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 hypothesis’ 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 its 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 indivi...