Out in the open: behavior’s effect on predation-risk and thermoregulation by aposematic caterpillars

Warning coloration should be under strong stabilizing selection but often
displays considerable intraspecific variation. Opposing selection on color
by predators and temperature is one potential explanation for this seeming
paradox. Despite the importance of behavior for both predator avoidance
and thermoregulation, its role in mediating selection by predators and
temperature on warning coloration has received little attention. Wood
tiger moth caterpillars, Arctia plantaginis, have aposematic coloration,
an orange patch on the black body. The size of the orange patch varies
considerably: individuals with larger patches are safer from predators,
but having a small patch is beneficial in cool environments. We
investigated microhabitat preference by these caterpillars and how it
interacted with their coloration. We expected caterpillar behavior to
reflect a balance between spending time exposed to maximize basking and
spending time concealed to avoid detection by predators. Instead, we found
that caterpillars preferred exposed locations regardless of their
coloration. Whether caterpillars were exposed or concealed had a strong
effect on both temperature and predation risk, but caterpillars in exposed
locations were both much warmer and less likely to be attacked by a bird
predator (great tits, Parus major). This shared optimum may explain why we
observed so little variation in caterpillar behavior and demonstrates the
important effects of behavior on multiple functions of coloration.