Body size does not affect crypsis: evidence from 3D-printed turtle models

To avoid predator or prey detection, cryptic animals can use background-matching coloration or disruptive patterns with a high-contrast. While it has been suggested that larger individuals may be less cryptic, this idea remains controversial, as studies that control for confounding visual factors, such as body shape, and directly quantify crypsis are lacking. To clarify the role of body size in crypsis, we conducted a controlled experiment using 3D-printed shell models of three turtle species, each displaying either background matching or disruptive coloration, while also simulating natural variation in body size and shape. The efficacy of crypsis was quantified using the Receptor Noise Limited Model to calculate chromatic and luminance Just Noticeable Differences (background matching), and the efficacy of disruptive coloration using the Gabor edge disruption ratio method to assess. The relationship between body size and crypsis efficacy then examined was evaluated using three predator vision models. After controlling for shape parameters and carapace height, no significant correlations were detected between carapace length and chromatic or luminance Just Noticeable Differences or with the Gabor edge disruption ratio. These results demonstrate that body size does not influence the background matching or disruptive coloration efficacy in these three turtle species. To the best of our knowledge, this is the first systematic study to evaluate the relationship between body size and crypsis in vertebrates, providing new insights into the interactions among morphology, visual perception, and anti-predator strategies. Background matching and disruptive coloration are key crypsis strategies; however, the role of body size in crypsis remains unclear. While a few predation experiments have suggested that crypsis efficacy decreases with size, theoretically, the core mechanisms of background matching and disruptive coloration should primarily rely on body color and pattern, not body size. However, few empirical studies have controlled for confounding factors, such as body shape, to test this relationship. In this study, we used 3D scanning and printing to produce model turtles, which are slow-moving animals with limited anti-predator behaviors. We then evaluated crypsis efficiency in multiple predator vision models. The results of this study provide the first systematic evidence that body size does not affect crypsis. This study helps resolve a long-standing controversy and offers new empirical insights into the mechanisms of visual camouflage in animals.