Small prey fight back: post-capture defences shape prey–predator size relationships

Size relationships between prey and predators are a key topic in ecology. A positive correlation often exists between the sizes of prey and predators. Although predators can consume much smaller prey, some preferentially target larger prey and avoid very small ones. Prey size preferences can be shaped by prey quality, alternative prey availability, and handling costs. Additionally, the defensive behaviour of small prey may prevent ingestion after capture, although this hypothesis has not been experimentally tested. In this study, I observed the behavioural responses of the predatory fish Silurus asotus (Siluriformes: Siluridae) to eight aquatic beetle species (Coleoptera: Gyrinidae, Dytiscidae, Hydrophilidae) of varying body sizes under aquarium conditions. The rate of successful predation varied from 20% to 90% among beetle species. Prey size significantly affected predation success: small beetles were more frequently spat out by fish after capture. When the locomotion of the small beetle Regimbartia attenuata was experimentally restricted by leg amputation, most individuals were readily consumed. These results demonstrate that small beetles use their legs to move rapidly within the fish's mouth or cling to the internal surfaces, thereby preventing ingestion and eliciting rejection. Thus, post-capture defences of small prey can influence prey size preferences in whole-swallowing predators.