Background matching and camouflage efficiency predict population density in four-eyed turtle (Sacalia quadriocellata)

• The carapace color of four-eyed turtle (Sacalia quadriocellata) most closely matches the substrate of midstream of Hezonggou stream.
• The camouflage efficiency is highest for the turtles that inhabit midstream.
• The turtles may enhance camouflage efficiency by selecting microhabitat that best match their carapace color.
• This may explain the high population density of the turtles in the midstream.

Background matching is an important way to camouflage and is widespread among animals. In the field, however, few studies have addressed background matching, and there has been no reported camouflage efficiency in freshwater turtles. Background matching and camouflage efficiency of the four-eyed turtle, Sacalia quadriocellata, among three microhabitat sections of Hezonggou stream were investigated by measuring carapace components of CIE L*a*b* (International Commission on Illumination; lightness, red/green and yellow/blue) color space, and scoring camouflage efficiency through the use of humans as predators. The results showed that the color difference (ΔE), lightness difference (ΔL*), and chroma difference (Δa*b*) between carapace and the substrate background in midstream were significantly lower than that upstream and downstream, indicating that the four-eyed turtle carapace color most closely matched the substrate of midstream. In line with these findings, the camouflage efficiency was the best for the turtles that inhabit midstream. These results suggest that the four-eyed turtles may enhance camouflage efficiency by selecting microhabitat that best match their carapace color. This finding may explain the high population density of the four-eyed turtle in the midstream section of Hezonggou stream. To the best of our knowledge, this study is among the first to quantify camouflage of freshwater turtles in the wild, laying the groundwork to further study the function and mechanisms of turtle camouflage.