Thank mothers for large ape brains

Humans, apes and monkeys have their mothers to thank for their large brains.

It takes a lot of energy to make and run a brain, so large ones should only have developed in animals with fast metabolisms. But according to Vera Weisbecker of the University of Cambridge and Anjali Goswami of University College London, that's only part of the story.

The pair looked at the brains of 197 marsupials and 457 placental mammals, and could find a link between metabolic rate and brain size only in placental mammals. This suggests that parenting strategies play a key role.

"Placental babies are connected to their mothers via the placenta for a long time," says Weisbecker. "So if she has a high metabolic rate, the baby is more likely to benefit." By contrast, marsupial babies are born while they are still very small, then spend a long time feeding off their mothers' milk – a slower way to grow a large brain. Placentas offer a continuous supply of rich nutrients.

However, the pair found no difference in the average brain sizes of marsupials and placental mammals – as long as they excluded primates. These, it seem, got their disproportionately large brains from a double maternal boost. They are supplied with large amounts of energy by their mothers during gestation, and then receive additional months or even years of care after birth.