Data from: A farewell to EQ: A new brain size measure for comparative primate cognition

Both absolute and relative brain size vary greatly among and within the major vertebrate lineages. Scientists have long debated how larger brains in primates and hominins translate into greater cognitive performance, and in particular how to control for the relationship between the non-cognitive functions of the brain and body size. One solution to this problem is to establish the slope of cognitive equivalence, that is the line connecting organisms with an identical bauplan but different body sizes. The original approach to estimate this slope through intraspecific regressions was abandoned after it became clear that it generated slopes that were too low by an unknown margin due to estimation error. Here, we revisit this method. We control for the error problem by focusing on highly dimorphic primate species with large sample sizes and fitting a line through the mean values for adult females and males. We obtain the best estimate for the slope of ca 0.27, a value much lower than those constructed using all mammal species, and close to the value expected based on the genetic correlation between brain size and body size. We also find that the estimate of cognitive brain size based on cognitive equivalence fits empirical cognitive studies better than the encephalization quotient (EQ), which should therefore be avoided in future studies on primates, and presumably mammals and birds in general. The use of residuals from the line of cognitive equivalence may change conclusions concerning the cognitive abilities of extant and extinct primate species, including hominins.

各主要脊椎动物类群内部及类群之间，脑的绝对尺寸与相对尺寸均存在显著差异。长期以来，科学家们一直在争论灵长类与古人类的更大脑容量如何转化为更优异的认知能力，尤其探讨如何控制脑的非认知功能与躯体尺寸之间的关联以排除干扰。针对这一难题，学界提出了认知等效斜率的解决方案：即连接具有相同基本体型构造但躯体尺寸各异的物种的直线。早期通过种内回归估算该斜率的方法因被证实存在估算误差，导致所得斜率未知幅度地偏低，最终被学界弃用。本文重新审视了该方法：通过聚焦于样本量充足、雌雄二态性显著的灵长类物种，并基于成年雌雄个体的平均数值拟合直线，以此修正误差问题。最终得到该斜率的最优估算值约为0.27——该数值远低于基于全部哺乳类物种得到的结果，且与通过脑容量与躯体尺寸的遗传相关性预期得到的数值相近。此外，我们发现基于认知等效性得到的认知脑容量估算结果，比脑化指数（encephalization quotient, EQ）更契合实证认知研究，因此未来针对灵长类、乃至整体哺乳类与鸟类的相关研究中应避免使用脑化指数。基于认知等效直线的残差分析，或可改变关于现生及灭绝灵长类（包括古人类）认知能力的相关研究结论。