Brawn went before brains in Paleocene placental mammals

Mammals have the largest brain to body size ratio among vertebrates, but how did this unprecedented encephalization emerge during their evolution? A new study by Ornella Bertrand and colleagues concludes that body sizes among placental mammals were the first thing to increase after the animals expanded across the globe after the dinosaurs’ extinction. Body mass increased at such a fast rate, they say, that at first there was a decrease in relative brain size among placental mammals of the Paleocene (about 66 to 56 million years ago). During the Eocene (about 55 to 34 million years ago), however, brain size began to increase among placental mammals again. The researchers traced these trends with the help of computerized tomography (CT) scans of recently discovered Paleocene mammal skulls from the San Juan Basin of New Mexico and the Denver Basin of Colorado, along with CT scans of other mammal skulls from this time span. Along with the size of the full brain, Bertrand et al. also measured the size of individual sensory components in the skull, such as the neocortex and olfactory bulbs, to look for changes in those components over time. They conclude that placental mammal species first increased their body size to expand into new ecological niches left behind by the dinosaurs, and then expanded their brains during the Eocene as enhanced cognition became more important in saturated ecosystems filled with competitors. In particular, placental mammals of the Eocene grew their neocortex and components of the brain involved in eye movement, at the expense of olfactory bulbs, the researchers suggest. In a related Perspective, Felisa Smith discusses how this road to encephalization among mammals could shed light on the importance of ecology, among other factors, in macroevolution.