Competition within species determines the value of a mutualism between species

Social interactions within species, and mutualisms between species are both well characterised, but their influence on each other is poorly understood. We determined how interactions among burying beetlesNicrophorus vespilloidesinfluence the value of their interactions with the mitePoecilochirus carabi.Beetles transport these mites to carrion, upon which both species breed. We show that mites help beetles win intraspecific contests for this scarce resource: mites raise beetle body temperature, which enhances beetle competitive prowess. However, mites confer this benefit only upon smaller beetles, which are otherwise doomed by their size to lose contests for carrion. Larger beetles need no assistance to win a carcass and lose reproductive success when breeding alongside mites. We conclude that social interactions within species explain whether interactions with another species are mutualistic or parasitic.One Sentence SummarySocial interactions within species can explain whether interactions with a second species are mutualistic or parasitic.