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. 逃离捕食者关乎生死存亡，被捕食者在长期捕食风险压力下需适应性调整自身生理状态，此类调整或会对逃逸能力产生影响。现有理论模型假设逃逸性能与体重相关，即受惊扰的被捕食者会策略性维持最优体重以提升逃逸能力。针对体重依赖性捕食风险（mass-dependent predation risk, MDPR）假说的相关实验已证实，过往遭遇捕食风险的经历会影响被捕食者的体重，以及其关于躲避行为的决策。另有针对体重自然变化的研究表明，体重本身也会对逃逸能力产生影响。目前尚无单一实验验证上述所有要素是否确实存在关联，而这正是支撑MDPR假说的关键必要条件。 2. 我们针对饲养于半自然环境中的褐头牛鹂（brown-headed cowbirds）开展重复测量实验，对MDPR假说的全部要素进行验证。实验中我们向受试个体分别呈现捕食者与非捕食者的线索。结果显示，暴露于捕食者线索会影响受试个体的体重、脂肪含量、胸肌厚度以及躲避行为（起飞角度与速度），但并未对以飞行能力衡量的逃逸生理能力产生影响。对个体差异的分析表明，无论雌雄个体，其飞行能力均与体重下降无关联，与雄性的体重上升也无关联，仅当雌性体重大幅增加时，其飞行能力才会显著下降。 3. 随后我们在实验室中开展体重操控实验，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。我们设置了三组处理：无体重变化的对照组、<10%的中度体重下降组，以及>10%的极端体重下降组——现有研究表明，极端体重下降应能提升飞行表现。但实验结果显示，无论接受何种处理，受试个体的飞行能力均未发生改变。对个体差异的分析同样得出了与首个实验完全一致的结果模式。 4. 我们由此得出结论：被捕食者或许会根据捕食风险调整自身体重与躲避行为，但它们的逃逸能力始终保持稳定且不受体重影响，这可能是因为削弱自身逃逸能力大概率会带来灾难性后果。我们推测，动物仅会在与生命史阶段相关的少数特定情境下面临体重依赖性捕食风险的权衡，例如分娩与迁徙这类需要大幅增加体重的场景。