Lizard colour plasticity tracks background seasonal changes

Environmental heterogeneity on spatial and temporal scale fosters organism’s capacity to plastically alter coloration. Predation risk might favour the evolution of phenotypic plasticity in colour patterns, as individuals, which change colour throughout the year, could be able to improve their fitness. Here we explored the change in dorsal pigmentation of the Italian wall lizard (Podarcis siculus campestris) along three time points (March, July and October) during the period of activity. Lizard dorsal pictures were collected on the field, with the support of a reference chart to quantitatively estimate chromatic variables (hue, saturation and value, HSV). At the same time, pictures of grassy coverings (the most representative portion of the environment subjected to normal seasonal change), were collected. Our findings show that lizards are capable of altering dorsal coloration during seasonal change. They vary from green, at the onset of spring, to brownish in the middle of summer, and greyish colour in October. This modification closely followed environmental background colour variation and enhanced lizard crypsis during each season.


Introduction
Differences in cryptic coloration among animals have provided evidence for significant selective 31 advantages in specific contexts [1,2,3]. One possible trigger of chromatic variation is the habitat-32 specific background colour. When the level of predation is high, a body coloration matching the 33 background colour is common among prey species and has been reported in many studies [4,5]. The  Animal colour regulation may be fixed (i.e. constitutive), for the past effect of natural 43 selection, or expressed over different timescales (from seconds to years) and is regarded as a form 44 of phenotypic plasticity. In order to achieve an effective background matching, colour change can 45 also follow seasonal changes. This phenomenon has been observed in numerous invertebrates and 46 vertebrates. The tropical butterfly Bicyclus anynana shows seasonal variation in wing patterns, 47 following the temperature which anticipates seasonal change in vegetation [13]; the snowshoe hare 48 (Lepus americanus) goes through seasonal coat colour shifts from brown to white. This change has 49 been recorded to be strongly related to survival [14]. 50 Lizard coloration has been explored throughout numerous ecological and evolutionary 51 studies [15,16,17], however seasonal colour change has been rarely explored in this group [18,19].

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In this study, dorsal colour variation in Podarcis siculus campestris was recorded by sampling 53 individuals of the same population in three different months during the yearly period of activity. We 54 also explored the possible differences between sexes in colour variation, and whether it might   With the aim to explore the seasonal change in lizard dorsal colour, we adopted linear mixed 100 models which included SVL as covariate, and sex, month and their 2-way interaction as fixed 101 factors. The date of data collection was included as a random factor. We ran three models with hue, 102 saturation and value as response variables, respectively. In order to meet the assumption of residuals 103 homogeneity, we included a variance structure for a different spread per stratum (i.e. month). This

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In the Italian wall lizard, the observed sharp change in dorsal colouration throughout the 140 seasons could be associated with an anti-predatory adaptation. This is done by displaying a 141 colouration resembling that of those backgrounds in which they are the most exposed to potential 142 predators [25]. Interestingly, we found a similar seasonal trend in background mean values of hue, 143 thus revealing an environmental colour matching with lizard dorsal tonality. The grassy habitat, 144 which in our study area is the main microhabitat used by lizards for thermoregulation, radically 145 changes between spring and summer. The landscape goes from a bright and intense green in spring 146 to a brownish colour in summer due to the dry and arid grass (Fig. 2). In late summer, with the 147 onset of the first rains, the grass begins to regain a greenish colour, creating a mosaic of colours

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We also found interesting results concerning both sex and size/age in relation to colour 158 variation. Males and females did not show particular differences in chromatic variables throughout 159 the sessions. This suggests that dorsal colour variation may be a generalized phenomenon, without 160 implications related to differential sex-dependent strategies. However, saturation and value 161 appeared to be affected by size, thus indicating a possible effect of the individual's age on colour 162 expression, as known for other lizard species [28,29].    Competing interests 180 We declare we have no competing interests.

March
July October