Earlier this week, the Brooklyn Daily Eagle ran a story with the headline “Rats the Size of Burritos Feast on Chipotle’s Trash in Brooklyn Heights.”
The story detailed how the Brooklyn Heights Association, a group of residents, was pointing fingers at Chipotle’s piles of garbage that the rats seemed to veer toward for their diets. Chipotle, in turn, blamed the rats on the construction of a nearby residential and office building. When the reporter asked a construction site worker how big the rats were, he spread his hands to the width of a house cat.
Which made us wonder: Is there a slow uprising of Chipotle-fed, cat-sized rats that are about to take over the nooks and crannies of cities?
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Not so fast, said Matthew Combs, a Ph.D. candidate at Fordham University in the Department of Biological Sciences. “I see stories like this a lot, and often the comparison is to cats,” he said.
Combs has reason to be suspicious: His research is on urban rat populations, specifically on how variations in the urban environment (different architecture, how we organize ourselves in the city, and more) influences the ecology and evolution of rats.
In November, he published a paper in the journal Molecular Ecology that showed how rats genomically varied on the island of Manhattan based on diets and the island’s unique environments. A Midtown Manhattan rat’s more fatty diet and ability to skirt crowds of tourists were different from those of a rat in more peaceful Central Park.
But back to these Chipotle-loving rats. “I think the size of rats is often overestimated and exaggerated,” Combs said.
The normal size of a rat? “Around 2 pounds, just under a kilo, with a length maxing out at 8 to 10 inches. The tail might add another 7 inches,” Combs ticked off. “That’s the biggest you will see. The largest rat we’ve ever caught was about one-and-a-half pounds, that’s big! It’s shocking, but not as big as a cat.”
Menno Schilthuizen, an evolutionary biologist and the author of the recently released Darwin Comes to Town: How the Urban Jungle Drives Evolution, agreed. “I am not convinced these rats are really any different from average city rats,” Schilthuizen told The Daily Beast via email. “Having said that, if these Brooklyn Heights rats are really a population that is genetically different, then food supply probably provides the evolutionary benefit, and some kind of barriers with other rat populations may allow them to evolve their own local physique.”
OK, so the rat isn’t gargantuan and we don’t have to fear mutant rats taking over urban centers. But the Chipotle-only diet might have some truth in it.
And it might all come down to familiarity. Turns out rats go to Chipotle for the same reason why we might turn to fast food: It’s predictable, we know what it’ll taste like, and we know what the predicted health effects are (mixed for humans, but we digress). And these rats might not necessarily favor the flavor of Chipotle’s world-renowned burritos but might simply find them accessible and encompassing their nutritional needs.
That’s not to say Chipotle burritos don’t negatively affect rats the way they affect us. Rats, after all, are meant to be mobile creatures, and Chipotle’s higher caloric content makes it more likely the rats will get bigger. “The closer a rat is to a food source, the more fat it will be, the less energy it will have to expend to get to a food source,” Combs explained. “That’s why they’re [these Chipotle rats] are so fat—they don’t exercise.”
Which can ultimately hurt these Chipotle rats. “There’s still natural selection going on,” Combs said. “Those rats that are too large aren’t going to do as well and won’t be as good at avoiding predators or jump as far.”
Ah, what afflicts the fast food-heavy diet of humans affects rodents as well.
New York’s nightly lining of the streets with black garbage bags is as familiar a sight as scurrying rats, and Schilthuizen said the garbage’s placement right out there in the open, tempting rats with leftovers, isn’t necessarily helping prevent Chipotle rats.
“Well, the fact that garbage is left out on the street [is a big reason for the development of Chipotle rats],” Schilthuizen said. “In many other big cities, there are underground garbage containers, which the rats, gulls, and other ‘vermin’ cannot get into.”
The thing about rat diets that’s especially hard to figure out is that they’re so varied. “We know that rats are flexible in their diets, we call them generalists,” Combs said. But it could be behavioral plasticity, the idea that organisms adapt to environment by adjusting their stimuli—here, their taste for the Chipotle—or it could be simply that the rats find Chipotle safe and tasty and fulfilling.
Not to completely blame Chipotle for these rats, though; turns out the construction site offers a safe location to stay, with open soil, piles of wood, and ample hiding spots. “They can burrow into the earth and make nests,” Combs said.
A Chipotle next to a construction site? Heaven on earth, if you’re a rat.
That raises larger questions about diet, though: Are rats evolving to have more humanoid diets, and do those diets vary by geography? Could a rat in a poorer section of the city, for example, have a diet that’s fundamentally different from a rat in a more well-off section? That’s what Combs is researching next, looking at how urban rat diets differ from rural rat diets. A 2017 paper by Combs’ adviser, Jason Munshi-South, found that white-footed mice had different diets based on whether they were in New York City or around it (PDF).
The same might be true for rats but would probably take generations. Schilthuizen said having Chipotle rats versus, say, pizza rats, or maybe kale rats, could theoretically happen. “It’s possible that city rats begin to specialize on different kinds of food, resulting in the evolution of multiple species of city rat, each with its own food preference,” Schilthuizen said. But “it will take hundreds of years at least, and it’s unlikely that human food fads will stay stable for that long.”
When it comes to increasing urbanization around the world—along with climate change—there’s reason to believe that the rats that have invaded cities around the world might develop unique, city-specific characteristics that might make them more likely to survive their locales. And it’s a burgeoning public health crisis that Combs said is not being given enough attention.
“Rats are a product of their environment,” Combs said. “In New York, rats are eating trash and growing as large as they can. Rats carry several diseases that can be passed to humans. If you have rats near restaurants or running by populations, it’s something the city should take into account to mitigate the problem.”
