Mammals have the largest brains in relation to body size among vertebrates, but which came first? New research that examined the assumption that enlarging brains led to larger body sizes in mammalian evolution found instead that body size was the first to increase, followed by bigger brains.

UNM Biology Prof. Felisa Smith, an expert on body size evolution and president-elect of the American Society of Mammalogists, was asked by the prestigious journal Science to interpret the new findings, which looked at the explosion of mammalian diversification after dinosaurs went extinct.

“But did brain size also increase proportionately? The study, it turns out, showed that it didn’t,” Smith said. “Essentially mammals got bigger and ‘dumber’ first. Once these body size niches were all full, then there was strong selection on brain size and mammal brain size increased.”

Why?

“Brains are energetically expensive, which means that if you had two animals of the same size, the one with the larger brain would require much more energy (food) to survive. Because energy is often limiting for animals, this means that other activities, and especially reproduction, are scaled down. Indeed, animals with relatively larger brains for their bodies have lower reproductive rates,” Smith said.