Small mammals that either produce a lot of babies or eat a lot of bugs exhibited the greatest cranial capacity (i.e. brain growth) due to human-caused environmental changes over the past century, suggests the study, which is published in the latest Proceedings of the Royal Society B.

“Small mammals with high fecundity (such as deer mice and meadow voles) show the most pronounced differences in cranial capacity between urban and rural environments, having larger cranial capacity in urban environments,” lead author Emilie Snell-Rood told Discovery News.

“Insect-feeding species, like bats and shrews, show the most pronounced response to rural environments, showing an increase in cranial capacity in rural populations over the last 90 years,” added Snell-Rood, who is an assistant professor in the Department of Ecology, Evolution and Behavior at the University of Minnesota.