Overall Brain Size, and Not Encephalization Quotient, Best Predicts Cognitive Ability across Non-Human Primates

Abstract

For over a century, various neuroanatomical measures have been employed as assays of cognitive ability in comparative studies. Nevertheless, it is still unclear whether these measures actually correspond to cognitive ability. A recent meta-analysis of cognitive performance of a broad set of primate species has made it possible to provide a quantitative estimate of general cognitive ability across primates. We find that this estimate is not strongly correlated with neuroanatomical measures that statistically control for a possible effect of body size, such as encephalization quotient or brain size residuals. Instead, absolute brain size measures were the best predictors of primate cognitive ability. Moreover, there was no indication that neocortex-based measures were superior to measures based on the whole brain. The results of previous comparative studies on the evolution of intelligence must be reviewed with this conclusion in mind.

Endocranial volumes of primate species: scaling analyses using a comprehensive and reliable data set

“…If a line with this common slope is fitted to the logged species means of the complete data set, it becomes clear that all relatively large-bodied extant primates are also relatively large-brained. This finding may be connected with the results of Deaner et al (2007), who report that absolute brain size is the best predictor of domain general intelligence in primates.…”

“…Various lines of evidence suggest that brain size in primates is both Primate endocranial volumes 3 correlated with cognitive abilities (Reader and Laland, 2002;Deaner et al, 2007) and influenced by a variety of social, ecological, and physiological variables (Clutton-Brock and Harvey, 1980;Byrne and Whiten, 1988;Sawaguchi, 1990;Aiello and Wheeler, 1995;Martin, 1996). The scaling relationship between brain size and body mass in primates (and mammals generally) has also been a major topic of debate, in part because the influence of body mass must be considered in comparative analyses of brain evolution (e.g.…”

The Expensive Brain: A framework for explaining evolutionary changes in brain size

“…While there is no doubt that larger brains provide cognitive benefits (e.g. Sol et al 2005;Deaner et al 2007;Sol et al 2007), the present findings suggest that these benefits do not necessarily flow into increased adult survival, but may sometimes affect reproduction more.…”

“…Explaining variation in relative brain size is important, because the latter corresponds to intellectual or cognitive performance Lefebvre et al 2004;Sol et al 2005;Deaner et al 2007). Paleoanthropologists, in particular, are interested in this question, because the size of the human brain is roughly three times that of our closest relatives, common and pygmy chimpanzees, who have maintained more or less similar brain sizes since the split with the last common ancestor.…”

“…Focusing on the size of the brain as a whole rather than on specific regions is warranted from an energetic perspective. In addition, an individual's cognitive performance is linked to the size of its whole brain, corrected for the size of its body, both between species (Jerison 1973; Deaner et al 2007), and even within species, such as humans (McDaniel 2005). One reason for this may be that the composition of brains across species within a lineage shows obvious regularities that suggest that changes in one region are necessarily linked to a fairly large extent to changes in others (Finlay and Darlington 1995;Jerison 2001).…”