘tandem attempt’, male sexual attempt to grasp the models’ pronotum with his anal appendages to form the so-called tandem position, required for mating. Thus each focal male scored a ‘1’ in one of these categories (presence of behaviour) and a ‘0’ (absence) in all the others. When the focal male showed sexual response, or when the maximal presentation time (120 s) was reached, the male was caught and marked with a permanent pen to avoid multiple testing. Although behaviour is limited when glued on a grass stem, models are still able to show refusal displays, such as wing displays or abdomen curling. Occurrence of any kind of model behaviour was noted as binary data.

Materials and methods Model species and study sites

Statistical analyses

Nehalennia irene Hagen is a non-territorial North American damselfly with one male and two different female morphs. Andromorph females closely resemble males in blue lateral thorax coloration and melanin pattern of the last three abdominal segments (see Forbes et al., 1995, for more detail). Gynomorph females show a distinct yellow-green thorax and have a different abdominal pattern than males and andromorphs. Furthermore, andromorphs are overall closer in size to males than gynomorphs are (Iserbyt et al., 2011). In the present study, male-mate discrimination was tested in two populations in British Columbia (Canada). In both cases andromorphs constituted the majority female morph (>90%).

Generalised linear mixed models (PROC GLIMMIX in SAS 9.2, SAS Institute Inc., Cary, North Carolina) with a binomial error structure were applied to evaluate variation in male response to the different experimental models. We used the presented model, colour treatment, and their interaction plus occurrence of model behaviour as explanatory variables. Furthermore, site and presented individual were added as random variables to control for a possible variation between sites and to control for pseudo-replication.

Experimental design Male-mate preference was tested with manipulated and unmanipulated models: andromorphs, gynomorphs or males

Results The male behavioural responses ‘no response’ and ‘contact’ rarely occurred (respectively 3.3% and 7.3% out of 1200 presentations). ‘Tandem attempts’ (30.2%) and ‘fly away’ (59.2%) were therefore two almost mutually exclusive observational endpoints. Results of analyses with ‘fly away’ or ‘tandem

© 2011 The Authors Ecological Entomology © 2011 The Royal Entomological Society, Ecological Entomology, 36, 544–548

546 Arne Iserbyt and Hans Van Gossum

Fig. 1. Mean number (±SE) of male sexual responses towards manipulated and unmanipulated andromorph (A), gynomorph (G) and male (M) models of the damselfly Nehalennia irene.

attempts’ as dependent variables gave very similar but opposite results. Here, we only present results of the ‘tandem attempts’. Males clearly discriminated among the presented models (F2,958 = 13.03; P < 0.0001), however, model-specific preference did not differ across colour treatments (treatment × model: F8,954 = 0.36; P = 0.94; main effect treatment: F4,955 = 0.90; P = 0.47). Specifically, males strongly preferred to mate with andromorph models (mean ± SE out of five presentations: 1.96 ± 0.29) relative to either gynomorph (1.02 ± 0.18; Bonferroni’s corrected post-hoc comparisons: T958 = 5.10; P < 0.0001) or male models (1.54 ± 0.30; T958 = 2.11; P = 0.035; see Fig. 1). Gynomorph models were least preferred by males (relative to other presented males: T958 = −3.03; P = 0.0025). The occurrence of model behaviour had no effect on male mating preference (F1,957 = 2.23; P = 0.14). Discussion In short, our experiment clearly showed that males were differentially interested in the presented phenotypes. Sexual response by mate-searching males was highest towards andromorph females, lower towards conspecific males, and lowest towards gynomorph females. This preference order was remarkably consistent and independent of the presented models’ manipulated colour. To interpret these unexpected results, we will first review current knowledge on colour vision in odonates. Second, we will evaluate what is known on cues in male mate discrimination for polymorphic damselflies, and third, we will discuss the implications of our observations on the evolution and maintenance of intra-specific mimicry. First, odonates have well-developed colour vision (Briscoe & Chittka, 2001) and are able to perceive colour signals spanning a broad wavelength range from 350 to 700 nm (reviewed in Schultz et al., 2008). Spectrophotometric measures indicate that this potential for colour perception corresponds with the reflected spectra of the thorax cuticle. Furthermore, reflectance

spectra are remarkably similar between andromorphs and males, but different from gynomorphs (Fincke et al., 2007; Van Gossum et al., 2011). Therefore it does not surprise that body coloration is generally assumed to act as the most relevant trait for male mate discrimination in polymorphic damselflies (e.g. Gorb, 1998; Miller & Fincke, 1999; Joop et al., 2006; Van Gossum et al., 2011). Second, numerous studies showed that mate-searching males discriminate among phenotypes, rather than randomly approaching individuals (e.g. Miller & Fincke, 1999; Van Gossum et al., 2001a, 2011; Cordero Rivera & Sánchez-Guillén, 2007; Hammers & Van Gossum, 2008; Takahashi & Watanabe, 2009; Ting et al., 2009). As indicated before, males and female morphs differ in several phenotypic traits. At a proximate level, however, few studies attempted to quantify the exact cues that are relevant to mate-searching males. Previous preference experiments with manipulated phenotypes indicated that males initially discriminate among phenotypes based on sex-specific traits (abdominal melanin patterning) and additionally use morph-specific cues, such as body coloration in finetuning their search image (Gorb, 1998; Miller & Fincke, 1999). However, phenotype-specific body coloration and abdominal melanin pattern can differ substantially among polymorphic species (Joop et al., 2006). In some species, reflectance spectra and melanin patterning of andromorphs and males are so similar (Van Gossum et al., 2008, 2011), that a mate-searching male would likely need to rely on other cues to recognise these females as potential mates. In support, male sexual interest differed for presented dead and alive individuals, indicating the relevance of a potential mates’ behaviour (Cordero et al., 1998; Cordero Rivera & Andrés, 2001; Andrés et al., 2002). For N. Irene, however, no morph-specific behavioural differences were observed in a similar population where andromorphs were the most frequent morph as well (Iserbyt & Van Gossum, 2009). Furthermore, although behaviour of glued individuals is limited, yet still possible in the present study, we did not observe an effect of model behaviour on male-mate preference. However, behaviour can be measured in various ways, and we


Mate preference and intra-specific mimicry 547 acknowledge that further research focus is needed to evaluate the relevance of signalling a mates’ resistance or tolerance to mating (cf. Gosden & Svensson, 2009). Nonetheless, we showed male preference to differ among presented phenotypes and as it seems, independent of body coloration and perhaps behaviour. The consistency in our results suggests that signals, other than the manipulated ones, are more relevant for males to be attracted. We only painted those body parts that differed most obviously among phenotypes for the human eye, i.e. the last three abdominal segments and lateral sides of the thorax. Possibly more subtle differences, such as the compound eye colour or slight difference in metallic lustre of the dorsal black parts, are relevant for males to discriminate among phenotypes (cf. Bick & Bick, 1965). Perhaps differences in ventral body coloration may also be relevant, depending on the males’ angle of approach (Gorb, 1999). Furthermore, males and female morphs differ in body size and shape (Abbott & Gosden, 2009; Bots et al., 2009; Iserbyt et al., 2011). However, these differences are much smaller between female morphs than between sexes. Given the observed male preference for andromorphs > males > gynomorphs, we consider it unlikely that morphological differences act as the only cue in explaining our observed pattern in mating preference. Further, odonate antennae bear sensory structures that most likely enable them to perceive odours (Rebora et al., 2010). Therefore, it may not be ruled out that chemical signals are a cue in mate-searching males, as also suggested by Cordero et al. (1998). However, odonates are generally assumed to detect potential mates relying on visual cues and the presence of sexual pheromones has never been documented in any odonate species (Corbet, 1999). Clearly, further research is warranted to identify which cues matter for mate discrimination in the present study system. Third, we performed our experiment in the Western edge of the species’ distribution range (Walker, 1953), a region that is characterised by consistent and high relative andromorph frequencies (Van Gossum et al., 2007; Iserbyt et al., 2009). Recent empirical evidence in such populations suggests a stronger selection to improve mimetic similarity of andromorphs to males (Iserbyt et al., 2011). On the other hand, the present study area is also characterised by low population densities (Iserbyt et al., 2009) and low genetic diversity (Iserbyt et al., 2010). Further, in line with predictions from the signal detection theory, we observed the rare gynomorphs to be least preferred, and that male–male mating attempts frequently occurred. These observations raise the question whether generally proposed explanations in the context of female polymorphic damselflies may not apply in the present study region (see also Van Gossum & Sherratt, 2008). Male conspicuousness may be evolved as a signal to reduce male–male harassment (Sherratt & Forbes, 2001) and andromorphs may generally be functional male mimics that experience reduced male harassment compared with gynomorphs when beyond their optimal mating rate (Van Gossum et al., 2011). However, these explanations may not apply for populations of N. irene in Western Canada. Perhaps, mimicry may have broken down in these populations where andromorphs largely outnumber gynomorph females (cf. Holen & Johnstone, 2004; but see

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