Numbers of neurons as biological correlates of cognitive capability

What impact does variation in brain size have on the behavioral or cognitive capabilities of different species? Answering this seemingly simple question has been hampered by difficulties in defining and measuring the relevant variables in the brain, on the one hand, and in quantifying behavior in a way that can be compared across species, on the other. A new method of counting cells has made it easy to obtain direct estimates of the numbers of neurons that compose different brain structures. Crossing these numbers with the first large-scale quantitative studies of cognitive capabilities across species suggests that absolute numbers of neurons in the mammalian cerebral cortex, or in the bird pallium, are good correlates of cognitive diversity: the more the neurons, regardless of brain or body size, the better a species performs at a same task.