Data from: Too important to tamper with: predation risk affects body mass and escape behaviour but not escape ability

1. Escaping from a predator is a matter of life or death, and prey are expected to adaptively alter their physiology under chronic predation risk in ways that may affect escape. Theoretical models assume that escape performance is mass-dependent whereby scared prey strategically maintain an optimal body mass to enhance escape. Experiments testing the mass-dependent predation risk (MDPR) hypothesis have demonstrated that prior experience of predation risk can affect body mass, and the behavioural decisions about evasive actions to take. Other studies on natural changes in body mass indicate that mass can affect escape. No single experiment has tested if all of these components are indeed linked, which is a critical necessary condition underpinning the MDPR. 2. We tested all components of the MDPR in a repeated-measures experiment by presenting predator and non-predator cues to brown-headed cowbirds housed in semi-natural conditions. Exposure to predator cues affected body mass, fat, pectoral muscle thickness, and evasive actions (take-off angle and speed), but not the physiological capacity to escape, as measured by flying ability. Examining individual variation revealed that flying ability was unrelated to mass loss in either sex, unrelated to mass gain in males, and only females that gained a very large amount of mass flew poorly. 3. We next conducted a body mass manipulation in the lab to rigorously test whether small to large perturbations in mass can ever affect flying ability. We induced either no change in mass (control), a moderate reduction of <10%, and a more extreme reduction of >10% which the literature suggests should enhance flight. Flying ability was maintained regardless of treatment. Examining individual variation revealed the same precise patterns as in the first experiment. 4. We conclude that prey may alter their mass and evasive actions in response to predation risk, but their escape ability remains robust and inelastic, presumably because dis-abling oneself is likely to lead to disastrous consequences. We suggest that animals may only face a mass-dependant predation risk trade-off in a narrow set of circumstances linked to life history stages that require large amounts of mass gain, e.g. parturition and migration.

1. 逃离捕食者乃是生死攸关之事，猎物在长期捕食风险下需自适应地调整自身生理状态，此类调整或会影响其逃逸能力。理论模型均假定逃逸性能与体重相关，受惊的猎物会策略性地维持最优体重以提升逃逸能力。检验体重依赖型捕食风险（mass-dependent predation risk, MDPR）假说的实验已证实，过往的捕食风险经历会影响猎物体重，以及其针对逃逸行为的决策。另有针对体重自然变化的研究表明，体重会对逃逸能力产生影响。但目前尚无任何一项实验验证上述所有环节是否确实存在关联，而这正是支撑MDPR假说的关键必要条件。2. 本研究通过向半自然环境中饲养的棕头牛鹂呈现捕食者与非捕食者线索，在重复测量实验中检验了MDPR假说的所有环节。接触捕食者线索会影响猎物的体重、脂肪含量、胸肌厚度以及逃逸行为（起飞角度与速度），但不会影响以飞行能力衡量的逃逸生理能力。对个体差异的分析显示，无论雌雄，其飞行能力均与体重下降无关联；雄性的飞行能力也与体重增加无关；仅当雌性体重大幅增加时，其飞行能力才会出现下降。3. 随后，本研究在实验室中开展了体重操控实验，以严格检验体重的小幅至大幅波动是否会对飞行能力产生影响。实验设置了三种处理：体重无变化（对照组）、体重适度下降（<10%），以及文献表明可提升飞行能力的大幅体重下降（>10%）。无论接受何种处理，实验对象的飞行能力均未发生改变。对个体差异的分析结果与首次实验中的精确模式完全一致。4. 本研究得出结论：猎物或会根据捕食风险调整自身体重与逃逸行为，但其逃逸能力始终保持稳健且不受体重影响，推测原因在于主动削弱自身状态大概率会带来灾难性后果。本研究认为，仅在少数与需要大幅增加体重的生活史阶段（如分娩与迁徙）相关的情境中，动物才会面临体重依赖型捕食风险的权衡取舍。