Hainan four-eyed turtles actively select suitable stones to masquerade according to their own morphology

Masquerade is a form of camouflage in which animals use their body size, shape, and coloration to resemble inanimate objects in their environment to deceive predators. However, there is a lack of experimental evidence to show that animals actively choose objects that match these body parameters. To explore how the Hainan four-eyed turtle, Sacalia insulensis, masquerades using suitable stones, we used indoor video surveillance technology to study the preferences of juvenile S. insulensis for stones of different sizes, shapes, and colors. The results indicated that under normal conditions, the turtles preferred larger oval or round brown stones. When disturbed (swinging the arm of a researcher back and forth above the experimental setup every hour to mimic a predator), the turtles showed a preference for brown stones that were closer to their size and oval in shape. These findings suggest that juvenile S. insulensis prefer stones that resemble their carapace size, shape, and color to masquerade when undisturbed, and that this preference is reinforced when they masquerade to reduce the risk of predation. To the best of our knowledge, this is the first study to provide evidence that vertebrates can recognize object shapes and selectively choose objects that resemble their own shape for masquerading to reduce predation risk.