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. 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.

'Humans are creating all these totally new environments compared to what animals have seen in evolutionary history.'

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.”

If so, it might be driven by high population densities of salamanders in the city. Aggressive neighbors don’t tend to be good neighbors. Through that lens, city animals could be domesticating themselves, a process that can occur without direct human intervention.

Even more fundamentally, muted stress responses have been found in many species of urban animals. When surprised or threatened, their endocrine systems release lower-than-usual amounts of stress hormones. It’s a sensible-seeming adaptation. A rat that gets anxious every time a subway train rolls past won’t be very successful.

“They’re clearly attenuating their physiological response to stress, probably because they’re constantly inundated with noise, traffic, and all kinds of environmental stresses in cities,” said biologist Jonathan Atwell of Indiana University. “If they were ramping that response up all the time, it would be too costly.”





A challenging question is whether traits like these represent inherited biological changes or what researchers call phenotypic plasticity: the ability to make on-the-fly adjustments to circumstance.

Some adaptations, such as the swath of genetic mutations that Munshi-South identified in New York City’s white-footed mice, are clearly heritable. Others are learned. In many cases, both processes are likely involved, said Atwell, who studied the question in his research on songbirds called dark-eyed juncos around San Diego.

The San Diego juncos sing at higher frequencies than those living in rural, traffic-free settings. When Atwell raised some of their chicks in a quiet place, that rise in song frequency dropped by about half, suggesting an even split between heritability and plasticity.

Where things get really interesting, though, is with social learning and animal culture — all those animal habits and abilities that are not inborn, but taught. “I suspect that often it’s not their cognitive abilities evolving, but cultural evolution going on,” said Atwell. “Anytime animals can learn behavior from one another, I think there might be cultural evolution.”

Urban squirrels, for example, seem to have adjusted to vocalization-drowning ambient noise by making tail-waving a routine part of communications. Perhaps this was instinctive in a few animals, then picked up by others. Likewise, squirrels might learn about traffic by seeing others get run over, said Snell-Rood. Rats could see brethren die after eating poisoned bait, then teach pups to avoid the traps.

'You could imagine some kind of speciation over long periods of time.'

Not all changes in urban animals will represent adaptations to urban living, however.

Most genetic mutations are neither beneficial nor harmful, at least not right away. They simply happen and, over long periods of time, accumulate in populations through what’s known as genetic drift. In isolated groups, drift’s effects are magnified, as are so-called founder effects, in which entire populations bear the genetic imprint of a few early animals. For these creatures, urban adaptations won’t necessarily represent adjustments to city life, but simple happenstance.

How might this play out in deep time? If humans can keep civilization intact long enough, will urban animal populations eventually become their own distinct species — bold, relaxed, and clever, with a store of learned information about our habits, and perhaps a few other traits that arise by chance?

Nobody knows, said Snell-Rood, but “you could imagine some kind of speciation over long periods of time.” She noted, though, that not all the changes seen in urban animals are necessarily permanent. The big brains of those city-dwelling Minnesota mammals, for instance, seemed to shrink after a few decades of urban adaptation.

“The way I interpret it is that during the initial colonization, it pays to be smart,” Snell-Rood said. Once city life becomes predictable, “you can go back to having a smaller brain.”