Sexual dimorphism in conspicuousness and ornamentation in the enigmatic leaf-nosed lizardCeratophora tennentiifrom Sri Lanka

Abstract

Measures of physiological performance capacity, such as bite force, form the functional basis of sexual selection. Information about fighting ability may be conveyed through a structural feature such as a rostrum (i.e. horn) or a colour signal and thereby help reduce costly conflict. We quantified sexual dimorphism in key traits likely to be the targets of sexual selection in Tennent's leaf‐nosed lizard (Ceratophora tennentii) from Sri Lanka, and examined their relationship to bite force and body condition. We found body length and bite force to be similar for males and females. However, head length was significantly greater in males and they had significantly more conspicuous throats and labials (chromatic contrast and luminance) than females. Males also had a proportionally larger rostrum, which we predicted could be an important source of information about male quality for both sexes. Rostrum length was correlated with throat chromatic contrast in males but not females. Nonetheless, the rostrum and aspects of coloration did not correlate with bite force or body condition as we predicted. We have no information on contest escalation in this species but if they rarely bite, as suggested by a lack of difference in bite force between males and females, then bite force and any associated signals would not be a target of selection. Finally, males and females had similar spectral reflectance of the mouth and tongue and both had a peak in the ultra‐violet, and were conspicuous to birds. Lizards only gaped their mouths during capture and not when threatened by a potential predator (hand waving). We hypothesize that conspicuous mouth colour may act as a deimatic signal, startling a potential predator, although this will need careful experimental testing in the future.

Comparing colours using visual models

“…7). Here we draw on the data of Whiting et al . (2015), who recorded the spectral reflectance of 29 male and 27 female C. tennentii from four body regions (throat, labials, mouth-roof, and tongue).…”

“…The bootstrapped 95 % C.I’s for mean distances between groups in colour space (right panels). Partly reproduced, with permission, from Whiting et al . 2015.…”

“…Multi-dimensional plotting options for noise-converted coordinates are also available. Lizard colour data from Whiting et al . 2015 are available at http://dx.doi.org/10.6084/m9.figshare.1452908.…”

“…Maier, 1992; Vorobyev et al ., 2001), to estimate the magnitude of colour-differences and ultimately predict whether an observer could effectively discriminate pairs of colours (Chittka, 1992; Vorobyev & Osorio, 1998). This final point is critical to many tests of ecological and evolutionary hypotheses, such as the efficacy of camouflage (Pessoa et al ., 2014; Troscianko et al ., 2016), the presence of polymorphism or dichromatism (Schultz & Fincke, 2013; Whiting et al ., 2015), the accuracy of mimicry (O'Hanlon et al ., 2014; White et al ., 2017), the extent of signal variability among populations or species (Delhey & Peters, 2008; Rheindt et al ., 2014), or the effect of experimental manipulations (Barry et al ., 2015; White & Kemp, 2017). At the heart of these inquiries lies the same question: how different are these colours to the animal viewing them?…”

“…Finally, we can integrate behavioural and psychophysical data into models in order to predict whether an observer could effectively discriminate pairs of colours, or if they would instead be perceptually indistinguishable (Chittka, 1992; Vorobyev et al , 2001; Vorobyev & Osorio, 1998). This final point is critical to many tests of ecological and evolutionary hypotheses pertaining to, for example, the efficacy of camouflage (Pessoa et al , 2014; Troscianko et al , 2016), the precision of mimicry (O’Hanlon et al , 2014; White et al , 2017), the extent of signal variability among populations or species (Delhey & Peters, 2008; Rheindt et al , 2014), the presence of polymorphism or dichromatism (Schultz & Fincke, 2013; Whiting et al , 2015), or the effect of experimental treatments (Barry et al , 2015). At the heart of these diverse inquiries lies the same question: how different are these colours (or more precisely, these samples of colours) to the animal viewing them?…”