Female European green lizards (Lacerta viridis) prefer males with high ultraviolet throat reflectance
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- Bajer, K., Molnár, O., Török, J. et al. Behav Ecol Sociobiol (2010) 64: 2007. doi:10.1007/s00265-010-1012-2
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The role of ultraviolet (UV) signals in intraspecific communication has been identified in a number of vertebrate taxa. In lizards, the signalling role of UV has only been shown in male–male competition and male mate choice. Here, we investigated whether male UV colour can be a basis of female association preference in European green lizards (Lacerta viridis), a species where males develop blue nuptial throat colouration with high UV reflectance. We experimentally manipulated the UV colour of male pairs, where the members of the pair did not differ significantly in body length, body weight, head size, throat UV chroma and brightness or throat blue chroma and brightness measured prior to colour manipulation. By providing these pairs of males to females (only visual stimuli could be perceived by the females), we assessed the role of UV in female association preference irrespective of other potentially important visual traits. We found that unmated but receptive females preferred males of higher UV reflectance. Our results show for the first time that UV colour can be an important male signal in female preference in reptiles.
KeywordsLacerta viridisMate choiceMate preferenceSexual selectionSignallingUV
Even though sexual selection is a widely studied topic (see Andersson 1994), surprisingly little is known about the mechanisms and patterns of sexual selection in reptiles despite the bewildering array of colourful ornaments, complex behavioural displays and chemical signals they exhibit (e.g. West-Eberhard 1983; LeBas and Marshall 2000; López and Martín 2005a; Kopena et al. 2009; Martín and López 2009). Furthermore, while female mate choice has been observed in a wide variety of taxa (Andersson 1994), it has rarely been shown in lizards (Olsson and Madsen 1998). In non-territorial lizard species, males actively search for females, and mating is under strict male control (Fitze et al. 2005). As such, female mate choice is unlikely to manifest (but see Censky 1997); thus, females maximize their fitness with the aid of multiple copulations, sperm competition and sperm selection (Olsson and Madsen 1995, 1998). However, in territorial species, females can either choose their reproductive partner directly, i.e. according to the quality of the male (López et al. 2002; López and Martín 2005b; López et al. 2006; Martín and López 2008), or indirectly, based on the quality of the male’s territory (Calsbeek and Sinervo 2002).
Conspicuous colouration or other nuptial traits can signal male quality and are therefore suitable for reliably predicting the outcome of female mate choice (Andersson 1994). Only a few studies have found evidence of visual female mate choice in reptiles, primarily based on the size of males (Cooper and Vitt 1993; Martín and Forsman 1999). Some of the visual traits present in animals are not visible to humans and as such have not been studied in detail. For example, only recent studies have shown that ultraviolet (UV) signals are commonly used during sexual communication in birds and fish with UV perception (e.g. Bennett et al. 1996, 1997; Hunt et al. 1998; Cuthill et al. 2000; Smith et al. 2002; Siebeck 2004; Rick and Bakker 2008). Sexual dimorphism in UV colour has been shown in a lizard, Lacerta lepida (Font et al. 2009). Further, there is some evidence that UV colouration of males of certain green lizard species (Lacerta viridis and Lacerta schreiberi) are correlated with attributes that might provide basis for female mate choice (Václav et al. 2007; Martín and López 2009). However, while the role of UV in male–male competition (Stapley and Whiting 2006; Whiting et al. 2006) and in male mate choice (LeBas and Marshall 2000) has been demonstrated in lizards, the single study testing the role of male UV colouration in female mate choice has rejected its importance (LeBas and Marshall 2001).
The aim of the present study was to examine whether female European green lizards (L. viridis) distinguish between males based on their UV reflection. L. viridis males display a blue throat patch, which have a strong UV component, during the reproductive season. Although there are no available data on the visual system of L. viridis, the presence of UV-receptive visual pigments appears to be both frequent and conservative in reptiles (Fleishman et al. 1993, 1997; Ellingson et al. 1995; Loew et al. 1996; Sillman et al. 1997; Vorobyev 2003). To investigate the role of male UV colouration in female preference in L. viridis, we conducted experimental tests by manipulating the UV reflectance of males. By offering morphologically matched male pairs with manipulated UV reflectance to receptive unmated females, we tested the hypothesis that UV reflectance in male L. viridis is a signal that is used as a cue in female association preference. Note that the lizards did not mate in our experiment; hence, we use “female association preference” throughout the paper instead of female mate preference or female mate choice.
Study species and field sampling
L. viridis is a medium sized (snout–vent–length [SVL] = 80–120 mm) and widespread lizard throughout Europe (Gasc et al. 1997). Males are green with various spots, have a yellowish abdomen and develop blue nuptial coloration on their throats during the reproductive season (e.g. Václav et al. 2007) during which they defend territories. This blue patch on the throat has a strong UV reflectance (see “Results”). L. viridis males emerge from hibernation at the end of April, followed approximately 10 days later by the females. There are no data available on the cytological features of the annual sexual cycle of this species, but in two lacertid species (Lacerta agilis and Zootoca vivipara) occupying climatically similar habitats to L. viridis hibernation is immediately followed by vitellogenesis (Saveliev et al. 2006).
In 2007, 40 adult males and 20 adult females were captured from the study area (a forest–scrub–grassland mosaic near Tápiószentmárton, Hungary; 47°20′25″ N, 19°47′11″ E) by noosing. Only individuals with intact or fully regenerated tails were used in the experiment. To ensure females being receptive, we captured unmated females as soon as they appeared in the field. The status of females (mated vs. unmated) can easily be assessed in the field by visually checking the presence of mating scars (e.g. Fitze et al. 2005). After capture, morphological and colour data were taken, and animals were kept individually from 1 to 9 days in outdoor plastic terraria (30 × 40 × 20 cm; width × length × height, respectively) until the experiments were started. Animals were fed daily with mealworms (Tenebrio molitor) and crickets (Gryllus bimaculatus) and provided with fresh water ad libitum throughout the experimental period. We also used colour data (in one particular analysis, see “Results”) from males (N = 28) captured in 2008 for other scientific reasons.
For each lizard, we measured body weight (BW) with a digital scale to the nearest 0.01 g and SVL with a digital calliper to the nearest 0.1 mm. We also measured head length, head width and head depth to the nearest 0.1 mm.
Males (N = 40) were paired according to their SVL, with a maximum difference of 2 mm. We ran a principal components analysis (PCA) on head length, head width and head height in order to characterize head size with one (or more independent) variable(s). The PCA resulted in one PC with an eigenvalue >1 (2.622), describing head size (factor loadings on all original variables were <−0.9), which was used in subsequent analyses. We used paired t-tests to examine whether males (UV-reduced vs. control) within one pair differed in any morphological variables (SVL, BW, head size) prior to UV treatment. As the male pairs were chosen to be of similar SVL, we compared the remaining variables without correction for SVL. There were no significant differences in SVL (t15 = −0.73, p = 0.477), BW (t15 = −0.72, p = 0.485) or head size (t15 = −0.16, p = 0.874) between the manipulated vs. control males.
In order to provide male pairs that differ in throat UV, but not in any unmeasured background trait connected to UV, we manipulated male UV reflectance. The UV-reducing agents were Parsol® 1789 by Roche (4-tert, buty-4′-methoxy-dibenzoylmethane) that blocks near UV (320–400 nm) and Parsol® MCX by Roche (octyl methoxycinnamate) that blocks far UV (290–320 nm) (Andersson and Amundsen 1997) and which were mixed with duck preen gland fat (e.g. Korsten et al. 2007) and applied to male throats. A randomly chosen male from each pair was treated with fat containing the UV-reducing agents (UV-reduced) while the other male was treated with pure fat (control) using a fine paintbrush.
Association preference experiment
After the experiments, all males and females were released at the exact site of capture. None of the experimental animals suffered any injury, and no negative effects of the lizards’ physical condition had been observed.
First, we compared the UV brightness and UV chroma (relative UV intensity) ranges of the UV-reduced and control males to their own pre-manipulated (=intact) ranges and to those of males caught during the reproductive season of 2008 to see whether the throats of our experimental males reflected the natural UV range. Here, we applied a general linear models (GLMs) with male groups (UV-reduced, control, 2007 intact, 2008 intact) as a factor and UV brightness or UV chroma as a dependent variable.
After the UV manipulation, UV and blue brightness and chroma were re-tested by paired t-tests between UV-reduced vs. control males to ensure that our treatment had the desired effect. Finally, female association preference was tested using a paired t-test on the number of times a female was observed on the UV-reduced vs. control male side of the arena. Because count data is often problematic to use in parametric tests, we also run a Wilcoxon matched-pairs test.
All tests were performed using STATISTICA for Windows v. 7.0 (StatSoft Inc., Tulsa, OK, USA).
The GLMs revealed that both mean UV brightness and UV chroma differed among UV-reduced, control, intact in 2007 and intact in 2008 males (UV brightness: F3,85 = 11.36, p < 0.001; UV chroma: F3,85 = 22.96, p < 0.001). In short, it seems that both of our experimental groups represented natural absolute and relative UV reflectance because the 95% confidence intervals for both treatments fall within the range of the intact lizards for both UV brightness and UV chroma (Fig. 2a, b).
Finally, we found that the females’ spatial distribution was significantly affected by our treatment (t15 = −3.02, p = 0.008; Fig. 4e). The Wilcoxon matched-pairs test gave similar result (Z = 2.66, N = 16, p = 0.008). Female L. viridis preferred control males over UV-reduced males or, in other words, they were spatially associated with males having higher UV reflectance.
We found that receptive, unmated L. viridis females preferred to be associated to males with higher relative UV reflection on their throat over males with experimentally reduced UV reflectance. Due to the experimental setup, females could only choose based on a male’s visual traits, as chemical communication was not possible. Since male pairs did not differ significantly in other morphological characters, females could not have chosen males on the basis of these. However, an additional effect stemming from the differences in blue brightness—but not chroma—cannot be excluded. Female mate preference based on UV reflectance has been repeatedly shown in birds (Bennett et al. 1996, 1997; Andersson and Amundsen 1997; Andersson et al. 1998; Pearn et al. 2001), fish (Smith et al. 2002; Boulcott et al. 2005) and even invertebrates (Kemp 2008; Li et al. 2008), but to the authors’ knowledge this is the first case that it has been shown in a reptile.
Although some authors have concluded that female lizards generally do not choose their mates (Olsson and Madsen 1995; Tokarz 1995), there is some evidence for female mate choice in this taxon. Male body size is a trait that had been shown to play a role in female preference and to affect male reproductive success (e.g. Stamps 1983; Ruby 1984; Cooper and Vitt 1993; Martín and Forsman 1999). Only a handful of studies have examined colour-based mate choice among lizards. For instance, in a multivariate approach, Hamilton and Sullivan (2005) showed that, besides male body size and head size, body and tail colouration also attracted Urosaurus ornatus females. Colour-based female mate preference has also been suggested by interpopulation comparisons (Baird et al. 1997; Kwiatowski and Sullivan 2002) and correlative field studies (Salvador and Veiga 2001; Salvador et al. 2008). Yet, these studies used highly subjective colour definitions without exploring the role of UV colour in mate choice (but see LeBas and Marshall 2000, 2001). The only study supporting the role of UV in mate choice in reptiles is that of LeBas and Marshall (2000) in the agamid lizard, Ctenophorus ornatus, where males preferred females with higher UV chroma. However, the role for UV signalling in advertising fighting ability, and thus male rank, was detected in Platysaurus broadleyi (Stapley and Whiting 2006; Whiting et al. 2006).
Structural colour (caused by interference effects; such as UV) signals were previously not accepted as quality signals because their development was regarded as independent of the costly biochemical pathways or resources connected to the development of pigment-based colour (Jawor and Breitwisch 2004). Nevertheless, structural colour has been shown to be condition dependent and costly (Olsson 1993; Simmons and Bailey 1993; McGraw et al. 2002; Siefferman and Hill 2005) and to be affected by the environment (Figuerola and Senar 2005; Penteriani et al. 2006), suggesting that structural colours can be reliable signals just like pigment colours (e.g. Senar et al. 2003). So, why would L. viridis females prefer males with high throat UV reflection? The most likely—and non-exclusive—explanations are that high UV reflectance signals either (1) the male quality directly (either genetic quality or actual health status) or (2) the quality of the male’s territory. It has been shown that multiple colour signals of the same individual can transfer information about dominance, health status or pairing status at the same time (Martín and López 2009). In another L. viridis population, Václav et al. (2007) showed that different aspects of colour—but not UV—were related to tick infection. It is also noteworthy that, in a different study on L. viridis, we found that good environmental conditions positively affected the development of throat UV colouration in males (Bajer et al., unpublished data), suggesting that developing high UV reflectance depends on individual quality and thus can be an honest signal. While detailed data on the mating system of L. viridis is lacking, according to our field observations (the pair can be together for more than 30 min before the actual mating happens, and the size difference within the mating pair is often negligible) females have time to assess their mates and possible means to express their choice.
In summary, our results have three levels of implications for sexual selection in lizards. First, we found female preference towards male attributes in a widespread lacertid lizard. This is an important finding, because it is widely accepted that female lizards have no control over mating (Olsson and Madsen 1995; Tokarz 1995) and that females can choose between sires of their offspring only at the sperm level (Olsson and Madsen 1995, 1998). Second, we showed that female preference in this case is based on a visual ornament and not on body size or the size of certain body parts only. Data supporting such a phenomenon are rare in the literature, and studies where colour is properly measured and female choice is evaluated under standardized manipulative experiments are extremely scarce. Third, this is the first study to suggest that UV-colour-based female preference for males exists in reptiles.
We thank Közép- Duna- Völgyi Környezetvédelmi, Természetvédelmi és Vízügyi Felügyelőség for permission to conduct this study (Project no.: 15954-2/2008). We are highly indebted to Gergely Hegyi and Natasha LeBas for their constructive comments leading to improvements of the manuscript and Johan Kotze for correcting the English. This work was supported by an OTKA (Országos Tudományos Kutatási Alapprogramok; ref. no. F68403) grant to G. H.