Direct Fitness Correlates and Thermal Consequences of Facultative Aggregation in a Desert Lizard

Social aggregation is a common behavioral phenomenon thought to evolve through adaptive benefits to group living. Comparing fitness differences between aggregated and solitary individuals in nature – necessary to infer an evolutionary benefit to living in groups – has proven difficult because communally-living species tend to be obligately social and behaviorally complex. However, these differences and the mechanisms driving them are critical to understanding how solitary individuals transition to group living, as well as how and why nascent social systems change over time. Here we demonstrate that facultative aggregation in a reptile (the Desert Night Lizard, Xantusia vigilis) confers direct reproductive success and survival advantages and that thermal benefits of winter huddling disproportionately benefit small juveniles, which can favor delayed dispersal of offspring and the formation of kin groups. Using climate projection models, however, we estimate that future aggregation in night lizards could decline more than 50% due to warmer temperatures. Our results support the theory that transitions to group living arise from direct benefits to social individuals and offer a clear mechanism for the origin of kin groups through juvenile philopatry. The temperature dependence of aggregation in this and other taxa suggests that environmental variation may be a powerful but underappreciated force in the rapid transition between social and solitary behavior.

社会聚集行为（social aggregation）是一种普遍的行为现象，学界普遍认为其演化源于群体生活带来的适应性收益。若要推断群体生活的进化收益，需比较自然环境中群居与独居个体的适合度（fitness）差异，但这一工作始终颇具挑战——因群居物种往往为专性社会性且行为复杂。然而，这些差异及其背后的驱动机制，对于理解独居个体如何向群居生活过渡，以及新生社会系统随时间变化的方式与原因，均至关重要。本研究针对一种爬行动物——沙漠夜蜥（*Xantusia vigilis*）——的兼性群居行为（facultative aggregation）展开研究，证明其兼具直接繁殖成功率与生存优势；且冬季集群的热收益对小型幼体的益处尤为显著，这将有利于延缓后代扩散并形成亲缘群体。不过，借助气候预测模型，本研究估算未来因气温升高，夜蜥的群居行为可能减少50%以上。本研究结果支持“向群居生活的过渡源于群居个体的直接收益”这一理论，并为通过幼体恋巢性（juvenile philopatry）形成亲缘群体提供了清晰的机制解释。该物种及其他类群的群居行为对温度的依赖性表明，环境变化可能是驱动社会与独居行为快速转变的一股强大却未被充分重视的力量。