As humans change the environment, various species are forced to adapt or perish. Some cliff swallows learn to avoid swift highway traffic. Some Atlantic tomcod find ways to resist river pollution. A few members of Congress even recognize the need to address climate change. Survival of the fittest, indeed.
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Fewer changes to the environment have been as extreme as the shift to cities, but since that process occurred so long ago in developed countries, the response by various animals is hard to measure. Well, an imaginative new study helps clarify that question. Turns out animals did adapt to the human push for cities: with bigger brains.
To reach that conclusion, evolutionary researchers Emilie C. Snell-Rood and Naomi Wick of the University of Minnesota gathered dozens of skulls from the Bell Museum of Natural History. They focused on 10 species of mammals that lived in either rural or urban areas of the state during the past 100 years. Measuring the skulls allowed Snell-Rood and Wick to estimate cranial capacity, and by extension, brain size.
Two rodent species—the white-footed mouse and the meadow vole—had about 6 percent greater cranial capacity among the urban regions of Minnesota than the rural parts of the state. Two other species from cities had brains that also seemed bigger by comparison, though there weren't enough skulls to produce statistical significance. Across all species, write the researchers, "there was a significant tendency for urban populations to have larger cranial capacity when controlling for body size."
Their findings were published last week in the journal Proceedings of the Royal Society B:
In conclusion, our results provide some support of the hypothesis that urbanization selects for increased cognition, at least for a handful of species.
Snell-Rood and Wick note two potential explanations for why the species in cities would have larger skulls than those in the country. One is that only animals with bigger brains were capable of surviving in cities in the first place. The other is that as species lived in cities over a long period, their brains actually became bigger to handle everyday challenges of the urban environment.
The researchers couldn't be certain which adaptive process occurred, in part because Minnesota became industrialized in the middle-to-late 19th century, and the skull samples came from the 1920s and '30s.
However, other parts of the study do suggest that the animals brains themselves evolved in response to human development. The cranial capacities of four rural species, when compared to themselves and not to the skulls of their urban relatives, did increase over time. The likely reason for this shift is that rural areas also became more complicated: for instance, as humans cleared forests to make way for farms, bats had to fly farther to find food, which required brains large enough to carry more spatial knowledge.
Snell-Rood and Wick failed to find this same within-species skull shift for city animals over time. But again, their skull collection didn't go back to the very dawn of Minnesota cities. In other words, they may simply have missed the selective moment when these cognitive adaptations occurred.
What the study underscores for certain is that as people alter the environment—whether drastically, as with the jump from forests to cities, or more gradually, as with forests to farms—animals fit to survive must find ways to deal with the change.