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, during the day, juvenile S. insulensis preferred larger
oval or round stones, while at night, they preferred oval stones that were
closer to their own size, with no significant preference for stone color
during either time. When disturbed (by a researcher swinging their arm
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 and
shape to masquerade when undisturbed, and that this preference is
reinforced when they masquerade to reduce the risk of predation. The
preference for stones that resemble their carapace color is significant
only when there is a disturbance. To the best of our knowledge, this is
the first study to provide evidence that vertebrates can selectively
choose objects that resemble their own morphology for masquerading to
reduce predation risk.