Hard exoskeleton of beetles as armour against avian predators

The robust exoskeleton of beetles, Coleoptera, is believed to have protective advantages, contributing to their evolutionary success. However, empirical evidence of the defensive capabilities of this exoskeleton remains surprisingly scarce. Here, we demonstrate the critical role of the robust beetle exoskeleton in protecting against avian predation. We found that flower chafers (Scarabaeidae, Cetoniinae) have more robust bodies than other scarab species. Laboratory experiments with naïve Japanese quail, Coturnix japonica, revealed that some individuals of intact Cetoniinae survived attacks without serious injury, whereas all individuals of soft scarab species or elytra-removed Cetoniinae were consumed. The survival rate of intact Cetoniinae increased in complex environments because the combination of their stiffness and elliptical shape made it difficult for quail to handle the prey. Field experiments with wild white-cheeked starlings, Sturnus cineraceus, and Eurasian tree sparrows, Passer montanus, demonstrated that most individuals of Cetoniinae species were ignored, whereas soft species were readily preyed upon. Further, when we presented the starlings with the Cetoniinae species Protaetia orientalis that had artificially softened bodies and altered appearance, the starlings readily preyed upon them. This observation suggests that P. orientalis is not chemically defended. Moreover, wild birds can visually discriminate hard species because of prior experience with the unprofitable prey. These results collectively provide evidence that the robust exoskeleton of beetles protects them from predatory attack.