In August, I joined a trio of scientists on an expedition to a recently recognized hot spot of evolution: Not a geologically young archipelago of volcanic islands like the Galapagos, nor some previously unexplored tract of rainforest, but a corner of Highbridge Park in Washington Heights. Jason Munshi-South, an evolutionary biologist who teaches at Fordham University, waved to me from our agreed meeting spot at the intersection of 167th Street and Edgecombe Avenue. Beside him were two of his research collaborators, Stephen Harris, a PhD candidate in biology at CUNY, and Erin Dimech, a master’s student in conservation biology at Columbia.
A day earlier, they had set traps baited with birdseed. Now it was time to collect their specimens.
Once inside the park, we left the paved path and carefully made our way down a steep leaf-littered hill in that step-n-slide kind of way, dodging poison ivy and using trees for support. Human detritus was everywhere: flattened water bottles, empty potato chip bags, shattered compact discs — the occasional needle. One hundred small red flags poked through the underbrush here and there. Beside each one was a rectangular aluminum box the size of a Jenga set.
With the snap of taut latex, the researchers pulled on purple gloves, split up, and started to check the containers. About 20 minutes into the search, Harris stopped in place: “I think this one has something,” he said. He tipped the box on one end and slowly opened its trap door. A little brown mouse crouched inside. It looked perfectly ordinary to the untrained eye, but for the scientists, who would later analyze the critter’s anatomy, right down to its DNA, the mouse held secrets of how our manmade urban landscapes change the very biology of the city’s wildlife.
Munshi-South never anticipated that he would study evolution in New York City’s parks. He began his career as a biologist traipsing after elephants, parrots, and snakes in the jungles of Africa and Borneo. He was particularly interested in how logging and drilling for oil forced animals to adapt to a changing landscape. In 2007, however, he accepted an attractive assistant professorship at Baruch College, which meant moving to NYC. Despite trading the rainforest for skyscrapers and rivers of asphalt, he was determined to continue studying wild animals. What began as a few exploratory trips with a lone undergraduate to capture mice in Central Park turned into several long-term research projects. It’s been a fascinating, learn-as-you-go adventure, with more drama than you might expect. One time, in a patch of forest on the Brooklyn-Queens border, the police accosted Munshi-South and his colleagues, guns drawn. A passerby had apparently mistaken some of their equipment — jugs of ethanol and a blue tarp — for the accoutrement of a mobile drug lab.
Biologists now recognize Munshi-South as a pioneer of an emerging science that sees the city itself as a major engine of evolution. “Evolution can happen much more rapidly in cities than had been appreciated,” Munshi-South says. Creatures change with their environment. A group of finches swept onto an unfamiliar island by a storm; pine trees routinely ravaged by lightning-sparked fires; freshwater fish faced with a prolonged period of drought — generation by generation, each must adapt to nature’s vicissitudes. But some of the greatest pressures to evolve exist in places where nature appears to have been all but evicted.
How New York City has changed the white-footed mouse is a perfect example. Before the 1700s, the white-footed mouse (Peromyscus leucopus) roamed the vast woodlands that once covered the five boroughs. New York City’s expansion sliced up that native habitat into scattered fragments of greenery, isolating numerous groups of mice, who fear the open roads and highways and stick to much safer areas thick with foliage. Such geographic isolation drives evolution and can even cleave a single species in two.
To find evidence of such adaptation, Harris, Munshi-South, and their colleagues have been catching mice in NYC’s remaining islands of wilderness and taking snips of their tails, from which they extract DNA. They have discovered that segregated populations of this one species have diverged onto different evolutionary paths. The mice have become so genetically distinct from one another that if you show Munshi-South a DNA sequence randomly selected from a white-footed mouse in NYC, he can tell you where it lives. At the same time, city mice as a whole also seem to be evolving new traits that mice from rural areas outside the city lack: genetic mutations that may help them neutralize toxic metals in polluted soil, for example, or speed up their sperm in response to the intense sexual competition in their overcrowded metropolitan homes.
The city has likely been a catalyst for more explicit changes in the mice as well — that’s what Munshi-South and his collaborators are searching for next. In the city’s parks, where there is still plenty of shrubbery, the mice tend to be more acrobatic, sprinting along logs and vaulting over tangled vines. In the suburbs, where deer strip away much of the vegetation, mice mostly scurry over flat ground. Munshi-South predicts that after enough generations, the length of the mice’s tailbones and limbs might have shifted to suit these different styles of locomotion, something the researchers can demonstrate by meticulously measuring enough of their skeletons. In a separate study, Dimech is investigating bacteria that live inside the rodents’ gut and are known to increase the production of T cells, an indispensable legion of immune system warriors. The demographics of gut bacteria shift depending on diet and lifestyle, so Dimech suspects that mice from urban, suburban, and rural areas will have different numbers of these immune-boosting bacteria, which will in turn alter their health.
Mice aren’t the only creature that have endured the city’s evolutionary pressures: In another series of studies, Munshi-South and an ecologist with the city’s Parks and Recreation Department, Ellen Pehek, have focused on the dusky salamander (Desmognathus fuscus), a native amphibian that hadn’t been observed in the city for decades. Their investigation started after Pehek found a curious research note from 1945 detailing the discovery of 11 dusky salamanders “under rocks lying in and adjacent to several small springs” located on “the bluffs overlooking the Harlem River, between 181st and 190th Sts.” When Pehek visited the same spot in 2005 and again in 2010, she was amazed to discover that the salamanders were still living and breeding there. Ecologists now know that these salamanders cling to a handful of places around New York City where clean water seeps from the ground. Their future is highly tenuous, however. Based on surveys and genetic analysis, Pehek and Munshi-South have learned that each of the scattered salamander populations is so small, inbred, and homogenous that it likely lacks the genetic diversity required to keep adapting. Unless the city establishes protected areas for the salamanders, boosting their numbers, they may soon go extinct.
I find Munshi-South checking traps at the bottom of the hill. A handful are flipped on their sides or tossed a short distance from their flags. “Sometimes you get just one raccoon that messes with all of them,” he says. He shows me a trap with warped metal. “They chew the corners.”
The edge of the park is bordered by a thin strip of turf grass and an adjacent sidewalk. “So this is what’s called an edge effect,” Munshi-South said. “There’s all this sunlight right at the edge, so it dries out and you get a lot of invasive plants like vines. But just before the edge, it tends to be pretty dense bush that the mice like. We have trapped here before with good success.” I looked past the sidewalk to Harlem River Drive, where cars roared past with terrifying speed.
After checking more traps, we rendezvous with Dimech and Harris halfway up the hill. The day’s search has yielded five mice in total — only half of what they wanted, but the researchers are used to disappointment and unpredictability. “The trap success is usually anywhere from 7 percent to like 50, depending on the site and time of year,” Munshi-South says.
As the weather cooled this fall, the researchers headed inside to begin analyzing their findings, a process that will extend well into the spring. This year, they hope to add rats and coyotes to their evolutionary menagerie. Munshi-South suspects that, like white-footed mice, geographically segregated populations of NYC rats have become genetically distinct from one another — possibly on a block-by-block level — and that all the efforts to poison and deter rats may also have steered their evolution, selecting for rodents that are more resilient. Other scientists are beginning to enter the field as well: “It’s really starting to catch on,” Munshi-South says. At Barnard, Rebecca Calissi is preparing to study how city life changes the brains and immune systems of pigeons. Researchers in Australia recently discovered that golden-orb-weaving spiders living in cities have evolved bigger bodies and larger ovaries for increased reproductive capacity. And biologists in the Netherlands have learned that birds in urban areas sing at a higher pitch to be heard above all the hustle and bustle.
Eventually, this new generation of urban ecologists hopes to learn something fundamental about evolution by comparing the way urbanization puts pressure on animals to adapt around the globe. “What we really want to understand,” Munshi-South says, “is whether this is happening independently in cities all across the world in idiosyncratic ways. Or are we creating urban environments that more or less drive species to evolve along roughly the same paths? In other words, is there a syndrome?”