Department of Experimental Psychology, Radcliffe Observatory Quarter, University of Oxford, Oxford, OX2 1GG, UK.
Department of Earth and Environmental Sciences, Michael Smith Building, University of Manchester, Manchester, M13 9PT, UK.
Biol Rev Camb Philos Soc. 2021 Oct;96(5):1889-1906. doi: 10.1111/brv.12730. Epub 2021 May 4.
Compared to most other mammals and birds, anthropoid primates have unusually complex societies characterised by bonded social groups. Among primates, this effect is encapsulated in the social brain hypothesis: the robust correlation between various indices of social complexity (social group size, grooming clique size, tactical behaviour, coalition formation) and brain size. Hitherto, this has always been interpreted as a simple, unitary relationship. Using data for five different indices of brain volume from four independent brain databases, we show that the distribution of group size plotted against brain size is best described as a set of four distinct, very narrowly defined grades which are unrelated to phylogeny. The allocation of genera to these grades is highly consistent across the different data sets and brain indices. We show that these grades correspond to the progressive evolution of bonded social groups. In addition, we show, for those species that live in multilevel social systems, that the typical sizes of the different grouping levels in each case coincide with different grades. This suggests that the grades correspond to demographic attractors that are especially stable. Using five different cognitive indices, we show that the grades correlate with increasing social cognitive skills, suggesting that the cognitive demands of managing group cohesion increase progressively across grades. We argue that the grades themselves represent glass ceilings on animals' capacity to maintain social and spatial coherence during foraging and that, in order to evolve more highly bonded groups, species have to be able to invest in costly forms of cognition.
与大多数其他哺乳动物和鸟类相比，类人猿具有异常复杂的社会结构，其特征是有密切联系的社会群体。在灵长类动物中，这种现象被概括为“社会大脑假说”：各种社会复杂性指标（社会群体规模、梳理群体规模、战术行为、结盟形成）与大脑大小之间存在很强的相关性。迄今为止，这一直被解释为一种简单、单一的关系。我们利用来自四个独立脑数据库的五个不同脑容量指数的数据表明，群体规模与大脑大小的分布可以最好地描述为一组四个截然不同、非常狭窄定义的等级，这些等级与系统发育无关。将属分配到这些等级的方式在不同的数据集中和脑指数中都非常一致。我们表明，这些等级对应于有联系的社会群体的逐步进化。此外，我们还表明，对于那些生活在多层次社会系统中的物种，每种情况下不同分组级别典型大小都与不同等级相对应。这表明这些等级对应于特别稳定的人口统计学吸引子。我们利用五个不同的认知指数表明，等级与社会认知技能的提高相关，这表明管理群体凝聚力的认知需求在等级上逐渐增加。我们认为，这些等级本身代表了动物在觅食过程中维持社会和空间一致性的能力的天花板，而且为了进化出更紧密联系的群体，物种必须能够投资于昂贵的认知形式。
