Structural and colored disruption as camouflage strategies in two sympatric Asian box turtle species (Cuora spp.)

Abstract

Disruptive coloration is a common camouflage strategy that breaks body outlines and ostensibly blends organisms into complex backgrounds. However, contrasting false edges caused by an animal's structure can also break body outlines, although there is no empirical evidence to support this strategy. Here, we examined the Gabor edge disruption ratio (GabRat) of two species, the keeled box turtle (Cuora mouhotii) and the Indochinese box turtle (C. galbinifrons), on preferred (e.g., deciduous leaves) and non-preferred (i.e., grass) substrates. We quantified edge disruption in different substrates to compare interspecific differences in the GabRat values of disruptive coloration among the turtles' preferred and non-preferred (control) substrates. We found that both species exhibited higher GabRat values on preferred substrates, but interestingly, the keeled box turtle, with a uniformly colored carapace containing flat scutes and two keels, had a higher GabRat value than the Indochinese box turtle, characterized by two yellow stripes on its carapace. Our results indicated that the strong brightness gradients caused by the directional illumination of the flattened and keeled carapaces created disruptive coloration in the keeled box turtles, whereas a high chroma contrast created disruptive coloration in the Indochinese box turtles. For these turtles, the structural modifications resulted in variations in lightness that led to higher levels of disruption than the chromatic disruption of the Indochinese box turtle. Our study provides, to our knowledge, the first evidence of disruptive camouflage in turtles and the first comprehensive test of structural and colored disruption in vertebrates.