For scientists, the most common method of assessing an animal’s intelligence is by looking at its relative brain size, with respect to its body size. The human brain, for instance, is small compared to other animal’s brain, however it’s exceptionally large when considering our body mass. A new study, which analyzed the relative brain size of a myriad of species across time, found that relative brain size is not necessarily as dependent on evolutionary selection on the brain as previously thought.

Fighting male elephant seals with on the background the carnivoran phylogeny displaying varying trends of brain and body size evolution. (c) UCL

In their study, the scientists at University College London (UCL) found that the most significant factor in determining relative brain size is often evolutionary pressure on body size, and not brain size. They came to this conclusion after they built data with brain and body mass for hundreds of modern and extinct bats, carnivorans, and primates, and then charted these data’s evolution over time for each species at hand.

It seems, aside from bats, most species increase in body size faster than brain size. Curiously, in the bats’ evolutionary history, they decreased body size much faster than brain size, leading to an increase in relative brain size. This allowed them to develop an improved flying maneuverability while maintaining the brainpower to handle foraging in cluttered environments.

“When using brain size relative to body size as a measure of intelligence, the assumption has always been that this measure is primarily driven by changes in brain size. It now appears that the relationship between changes in brain and body size in animals is more complex than has long been assumed,” lead author Jeroen Smaers said.

“Changes in body size often occur independently of changes in brain size and vice versa. Moreover, the nature of these independent changes in brain and body size, are different in different groups of animals,” Smaers continued.

For primates, brain size increase  marginally outpaced those in body size. Carnivoran evolution, on the contrary, relative brain size changes are generally more strongly associated with body size rather than selection on brain size and cognition.

The researchers suggest that body size acts as a more influential evolutionary factor than brain size. Findings were published in the journal  Proceedings of the National Academy of Sciences.