August 23, 2013
An opulent town mouse goes to visit his cousin in the country. The town mouse looks down on the country mouse, assuming all the city has to offer–the dining, the culture–make him the better rodent of the two. He insists that his cousin come visit him, and the country mouse reluctantly agrees. While dining the in the city, however, a pack of dogs attack the two mice, sending the cousins running. The country mouse realizes that the city is overrated and bids his cousin farewell, returning home to enjoy his life.
There may be a kernel of truth in this famous fable, it turns out–and for the very rodents it refers to. Like the town mouse, small mammals live alongside urbanite humans around the world. These metropolitan habitats are a far cry from the field or forest they originally evolved to thrive in. The city presents a myriad of obstacles–including pavement, cars, pesticides, dogs and countless other deathtraps–that may threaten a small creature’s survival. Therefore, scientists reason, animals who do manage to eke out a living in a hostile concrete jungle may be the brightest and sharpest of the bunch–essentially, the adaptable, know-it-all town mice of the furry world.
In the past, researchers showed that smarty-pants birds with bigger brains and feathered free-spirits with a more go-with-the-flow attitude are better able to cope with human-induced issues they may encounter, and also perform the best in urban environments. Whether the town animal is one who possesses a survivor’s edge to begin with, or whether the city itself shapes its smallest residents over time, however, remains unknown.
Researchers from the University of Minnesota put their cards on the latter hypothesis. Urban environments, they expected, are actively transforming populations of four-legged country bumpkins into street-smart townies. Furthermore, the longer a population of animals spends in the city, they thought, the more brain wealth that established lineage would likely acquire.
To test the validity of these guesses, the researchers set out to carefully measure the brain cavity sizes of thousands of skulls (a commonly used proxy for cognitive abilities) belonging to ten different species of small mammals, including voles, mice, squirrels, gophers, bats and shrews. They acquired museum specimens spanning the past 100 years of both Billybobs originally captured from rural sites and Rockefellers caught in urban locations in Minnesota. They used statistical tests to control for variables such as body size and gender, then analyzed their results to see if any differences emerged between the city slickers’ and the country folks’ smarts.
The results, described this week in Proceedings of the Royal Society B: Biology, surprised the researchers. Out of the ten species, only two of the urban populations–the white-footed mouse and the meadow vole–showed a significant cranial edge–both 6 percent greater–over their country cousins (though statistical tests did suggest that, with a larger sample size, big brown bats and masked shrews would also likely fall into this camp).
Not surprisingly, however, those species equipped with larger brains were the ones that have the highest reproductive rates, leading the researchers to speculate that they may have a generational advantage over their slower-to-make-babies neighbors since more babies equals more opportunities to shape new adaptations. Finally, when they combined all species into just two pots, urban and rural, and controlled for body size, they also noticed a general trend towards larger cranial capacity for urban dwellers in general.
Counterintuitively, the urban environment did not seem to shape animals’ skull sizes over the years. In other words, Mouse D. Trump Jr.’s brain was, statistically, the same size of Mouse D. Trump Sr.’s brain, even though 100 years separated the two. Over time, in fact, white-footed mice and big brown bats in the city environment actually seem to be losing their edge, showing slight declines in brain size over the years (perhaps that unwavering wakeup-commute-eat-sleep routine is dumbing them down?). On the other hand, rural populations of four species–two bats and two shrews–are coming up from behind, as LCD Soundsystem might put it. And American red squirrels of the backwoods aren’t so backwoods after all–they also showed a marginal inclination to become one of those kids “with better talent and better ideas” that those lackluster mouse and bat urbanites need to look out for.
While some of these findings did coincide with the authors’ assumption that city slickers should be smarter than their country counterparts, the research raises more questions than provides answers. It could be possible, for example, that the researchers didn’t have skulls spanning back far enough in time. By the early 1900s, when the first skulls from this study originated, some parts of Minnesota were already converted to urban areas, meaning cranial capacity adjustments in those species may have taken place earlier in time.
On the other hand, they hypothesize, it could be that one mouse’s field is another’s garbage can; perhaps animals just don’t need that much extra brain power to survive in their little niche in the world, regardless of whether the broader environment is built or grown.
As for those edgy rural species, they muse, perhaps those animals could be encountering even more significant changes and challenges over time than their urban counterparts. Logging, agricultural conversion, subdivisions and highways are all threatening Minnesota’s rural alcoves. Perhaps country animals are being forced to step up and adapt, or else get squished by a tractor or truck. Or, they speculate, maybe rural mammals are just getting a more well-balanced diet of leafy greens and farm-to-rodent produce than their city counterparts, who are forced to munch on stale fries and rotten kabobs.
Ultimately, they admit, only a manipulative field experiment–putting mice and voles into urban and rural environments and seeing what happens–would likely solve the question of the true drivers behind the town mouse and the country mouse’s smarts. In this case, at least, the town mice and voles are maintaining a superior edge, but the country bats, shrews and squirrels are proving that life away from the noise and pollution has its perks, too.
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