Orientation to the sun by animals and its interaction with crypsis

Abstract

Summary Orientation with respect to the sun has been observed in a wide range of species and has generally been interpreted in terms of thermoregulation and/or ultraviolet (UV) protection. For countershaded animals, orientation with respect to the sun may also result from the pressure to exploit the gradient of coloration optimally to enhance crypsis.Here, we use computational modelling to predict the optimal countershading pattern for an oriented body. We assess how camouflage performance declines as orientation varies using a computational model that incorporates realistic lighting environments.Once an optimal countershading pattern for crypsis has been chosen, we determine separately how UV protection/irradiation and solar thermal inflow fluctuate with orientation.We show that body orientations that could optimally use countershading to enhance crypsis are very similar to those that allow optimal solar heat inflow and UV protection.Our findings suggest that crypsis has been overlooked as a selective pressure on orientation and that new experiments should be designed to tease apart the respective roles of these different selective pressures. We propose potential experiments that could achieve this.

Optimizing countershading camouflage

“…[21][22][23][24] showed an advantage for countershading over uniform background-matching dark green. As mentioned in the Introduction, it was not the case that all these experiments were carried out on sunny days, nor were the prey carefully aligned to the sun [as in the present study and necessary for countershading to function in direct illumination (18)]. It is possible that, given that the prey were made from pastry dough, the boundary was smudged by handling, and so a partial gradient (not quantified) may have been created.…”

“…However, this argument does not apply to animals in the open, and we would definitely not interpret our results as supporting Thayer's proposition (12,13) that the near universality of countershading (the pattern) is explained by camouflage through self-shadow concealment. If a countershaded animal lives under direct illumination and does not orient with respect to the sun, or at least does not do so at times of high predation risk, we would have to conclude that its coloration has a function other than self-shadow concealment (17)(18)(19)(20). Unless the sun is overhead (i.e., midday in the tropics), the predicted effects on camouflage of not orienting toward the sun are always deleterious and, in direct illumination, potentially catastrophic (18).…”

“…If a countershaded animal lives under direct illumination and does not orient with respect to the sun, or at least does not do so at times of high predation risk, we would have to conclude that its coloration has a function other than self-shadow concealment (17)(18)(19)(20). Unless the sun is overhead (i.e., midday in the tropics), the predicted effects on camouflage of not orienting toward the sun are always deleterious and, in direct illumination, potentially catastrophic (18). For a bilaterally symmetrical animal, optimal countershading can only be achieved by orienting toward the sun because an optimal symmetrical camouflage requires an illumination field that is symmetrical with respect to the animal (18).…”

“…Unless the sun is overhead (i.e., midday in the tropics), the predicted effects on camouflage of not orienting toward the sun are always deleterious and, in direct illumination, potentially catastrophic (18). For a bilaterally symmetrical animal, optimal countershading can only be achieved by orienting toward the sun because an optimal symmetrical camouflage requires an illumination field that is symmetrical with respect to the animal (18). That an animal such as a caterpillar would move with the time of day so as to maintain equal illumination on both sides of its body is plausible but untested.…”

“…Such a comparative approach provides powerful correlative evidence for countershading as an adaptation to lighting conditions but, of itself, does not prove that camouflage is the function being optimized. One problem is that the predictions for UV protection are very similar to those for optimized camouflage (17)(18)(19)(20). Therefore, without measurement of predation rates on animals with or without the observed coloration, we cannot tell whether there is a causal effect on detectability.…”