Brains reflect sex differences

An international research team consisting of Patrik Lindenfors, Charles Nunn and Robert Barton examined data on primate brain structures in relation to traits important for male competition, such as greater body mass and larger canine teeth. The researchers also took into account the typical group size of each sex for individual primate species in order to assess sex-specific sociality – the tendency to associate with others and form social groups. The researchers then studied the differences between 21 primate species, which included chimpanzees, gorillas, and rhesus monkeys, using statistical techniques that incorporate evolutionary processes.

The authors found that sexual selection had an important influence on primates’ brains. Greater male-on-male competition (sexual selection) correlated with several brain structures involved with autonomic functions, sensory-motor skills and aggression. Where sexual selection played a greater role the septum was smaller, and therefore potentially exercised less control over aggression.

In contrast, the average number of females in a social group correlates with the relative size of the telencephalon (or cerebrum), the largest part of the brain. The telencephalon includes the neocortex, which is responsible for higher functions such as sensory perception, generation of motor commands and spatial reasoning. Primates with the most sociable females evolved a larger neocortex, suggesting that female social skills may yield the biggest brains for the species as a whole. Social demands on females and competitive demands on males require skills handled by different brain components, the authors suggest. The contrasting brain types, a result of behavioural differences between the sexes, might be a factor in other branches of mammalian brain evolution beyond anthropoid primates, too.