Group living is assumed to benefit prey by reducing predation risk due to dilution and detection effects. This increased safety could be exchanged against a reduced individual vigilance leading to increased foraging and fitness until costs of intra‐group competition offset this benefit. However, very few studies have been able to directly test the relationship between individual fitness and group size in non‐cooperative breeding species. Therefore, evidence that dilution and detection effects do indeed translate into increased fitness under predation risk remains mostly theoretical. Here, we aimed at empirically testing whether group size influences beneficial and costly behaviors, and ultimately the survival of individuals, in a prey species facing heavy predation pressure. We focused on plains zebra Equus quagga of Hwange National Park in Zimbabwe, a high lion density area. We collected behavioral data (i.e., individual vigilance and foraging, collective vigilance, and agonistic interactions) that we used as proxies to investigate the costs and benefits of grouping in this species. We then fitted multistate capture‐mark‐recapture and Cox proportional‐hazards models to estimate apparent survival of adults and young zebras, respectively, and assess whether a demographic signal of group size could be found. Our results show that collective vigilance increases with group size, a prerequisite of the detection effect, but individual vigilance does not consistently decrease with group size, which suggests that there is no group size effect on individual vigilance. Agonistic interactions were rare. These results suggest that there are no foraging costs of grouping here, and even some benefits such as a better collective vigilance. However, any benefits of being in larger groups did not translate into increased survival, as we found no effect of group size on the apparent survival of individuals. Since dilution and detection effects do not matter behaviorally and demographically in these plains zebras living under high predation risk, these results challenge our understanding of how predation shapes animal sociality: fitness benefits of dilution or detection should not be assumed.

中文翻译：

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斑马斑马提供的证据表明稀释和检测效果可能并不总是在行为和人口统计学上都重要

假定通过减少稀释和检测效应，降低种群捕食风险，可以使群体生活受益于猎物。可以用增加的安全性来换取降低的个人警觉性，从而提高觅食和适应性，直到组内竞争的成本抵消了这一好处。但是，很少有研究能够直接检验非合作繁殖物种个体适应性与群体规模之间的关系。因此，在掠食风险下，稀释和检测效果确实可以转化为适应性提高的证据大部分仍然是理论上的。在这里，我们旨在通过实证测试在面临巨大捕食压力的猎物物种中，种群规模是否会影响有益和昂贵的行为，并最终影响个体的生存。我们专注于斑马Equus quagga高狮子密度地区津巴布韦万基国家公园的全景。我们收集了行为数据（例如，个人警惕和觅食，集体警惕和激动互动），以此作为调查该物种分组的成本和收益的代理。然后，我们拟合了多状态捕获标记捕获和Cox比例风险模型，分别估计了成年斑马和年轻斑马的表观存活率，并评估了是否可以找到群体规模的人口统计学信号。我们的结果表明，集体警惕性随小组规模的增加而增加，这是检测效果的先决条件，但个人警惕性并不会随着小组规模的增加而持续降低，这表明小组规模对个人警惕性没有影响。激动性相互作用很少见。这些结果表明，这里没有进行分组觅食的费用，甚至没有诸如集体警惕性之类的好处。但是，在较大的群体中获得的任何好处并不能转化为存活率的提高，因为我们发现，群体规模对个体的表观存活率没有影响。由于在生活在高捕食风险下的这些斑马斑马中，稀释和检测效果在行为和人口统计学上均无关系，因此这些结果挑战了我们对捕食如何塑造动物社会性的理解：不应假定稀释或检测对健康有益。