How City Living Is Reshaping the Brains and Behavior of Urban Animals

When next you meet a rat or raccoon on the streets of your city, or see a starling or sparrow on a suburban lawn, take a moment to ask: Where did they come from, so to speak? And where are they going?
In evolutionary terms, the urban environments we take for granted represent radical ecological upheavals, the sort of massive changes that for most of Earth’s history have played out over geological time, not a few hundred years.
Houses, roads, landscaping, and the vast, dense populations of hairless bipedal apes responsible for it: All this is new, and animals are adapting, fast, all around us. A growing body of scientific evidence suggests that the brains and behaviors of urban animals are changing rapidly in response.
“A lot of biologists are really interested in how animals are going to deal with changes in their environments,” said biologist Emilie Snell-Rood of the University of Minnesota. “Humans are creating all these totally new environments compared to what they’ve seen in evolutionary history.”
Snell-Rood is one of many researchers who have updated the conventional narrative of urban animals, in which city life favors a few tough, adaptable jack-of-all-trades — hello, crows! — and those species fortunate enough to have found a built environment similar to their native niches, such as the formerly cliff-dwelling rock doves we now call pigeons and find perched on building ledges everywhere.
The long view, though, is rather more multidimensional. Cities are just one more setting for evolution, a new set of selection pressures. Those adaptable early immigrants, and other species that once avoided cities but are slowly moving in, are changing fast.
As Snell-Rood and colleagues describe in an August 21 Proceedings of the Royal Society B article, museum specimens gathered across the 20th century show that Minnesota’s urbanized small mammals — shrews and voles, bats and squirrels, mice and gophers — experienced a jump in brain size compared to rural mammals.
Snell-Rood thinks this might reflect the cognitive demands of adjusting to changing food sources, threats, and landscapes. “Being highly cognitive might give some animals a push, so they can deal with these new environments,” she said.
Brain size is, to be sure, a very rough metric, one that’s been discredited as a measure of raw intelligence in humans. For it to fluctuate across a whole suite of species, though, especially when other parts of their anatomy didn’t change, at least hints that something cognitive was going on.
Many other studies have looked at behavior rather than raw cranial capacity. In these, a common theme of emerges: Urban animals tend to be bold, not backing down from threats that would send their country counterparts into retreat. Yet even as they’re bold in certain situations, urban animals are often quite wary in others, especially when confronted with something they haven’t seen before.
“Maybe avoiding danger is an useful trait for some animals living in urban environments,” said biologist Catarina Miranda of Germany’s Max Planck Institute, who in a September Global Change Biology paper described her experiments with rural and urban blackbirds.
“Most of the birds that never approach new objects or enter new environments in this long period of time are urban,” Miranda said. “There are many new dangers in a town for a bird. Cars can run you over. Cats can eat you. Kids can take you home.”
Somewhat counterintuitively, bold urban animals also tend to be less-than-typically aggressive, a pattern documented in species as disparate as house sparrows and salamanders, the latter of which are a specialty of Jason Munshi-South, an evolutionary biologist at the City University of New York. The city’s salamanders — there aren’t many, but they’re there — “tend to be languid,” said Munshi-South. “If you try to pick them up, they don’t try to escape as vigorously as they do outside the city. I wonder if there’s been natural selection for that.”