Big brains do matter in new environments

Why are some species' brains so large despite the metabolic and developmental costs? Why would two species with the same body size have two different brain sizes? These have been central questions in the field of brain evolution for decades and are the driving questions behind the article by Sol et al. (1) in this issue of PNAS. Correlations among various measures of brain size, cognitive capacity, problem solving, and behavioral ecology have been recorded in many groups of animals. The conventional approach has been to interpret these correlations as proxies for “as-yet-unknown” causal relationships while relying on the assumption that large brains confer greater cognitive abilities. Such studies in recent years have benefited from enhanced methodological capacities to measure brain size (e.g., computed tomography), collect large behavioral databases, and apply powerful statistical tests to such data. Collectively, this body of work has brought to light some broad consistent patterns of association among brain size, cognition, and ecology across animal groups that can be summarized as follows.

Various measures of brain size (e.g., encephalization quotient, brain–body residuals, neocortex ratio) are positively correlated with

1. feeding innovation, learning, and tool use in birds and primates (2–10);

2. behavioral repertoire size in mammals (11);

3. social complexity in birds, primates, carnivores, and some insectivores (8, 12–16);

4. dietary complexity in primates (17–20); and

5. unpredictability of the environment in hominids (21, 22).