For each of the examined mammals, the researchers controlled for sex and body size. The animals therefore didn't just have bigger heads and bigger brains due to more human-provided food, for example.
In terms of the brainier city dwelling small mammals, "we hypothesize that the change in the urban environment is due to cognitive adaptations to the novel challenges associated with city and suburban living," Snell-Rood said.
Prior studies on birds have found that big-brained species tend to do better in cities. Snell-Rood suspects that the movement abilities of bird and small mammals give them an edge over species that are not so flexible.
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As for the big brained country critters, she said, "We hypothesize the changes in rural environments are due to increases in the size of foraging areas of insect-feeding species (due to human development), which is also selecting for greater cognition (e.g., for spatial memory)."
Some bats then appear to be evolving larger brains to deal with larger territories created, in part, by humans. Bigger brains come at a cost, however.