Exposure to potentially cannibalistic conspecifics induces an increased immune response

1. Within‐population infectious disease dynamics depend on multiple factors, including the ability of hosts to mount an effective immune response. These immune responses can be highly plastic, responding to pathogen risk, as well as the ecological context in which pathogens are encountered. 2. High conspecific density can stimulate immune activity, and recent research suggests that predators can cause indirect protective effects in their prey through the induction of increased immune responses. Comparatively little work, however, has investigated whether exposure to potentially cannibalistic conspecifics, representing both increased density and predatory pressures, will have similar effects on immune expression. 3. Using dragonfly larvae, the present study investigated whether exposure to potentially cannibalistic conspecifics altered the melanisation of simulated parasites. 4. Increased levels of melanisation were found in larvae regardless of whether that conspecific had recently engaged in cannibalism or not. Melanisation also increased as conspecific density increased, even if the conspecifics present were small, and therefore unlikely to pose a cannibalism threat. 5. The findings obtained in the present study indicate that conspecific presence is sufficient to affect immune responses in these insects even though they are relatively solitary compared with the phase‐polyphenic taxa typically associated with density‐dependent prophylaxis. Because melanisation is also important for wound healing, we suggest that the increased melanin response observed with increased conspecific density might act to induce heightened immunity when faced with potentially increased risk of infection, and also facilitate wound healing under threat of predation/cannibalism. Density‐dependent increases in immune response can happen in species with predictable phases of high population density. Increased density correlates with increased melanin immune activity in a relatively solitary insect species (Leucorrhinia intacta), regardless of recent cannibalism. Failed cannibalism attempts will typically cause wounding in L. intacta and it is hypothesised that increased melanin immunity in the presence of conspecifics is a response to wounding risk.