Bigger brains in mice and bats may be due to human activity

Human activity may be responsible for a sharp increase in brain growth in some small mammal species, according to a new study from University of Minnesota researchers.

Research showed cranial capacity in some species tracked human urban development, suggesting some animals have become smarter to adapt to environments created by and for people.

Past research on bird species has shown that birds with larger brains adapted best to urban living, but Emilie Snell-Rood, a behavioral and evolutionary biologist UM’s College of Biological Sciences sought to find out whether human environments drive change within species.

Researchers analyzed 100 years worth of skull specimens from ten species, including varieties of shrews, voles, bats, and squirrels, along with a mouse and gopher.

Snell-Rood and her colleagues found that two small mammals with high birth rates -- the white-footed mouse and meadow voles -- showed the most brain growth, with urban and suburban specimens showing a 6 percent higher cranial capacity than their rural counterparts.

On the other hand, bats, shrews and squirrels showed "showed marginally significant increases in relative cranial capacity over time in rural populations."

As human developments continues, and rural habitats are cleared for agriculture, researchers hypothesize that insect-eaters such as bats must now cover a larger territory to find food, selecting for higher cognition and a larger brain.

Researchers also controlled for overall body size and diet, suggesting a changed or increased diet was not in play.

Though the findings, published in the Proceedings of the Royal Society B., are not conclusive, Snell-Rood said, "we tended to not see changes in body size which suggests it’s not just nutritional, but rather an evolutionary response."

But, a larger brain, researchers say, also requires more food.

"Neural tissue is incredibly expensive metabolically. There are trade-offs in investing in brains and investing in reproduction, which may be why we see a reduction in cranial capacity over time in two of the species -- larger brains may be favored just during the initial colonization of the city."