The combined behavioural tendencies of predator and prey mediate the outcome of their interaction

• We examined the interaction of predator and prey behavioural types.
• We found that prey fitness depends on predator behavioural type and vice versa.
• Models with behavioural type × behavioural type interactions fitted significantly better than those without interactions.
• One needs to account for both predator and prey behavioural type to understand how behavioural types affect fitness.

Consistent individual differences in behaviour are present in most animal populations. Historically, the fitness consequences of behavioural types (e.g. bold and shy) have been investigated in one focal species at a time. Studies generally disregard variation in heterospecifics that interact with those types. However, intraspecific variation in behavioural types of multiple interactors could generate dependent effects of behavioural type × behavioural type in species interactions and might serve as a mechanism maintaining phenotypic variation in wild populations. Here we examined how behavioural types of both predators and prey jointly determine the outcome of a predator–prey interaction using the black widow spider, Latrodectus hesperus, and a syntopic field cricket, Gryllus integer. We assayed the antipredator (boldness) behaviour of individual G. integer and the boldness during foraging site selection of individual L. hesperus, as estimated by the spiders' tendency to settle in safe versus high-risk foraging environments. The best-fit models predicting prey survivorship and predator foraging success indicate that the behavioural tendencies of both predator and prey, as well as the interaction between them, jointly affect the outcome of staged predator–prey interactions. These models boasted R2 values 3–10 times those of models that considered behavioural type variation in either predator or prey alone. Our findings highlight the importance of considering the behavioural types of multiple interactors when testing the effects of behavioural types on species interactions and individual performance. More generally, our results strongly suggest that attention should be paid to multiple interactors if we are to understand the outcomes of species interactions.