Ecological transitions in grouping benefits explain the paradox of environmental quality and sociality

Both benign and harsh environments promote the evolution of sociality. This paradox—societies occur in environments of such contrasting quality—may be explained by the different types of benefits that individuals receive from grouping: resource defense benefits that derive from group-defended critical resources versus collective action benefits that result from social cooperation among group member. Here, we investigate cooperative behavior in the burying beetle Nicrophorus nepalensis along an elevational gradient where environmental quality (climate and competiton) varies with altitude. We show that climate (temperature) and competition (both intra- and interspecific) independently and synergistically influence sociality via different grouping benefits that vary along the gradient. At low elevations where interspecific competition for resources is intense, groups gain from the collective action benefit of increased interspecific competitive ability. In contrast, pairs have
higher fitness at intermediate elevations where intraspecific competition for resources is greatest because resource defense is the key grouping benefit. However, groups and pairs have similar fitness at high elevations, suggesting that there is no grouping benefit in such physiologically challenging environments. Our results demonstrate that sociality is favored for different reasons under a range of environmental conditions, perhaps explaining why animal societies occur in environments of such contrasting quality.

良性与严苛环境均会推动社会性行为的演化。这一悖论——即动物社会会存在于质量差异悬殊的环境中——或可通过个体从集群中获得的两类不同收益得到解释：一是依托群体守护关键资源所带来的资源防御收益，二是群体成员间社会合作产生的集体行动收益。本研究沿海拔梯度开展，探究尼泊尔埋葬甲（Nicrophorus nepalensis）的合作行为，该梯度下环境质量（气候与种内、种间竞争）随海拔高度发生变化。研究结果显示，气候（温度）与竞争（种内、种间竞争）分别通过沿梯度变化的不同集群收益，独立且协同地影响社会性行为。在资源种间竞争激烈的低海拔区域，群体可通过提升种间竞争能力的集体行动收益获益。与之相对，在资源种内竞争最为剧烈的中海拔区域，成对个体的适合度更高，因为此时资源防御是核心的集群收益。不过在高海拔区域，群体与成对个体的适合度并无显著差异，这表明在这类生理挑战严苛的环境中并无集群收益可言。本研究结果表明，社会性行为在多样环境条件下受不同机制青睐，这或许可以解释为何动物社会会存在于质量差异悬殊的各类环境中。