Slowly but Surely: Larger Brains Improve Immature Survival in Primates.

The high energy costs of brains suggest that a species' current brain size is adaptive. However, although the comparative data for mammals suggest a positive effect on fitness in larger-brained species because of higher adult survival and thus longer lifespan, it also reveals two negative effects, namely later age at first reproduction owing to slower development and a tendency towards reduced reproductive allocation owing to larger newborns. Here we suggest that what is missing is the positive impact of brain size on immature survival, causally linked to greater parental investment in larger-brained species. Using long-term demographic data on natural populations of 18 primate species, we find a strong positive brain size effect on immature survival, which is already apparent during the first year. We suggest this effect is caused by parental protection and provisioning, allowing young to survive better and mature slowly but surely. This survival effect may well be the strongest adaptive benefit of increased brain size. It remains unknown to what extent this effect generalizes to non-primates.