School of Biological Sciences, The University of Western Australia, Crawley, Western Australia 6009, Australia.
The UWA Oceans Institute, The University of Western Australia, Crawley, Western Australia 6009, Australia.
Proc Biol Sci. 2020 May 27;287(1927):20200477. doi: 10.1098/rspb.2020.0477. Epub 2020 May 13.
A three-dimensional body shape is problematic for camouflage because overhead lighting produces a luminance gradient across the body's surface. Countershading, a form of patterning where animals are darkest on their uppermost surface, is thought to counteract this luminance gradient and enhance concealment, but the mechanisms of protection remain unclear. Surprisingly, no study has examined how countershading alters prey contrast, or investigated how the presence of a dorsoventral luminance gradient affects detection under controlled viewing conditions. It has also been suggested that the direction of the dorsoventral luminance gradient (darkest or lightest on top) may interfere with predators' abilities to resolve prey's three-dimensional shape, yet this intriguing idea has never been tested. We used live fish predators (western rainbowfish, ) and computer-generated prey images to compare the detectability of uniformly pigmented (i.e. non-countershaded) prey with that of optimally countershaded prey of varying contrasts against the background. Optimally countershaded prey were difficult for predators to detect, and the probability and speed of detection depended on prey luminance contrast with the background. In comparison, non-countershaded prey were always highly detectable, even though their average luminance closely matched the luminance of the background. Our findings suggest that uniformly pigmented three-dimensional prey are highly conspicuous to predators because overhead lighting increases luminance contrast between different body parts or between the body and the background. We found no evidence for the notion that countershading interferes with predator perception of three-dimensional form.
三维体型对于伪装来说是一个问题，因为顶光会在物体表面产生亮度梯度。反荫蔽是一种动物在上表面最暗的图案形式，据认为它可以抵消这种亮度梯度并增强隐蔽性，但保护机制仍不清楚。令人惊讶的是，没有研究检查过反荫蔽如何改变猎物的对比度，也没有研究过在受控观察条件下，背腹亮度梯度的存在如何影响检测。有人还提出，背腹亮度梯度的方向（顶部最暗或最亮）可能会干扰捕食者分辨猎物三维形状的能力，但这个有趣的想法从未得到过验证。我们使用活体鱼类捕食者（西部彩虹鱼）和计算机生成的猎物图像，比较了均匀着色（即非反荫蔽）猎物与不同对比度的最佳反荫蔽猎物在背景下的可检测性。最佳反荫蔽的猎物对捕食者来说很难被发现，而且发现的概率和速度取决于猎物与背景的亮度对比。相比之下，非反荫蔽的猎物总是很容易被发现，即使它们的平均亮度与背景的亮度非常匹配。我们的研究结果表明，均匀着色的三维猎物对捕食者来说是非常明显的，因为顶光会增加不同身体部位之间或身体与背景之间的亮度对比。我们没有发现反荫蔽会干扰捕食者对三维形状感知的观点的证据。
