Data from: Conflict within species determines the value of a mutualism between species

Mutually beneficial interactions between species play a key role in
maintaining biodiversity and ecosystem function. Nevertheless, such
mutualisms can erode into antagonistic interactions. One explanation is
that the fitness costs and benefits of interacting with a partner species
vary among individuals. However, it is unclear why such variation exists.
Here we demonstrate that social behaviour within species plays an
important, though hitherto overlooked, role in determining the relative
fitness to be gained from interacting with a second species. By combining
laboratory experiments with field observations, we report that conflict
within burying beetles Nicrophorus vespilloides influences the fitness
that can be gained from interacting with the mite Poecilochirus 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 condemned by their size to lose contests for
carrion. Larger beetles need no assistance to win a carcass and then lose
reproductive success when breeding alongside mites. Thus the extent of
mutualism is dependent on an individual’s inability to compete
successfully and singlehandedly with conspecifics. Mutualisms degrade into
antagonism when interactions with a partner species start to yield a net
fitness loss, rather than a net fitness gain. This study suggests that
interactions with conspecifics determine where this tipping point lies.