Background colour matching increases with risk of predation in a colour-changing grasshopper

Abstract

Cryptic colouration can be adjusted to the local environment by physiological (rapid) change, and/or by morphological (slow) change. The threat-sensitivity hypothesis predicts that the degree of crypsis should respond to the risk of predation (assuming some cost to crypsis). This has not been studied for morphological colour changers, so we manipulated the colour of the rearing substrate (black vs. white) and the perceived risk of predation (higher vs. lower) for the grasshopper Sphingonotus azurescens. Over a period of several weeks, both nymphs and adults greatly adjusted the brightness of their body towards that of the substrate. Moreover, when individuals were exposed to a greater simulated predation risk (disturbance by hand), they became even more similar in brightness to their substrates, apparently augmenting their degree of crypsis. This study on a morphological colour changer shows that the degree of cryptic colouration (body brightness) is under individual control and appears to change adaptively in response to increased predation risk. In addition, based on analyses of systematic differences in colour in lab-reared offspring, we found indications that even in colour changers there is genetic variation in colouration among individuals, and that populations have diverged adaptively. Such integration of factors determining the cryptic phenotype improves our understanding of the natural selection and constraints imposed on crypsis, which influence both its optimization and evolution.