Institute of Marine Research, Bergen, Norway.
PLoS One. 2012;7(9):e44801. doi: 10.1371/journal.pone.0044801. Epub 2012 Sep 12.
Aggregation is thought to enhance an animal's security through effective predator detection and the dilution of risk. A decline in individual vigilance as group size increases is commonly reported in the literature and called the group size effect. However, to date, most of the research has only been directed toward examining whether this effect occurs at the population level. Few studies have explored the specific contributions of predator detection and risk dilution and the basis of individual differences in the use of vigilance tactics. We tested whether male and female (non-reproductive or with young) eastern grey kangaroos (Macropus giganteus) adopted different vigilance tactics when in mixed-sex groups and varied in their reliance on predator detection and/or risk dilution as group size changed. This species exhibits pronounced sexual dimorphism with females being much smaller than males, making them differentially vulnerable toward predators. We combined field observations with vigilance models describing the effects of detection and dilution on scanning rates as group size increased. We found that females with and without juveniles relied on predator detection and risk dilution, but the latter adjusted their vigilance to the proportion of females with juveniles within their group. Two models appeared to equally support the data for males suggesting that males, similarly to females, relied on predator detection and risk dilution but may also have adjusted their vigilance according to the proportion of mothers within their group. Differential vulnerability may cause sex differences in vigilance tactic use in this species. The presence of males within a group that do not, or only partially, contribute to predator detection and are less at risk may cause additional security costs to females. Our results call for reexamination of the classical view of the safety advantages of grouping to provide a more detailed functional interpretation of gregariousness.
聚群被认为通过有效探测捕食者和分散风险来提高动物的安全性。随着群体规模的增加，个体警觉性下降的现象在文献中被广泛报道，并被称为群体规模效应。然而，迄今为止，大多数研究仅致力于检验这种效应是否在种群水平上发生。很少有研究探索捕食者探测和风险分散的具体贡献以及个体在使用警戒策略方面的差异基础。我们测试了雄性和雌性（非繁殖期或有幼崽的）东部灰袋鼠（Macropus giganteus）在混合性别群体中是否采用了不同的警戒策略，以及它们在依赖捕食者探测和/或风险分散方面随群体规模变化的个体差异。该物种表现出明显的性别二态性，雌性比雄性小得多，使它们对捕食者的易感性存在差异。我们将野外观察与描述随着群体规模增加对扫描率的探测和稀释效应的警戒模型相结合。我们发现，有和没有幼崽的雌性依赖于捕食者的探测和风险的稀释，但后者会根据其群体中带有幼崽的雌性比例来调整自己的警觉性。两种模型似乎都同样支持雄性的数据，这表明雄性与雌性相似，依赖于捕食者的探测和风险的稀释，但也可能根据其群体中母亲的比例来调整自己的警觉性。差异易感性可能导致该物种在警戒策略使用方面的性别差异。群体中存在不参与或仅部分参与捕食者探测且风险较低的雄性，可能会给雌性带来额外的安全成本。我们的结果呼吁重新检验聚群的安全优势的经典观点，以对群居性提供更详细的功能解释。
