Absolute brain size: Did we throw the baby out with the bathwater?

Abstract

The recent study by Sherwood et al. (1) in this issue of PNAS is a study in irony. It is thus so because the authors used some of the most current and sophisticated histological and data-analytic tools to, in the end, unearth … a dinosaur. Now the authors did not do this in the literal sense. Rather, they laid bare an “extinct” measure of brain anatomy: that old fossil known as absolute brain size. Even as far back as the 1800s, Alexander Brandt (2) and, a bit later, Otto Snell (3) demonstrated the importance of scaling the brain to body size, signaling the impending demise of absolute brain size as a valid measure of brain capacity. The final death knell was struck in its most popular form by Harry Jerison, who reminded us once again that brain–body allometry plays a major role in the size of any species' brain. He effectively argued that the brain, like any other organ, scales with body size, and the validity of the use of brain size as a measure of intelligence or information processing capacity rests upon the size of the brain relative to the size of the body. He introduced the encephalization quotient (EQ) as a proper measure of relative brain size across species (4). EQ is a number that essentially quantifies how much larger or smaller the average brain size of a given species is relative to the expected brain size based on body size. Likewise, the residual of a brain weight–body weight regression across a sample of species provides similar information. And, although different authors have different theoretical justifications for the parameters of the brain–body size relationship across species [see Deaner et al. (5) for review], all measures of relative brain size are based on the common assumption that it is only meaningful to consider brain size if body size, or some relative measure, is taken into account. Therefore, nowadays, absolute brain size as a measure of cognitive capacity is considered obsolete.