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.

物种间的互利共生作用对维持生物多样性与生态系统功能至关重要。然而，这类互利关系可能退化为拮抗互作。现有解释认为，与伙伴物种互作所带来的适合度（fitness）成本与收益，在不同个体间存在差异，但目前尚不清楚该差异的成因。本研究证实，物种内部的社会行为——尽管此前被学界忽视——在决定从另一物种的互作中获得的相对适合度方面发挥着重要作用。我们结合实验室实验与野外观测发现，埋葬甲虫（Nicrophorus vespilloides）种群内部的冲突会影响其与刻点寄螨（Poecilochirus carabi）互作时所能获得的适合度。甲虫会携带这类螨类前往腐肉资源，二者均在此处繁殖。研究表明，螨类可帮助甲虫赢得针对该稀缺资源的种内竞争（intraspecific contests）：螨类通过提升甲虫的体温，增强其竞争能力。但该益处仅惠及体型较小的甲虫——这类个体仅凭自身往往会在腐肉资源的竞争中落败。体型较大的甲虫无需辅助即可赢得尸骸，但若与螨类一同繁殖则会损失繁殖成功率（reproductive success）。因此，互利共生的强弱取决于个体是否具备单独对抗同种个体（conspecifics）的能力。当与伙伴物种的互作开始带来净适合度损失而非净收益时，互利关系便会退化为拮抗互作。本研究提示，同种个体间的互作决定了这一临界点（tipping point）的位置。