Selfish herd effects depend on prey crypsis

Abstract

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 various 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 minimizing their ‘domain of danger’ (DOD)). This hypothesis assumes that an individual's predation risk is proportional to the size of its DOD; 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 plasticine model prey, we experimentally manipulated prey coloration as well as the DOD, and then tracked their ‘survival’ under natural field conditions. We found that an individual's predation risk increased with their DOD for conspicuous (red) prey, but decreased with the DOD in cryptic (green) prey. Our results are consistent with patterns in natural systems and indicate that the relationship between predation risk and DOD depends on additional factors like prey coloration.