Data from: Spatial patterning of prey at reproduction to reduce predation risk: what drives dispersion from groups?

Group-living is a widespread behaviour thought to be an evolutionary adaptation for reducing predation risk. Many group-living species, however, spend a portion of their life cycle as dispersed individuals, suggesting that the costs and benefits of these opposing behaviours vary temporally. Here, we evaluated mechanistic hypotheses for explaining individual dispersion as a tactic for reducing predation risk at reproduction (i.e. birthing) in an otherwise group-living animal. Using simulation analyses parameterized by empirical data, we assessed whether dispersion increases reproductive success by: (i) increasing predator search time, (ii) reducing predator encounter rates because individuals are inconspicuous relative to groups, or (iii) eliminating the risk of multiple kills per encounter. Simulations indicate that dispersion only becomes favourable when detectability increases with group size and there is risk of multiple kills per encounter. This latter effect, however, is likely the primary mechanism driving females to disperse at reproduction because group detectability effects are presumably constant year round. We suggest that the risk of multiple kills imposed by highly vulnerable offspring may be an important factor influencing dispersive behaviour in many species and conservation strategies for such species will require protecting sufficient space to allow dispersion to effectively reduce predation risk.

群居生活是一类广泛分布的行为模式，被视为降低捕食风险的进化适应策略。然而，多数群居物种会在生命周期的特定阶段以分散个体的形式存在，这表明这两种对立行为的成本与收益随时间动态变化。本研究针对群居动物在繁殖（即分娩）阶段以个体分散为策略以降低捕食风险的现象，评估了相关机制性假说。我们采用以实证数据设定参数的模拟分析方法，检验分散行为能否通过以下三种途径提升繁殖成功率：（1）延长捕食者的搜索时长；（2）降低个体与捕食者的遭遇概率——因单独个体相较于群居群体更不易被察觉；（3）消除单次遭遇捕食者时多只个体被捕杀的风险。模拟结果显示，仅当群体可探测性随群体规模扩大而升高，且单次遭遇存在多只个体被捕杀的风险时，分散行为才会成为有利策略。但其中后者（单次遭遇多只个体被捕杀的风险）大概率是驱动雌性在繁殖阶段分散的核心机制，因为群体可探测性的影响理论上全年恒定。我们提出，幼崽极易受到攻击所带来的单次多杀风险，可能是影响诸多物种分散行为的关键因素；针对这类物种的保护策略，需要预留足够的活动空间，以确保分散行为能够有效降低捕食风险。