Spatial patterns of differentiation in background matching strategies along the Quaternary range expansion route of the cryptic tree frog Hyla sarda

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Roberta Bisconti, Andrea Chiocchio, Giada Spadavecchia and Daniele Canestrelli, Department of Ecological and Biological Sciences, Università della Tuscia, Viterbo, Italy

Background/Question/Methods
Quaternary climatic changes have deeply impacted the range dynamics of temperate species. While the genetic legacy of range dynamics has been widely investigated, little is known about their consequences on the evolution of phenotypes. Antipredatory strategies offer intriguing opportunities to study phenotypic evolution in response to dispersal dynamics, since the ability to avoid predation is particularly crucial for populations at low demographic densities, such as those encountered at a range expansion front. Here we investigated, for the first time, the spatial differentiation of background color matching strategies along a well-characterized Late Pleistocene range expansion route of a temperate species, the Tyrrhenian tree frog Hyla sarda. H. sarda is a small, cryptically colored amphibian endemic to the Tyrrhenian islands, which colonized the Corsica island from the Sardinia island, exploiting a land bridge that connected these islands at the end of the last glaciation. Using a common-garden experiment, we explored whether individuals sampled in the source area (Sardinia) and individuals sampled in the expansion range (Corsica) differ in two components of the camouflage strategy: color change abilities and background choice behavior. Color change abilities were assessed through calibrated dorsal image analyses, after the exposure of the individual tree frogs to distinct monochromatic environments. Background choice behavior was assessed using dichromatic experimental arenas by analyzing the time spent by individual tree frogs on each chromatic substrate.
Results/Conclusions
We found a remarkable spatial pattern of differentiation in background choice and background matching abilities among populations of the Tyrrhenian tree frog Hyla sarda. Within dichromatic experimental arenas, tree frogs from Sardinia showed marked preferences for a green colored background. In contrast, individuals from Corsica showed preferences for brown backgrounds (the difference was highly statistically significant), irrespective of the location of the brown patches within the arena. On the other hand, greenness values of tree frogs exposed to monochromatic environments were significantly higher for Sardinia than for Corsica populations (P<0.001). Finally, we found greenness values and percent time spent on a green substrate in dichromatic environments were strongly correlated (Spearman rho: 0.98; <0.001). Our results suggest the intriguing hypothesis that the past biogeographic history of populations might be an overlooked major player in the shaping current spatial patterns of variation of camouflage-associated phenotypic traits.