Selfish herd effects depend on prey crypsis

Determining why some animals form groups while others remain solitary is a longstanding goal in behavioural ecology. Group formation can help mitigate predation risk through a variety of mechanisms, including risk dilution and group vigilance. The ‘selfish herd hypotheses’ proposes that prey can reduce their risk by minimizing the area around which all points in that area are closer to them than to another conspecific (i.e., by minimising their ‘domain of danger’). This hypothesis assumes that an individual’s predation risk is proportional to the size of their domain of danger, however the relationship between risk and proximity to conspecifics may depend on additional factors. Specifically, approaching conspecifics may be costly for prey that rely on crypsis because group formation increases detectability. Using model prey, we experimentally manipulated prey colouration as well as the domain of danger, then tracked their ‘survival’ under natural field conditions. We found that an individual’s predation risk increased with their domain of danger for conspicuous (red) prey, but decreased with the domain of danger in cryptic (green) prey. Our results are consistent with patterns in natural systems and indicate that the relationship between predation risk and domain of danger depends on additional factors like prey colouration.