We have long known that cities can have profound effects on people, but it is only recently that scientists have begun to comprehend the far-reaching effects that cities are having on the wild creatures that reside within them. This line of inquiry has given rise to a new scientific field known as eco-evolutionary dynamics that is revealing some fascinating truths, most notably that the wild species that survive in our urbanized world are doing more than simply adjusting their behaviours; they are actually evolving at a genetic level to the changes around them, and these changes are occurring at an accelerated rate that Darwin could never have imagined.
“The notion that evolution takes eons so we can’t see it in progress has been completely overturned,” says Colin Garroway, an evolutionary ecologist and assistant professor in the department of biological sciences at the University of Manitoba. “We can watch evolutionary change happening literally before our eyes, from one generation to the next.”
The opportunity to witness evolution in action has made urban environments an exciting new playground for research. Marc Johnson, an associate professor of biology at the University of Toronto Mississauga, and Jason Munshi-South, an associate professor of biology at Fordham University in New York, recently pored through the scientific literature looking for patterns among 192 studies that appear to show changes in evolutionary processes in response to urban environments. They published their results in 2017 in the journal Science.
Among the examples they cited were house finches in Tucson, Arizona, that are developing larger and stronger beaks so they can more easily eat from backyard bird feeders; crested anole lizards in Puerto Rico that are growing longer limbs and stickier toes to enable them to climb smooth-surfaced buildings; fish in the Eastern U.S. that have developed resistance to pollutants that allows them to survive in water that contains thousands of times the usual lethal levels of these toxins; and a new type of mosquito that lives in subways, feeds exclusively on humans and rats, and has different genes and breeding habits from those that live above ground. Says Dr. Johnson, “It’s clear that cities are much more important than we originally thought in driving the evolution of organisms that co-inhabit them.”
Cities affect wildlife in a myriad of ways – by noise, heat, fragmentation of habitat, food supply, disease and other factors that researchers don’t yet fully understand. “Human influence over evolution is more pervasive than we might have imagined,” says Sarah Otto, a highly respected theoretical biologist and holder of the Canada Research Chair in Theoretical and Experimental Evolution at the University of British Columbia. Dr. Otto notes the example of roadside-dwelling cliff swallows in Nebraska that responded to a rising death toll from collisions with passing cars by growing shorter wings, an adaption that lends them greater agility in the air. “You wouldn’t think cars could have such an influence over the mortality of birds, and yet they do,” she says.
For his part, U of M’s Dr. Garroway has been studying urban squirrels and their diet. In the forest, squirrels normally eat tree seeds, but in the city that has changed because of the abundance of junk food. “There are dozens and dozens of photos of squirrels eating pizza on Reddit,” he says. In fact, Dr. Garroway found that the squirrels used in his research were much heavier than he expected. “We’re now trying to determine if they are simply pleasantly plump or unhealthy.” The early evidence suggests the latter, with blood tests revealing high glucose levels.
That would be consistent with work done by Albrecht Schulte-Hostedde, who holds the Canada Research Chair in Applied Evolutionary Ecology at Laurentian University. Dr. Schulte-Hostedde has discovered that urban raccoons have blood glucose levels similar to diabetics, although it’s not possible at this point to predict what the consequences for the animals will be.
What is known is that intelligent animals tend to thrive in manmade environments – and there is also evidence to suggest that city-living boosts intelligence in some species. Emilie C. Snell-Rood, an associate professor of ecology, evolution and behaviour at the University of Minnesota, discovered that citified white-footed mice and meadow voles display a six-percent increase in brain capacity over their rural counterparts. Dr. Snell-Rood thinks this may reflect the cognitive demands of adjusting to changing food sources, threats and landscapes.
Suzanne MacDonald, a psychology professor at York University, believes something similar is happening with raccoons. Based on problem-solving experiments she has conducted, Dr. MacDonald theorizes that the complex demands of adjusting to the urban habitat is creating a type of “uber raccoon” that is smarter and more resourceful than its country cousins.
Crows are known to be intelligent creatures, but life in the city seems to be adding to their range of cerebral capabilities as well. Urban crows in Japan have learned to crack open walnuts by dropping them in front of passing cars. They do this at intersections, then wait for the light to turn to retrieve their prizes. City crows are also known to use French fries as bait to catch fish and to mimic cat meows to get hoodwinked homeowners to put out bowls of kibble.
The savvy of urban birds was illustrated by McGill University biologists Jean-Nicolas Audet, Simon Ducatez and Louis Lefebvre, who tested the problem-solving skills of bullfinches captured from rural and urban settings in Barbados. The bullfinches could see food inside a semi-transparent plastic box and could access the food by opening the lid or by pulling a drawer. In both cases, the urban birds solved the puzzle much faster. Their paper, published in 2015 in Behavioural Ecology, was the first to delineate clear cognitive differences between urban and rural birds.
Other types of city pressures also affect birds. Urban-dwelling peregrine falcons have become more nocturnal in their habits, using artificial lights to hunt night-flying birds and bats, while several types of songbirds change their songs to a higher pitch to ensure that their warbling is not lost in the urban din.
It is expected that the future spread of urbanization will continue to reshape and create new species in unexpected ways. For this reason, Dr. Johnson at U of T believes it is vital that we gain a better understanding of how often urbanization causes mutations and whether such mutations can affect human health. “This is the single most important question in the study of urban evolution,” he says.
Meanwhile, Dr. Otto at UBC worries about how dramatically humans are affecting the natural world. “We’re moving the tree of life in an entirely new direction and natural diversity is being lost. Evolution is one of the world’s most wondrous processes and the thought that it may now be heading sideways is terrifying to me.”
Like Dr. Johnson, Dr. Otto believes we must gain greater insight into exactly how we are altering the environment and the creatures around us. “We need to ensure that the changes we are making don’t eliminate the creatures that we love and which are beneficial to us. If we let these changes happen willy-nilly, we could end up facing some ugly surprises in the future.”