Color Change for Thermoregulation versus Camouflage in Free-Ranging Lizards

Abstract

Animal coloration has multiple functions including thermoregulation, camouflage, and social signaling, and the requirements of each function may sometimes conflict. Many terrestrial ectotherms accommodate the multiple functions of color through color change. However, the relative importance of these functions and how colorchanging species accommodate them when they do conflict are poorly understood because we lack data on color change in the wild. Here, we show that the color of individual radio-tracked bearded dragon lizards, Pogona vitticeps, correlates strongly with background color and less strongly, but significantly, with temperature. We found no evidence that individuals simultaneously optimize camouflage and thermoregulation by choosing light backgrounds when hot or dark backgrounds when cold. In laboratory experiments, lizards showed both UV-visible (300-700 nm) and near-infrared (700-2,100 nm) reflectance changes in response to different background and temperature treatments, consistent with camouflage and thermoregulatory functions, respectively, but with no interaction between the two. Overall, our results suggest that wild bearded dragons change color to improve both thermoregulation and camouflage but predominantly adjust for camouflage, suggesting that compromising camouflage may entail a greater potential immediate survival cost.

Geographic divergence and colour change in response to visual backgrounds and illumination intensity in bearded dragons

“…S1). Previous experiments on temperature-dependent colour change showed 1-2% change in UV reflectance across all individuals (Smith et al, 2016a), despite greater visible colour change than detected here. Separate experiments (Smith et al, 2016a) quantifying colour change during handling based on nearsimultaneous photographs and spectral measurements confirmed that the distribution of colours of lizards in RGB colour space was statistically similar to the distribution of colours in avian or lizard visual colour space (Smith et al, 2016a).…”

“…Previous experiments on temperature-dependent colour change showed 1-2% change in UV reflectance across all individuals (Smith et al, 2016a), despite greater visible colour change than detected here. Separate experiments (Smith et al, 2016a) quantifying colour change during handling based on nearsimultaneous photographs and spectral measurements confirmed that the distribution of colours of lizards in RGB colour space was statistically similar to the distribution of colours in avian or lizard visual colour space (Smith et al, 2016a). Those experiments were done using the same captive lizards from Mildura and Alice Springs as used in this study and validate the use of linearised and equalised RGB data from standard photographs to approximate variation perceived by avian predators or conspecifics (Smith et al, 2016a).…”

“…A temperature gradient between 25 and 50°C was maintained inside the terrarium during the light phase, allowing for behavioural thermoregulation. These temperatures are within the natural range experienced by bearded dragons in the wild (Smith et al, 2016a …”

“…S1). Although standard digital photographs only capture colour in the humanvisible spectrum and diurnal lizards such as bearded dragons (and many of their predators) are likely to be able to see in the UV (Olsson et al, 2013), these lizards and their backgrounds have minimal UV reflectance; photographs capture most of the colour variation (Smith et al, 2016a).…”

Barrens of gold: gonad conditioning of an overabundant sea urchin

“…Second, RGB values were equalised relative to the 20% grey standard in each image to account for slight variation in illumination (Stevens et al 2007). Further details of the linearisation and equalisation method are given in Stevens et al (2007), Garcia et al (2013) and Smith et al (2016). We then calculated the standardised difference between the R and G channels as (R − G) / (R + G + B), which was defined as x, and the G and B channels as (G − B) / (R + G + B), which was defined as y.…”