Torpor use in response to predation risk in a small, free-living bird: rest-phase energy savings moderate the costs of active-phase behavioural decisions

Animal decisions trade-off the mortality risks of starvation and predation, and anti-predator behaviours generally incur a cost of reduced energy intake. Torpor is a widespread physiological mechanism for reducing starvation risk among small mammals and birds. Here, we present a field-based experimental test of the hypothesis that energy savings from torpor use can also reduce predation risk by moderating the energy cost of anti-predator behavioural responses in a small bird during winter. We manipulated perceived predation risk in wild populations of the superb fairy-wren (Malurus cyaneus) by playback of conspecific alarm calls during the daytime active-phase and tested for effects on body temperature measured continuously by telemetry during the nocturnal rest-phase. We found that alarm call playback was associated with subsequent rest-phase torpor bouts that were significantly deeper (minimum skin temperature: 28.7 ± 1.7 °C vs. 30.0 ± 1.5 °C) and longer (duration in torpor: 6.0 ± 2.7 h vs. 3.8 ± 2.3 h) compared to control periods. By demonstrating the connection between resting energy expenditure and energy costs of behavioural decisions during activity, our study has implications for understanding both the ecological functions of torpor and the mortality risk consequences of behavioural responses by small birds to environmental challenges.动物的决策会权衡饥饿与捕食带来的死亡风险，而反捕食行为通常会伴随能量摄入减少的代价。蛰伏（Torpor）是小型哺乳动物与鸟类中广泛存在的生理机制，用于降低饥饿致死风险。本研究针对该假说开展野外实验验证：在冬季的小型鸟类中，利用蛰伏所节省的能量，可通过缓解活动期反捕食行为的能量消耗，进而降低捕食风险。我们通过在日间活动期播放华丽细尾鹩莺（Malurus cyaneus）野生种群的同种警戒叫声，以操控其感知到的捕食风险，并借助夜间休息阶段的连续遥测体温数据，检验该处理对体温的影响。研究结果显示，与对照组相比，播放警戒叫声后的受试个体，其休息期蛰伏深度显著更深（最低皮肤温度：28.7 ± 1.7 ℃ vs. 30.0 ± 1.5 ℃），且蛰伏时长显著更长（蛰伏持续时间：6.0 ± 2.7 h vs. 3.8 ± 2.3 h）。本研究揭示了休息期能量消耗与活动期行为决策的能量成本之间的关联，其结果有助于深入理解蛰伏的生态功能，以及小型鸟类应对环境挑战时的行为反应所带来的死亡风险后果。