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The town mouse may have a bigger brain than its country cousin. New research reveals that human behavior could be accelerating the evolution of animal brains by changing the habitats in which they live.

Scientists say that that the recent findings suggest that the brains of some animals have grown larger with the industrialization of their habitat, making city animals more intelligent than their rural counterparts.

Lead researcher Emilie Snell-Rood, an evolutionary biologist at the University of Minnesota, found that white-footed mice and meadow voles living in cities had brains that were on average 6 percent larger than those of animals collected from farms in the countryside.

Scientists say that the latest findings "suggest that animals are showing complex cognitive responses to both urban and rural areas with cranial capacity tracking human-induced change in some cases," according to a news release.

"Which species are going to be able to deal with 100 percent conversion of prairie to agriculture?" said Snell-Rood. "Which species are going to be able to deal with forests being supplanted by cities?"

She says that identifying which species are going to be able to cope with human-induced change will provide vital context when prioritizing conservation objectives.

Past research revealed a link between brain size and the ability to adapt to urban environment in birds. However, previous studies did not look to see whether changes in the environment caused the evolution.

In the latest study, researchers wanted to find "if dealing with a new environment is just a matter of species sorting- in which a particular species is 'pre-adapted' for success in a particular environment- or actually changes going on within species in addition to that species-level variation," according to the university release.

Snell-Rood and co-researcher Naomi Wick looked at specimens from the Bell Museum of Natural History collections dating back to the early 20th Century.  Researchers focused on the cranial capacity of 10 species including varieties of shrews, voles, bats, and squirrels, along with a mouse and gopher, from locations in and around the Twin Cities metro.

The finding showed that the urban populations of two of the species did, in fact, possess significantly greater cranial capacity. Researchers predicted that cranial capacity should also increase if cleverer animals were more favored in urban environments. 

Researchers said that the changes might be evolutionary.

"We tended to not see changes in body size which suggests it's not just nutritional, but rather an evolutionary response," said Professor Snell-Rood. She added that the study "reminds us of the fact that populations adapt, and that at least some species are tracking human-induced environmental change."

Researchers noted that some of the species examined in the study contradicted her theory.

We didn't see cranial capacity increases over time in the urban specimens...and actually, for two species we see a decline over time, while the bats and shrews show an increase in cranial capacity in rural populations," she said.

"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," she explained.

She said the larger cranial capacity seen in rural bats and shrews suggest that humans are changing rural environments in way that could be just as challenging as cities.