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If you happened to walk down Broadway in New York City in late May 2013, you may have seen something extraordinary: a man crawling over the sidewalks through garbage, occasionally stopping to suck up ants through a straw-like device called a pooter. This man was biologist Dr. Clint Penick, and he kept up this behavior for a week. “Not a single person asked me what I was doing,” Penick tells mental_floss. “I guess I wasn’t the weirdest thing they’d seen that day.”

Penick and his team at NC State collected 21 species of New York City ants to measure their stable isotopes and find out what the ants were eating. The researchers learned that some urban ant species are forgoing their usual diet of dead bugs in favor of Big Macs and milkshakes.

Everything we eat leaves its mark in our bodies in the form of stable isotopes. For example, corn—even in the form of corn oil, corn syrup, and corn-fed beef and chicken—is easy to spot. Animals that have eaten a lot of corn-based foods will have a much higher ratio of C13 to C12 isotopes than those that don’t. And let’s be clear: Americans are eating a lot of corn. A 2008 study measured stable isotopes in foods and drinks from Burger King, McDonalds, and Wendy’s meals all over America. They found corn in almost everything from Burger King and McDonald’s, and in every single item from Wendy’s.

Dr. Penick saw this as an opportunity. “You can take a hair sample from a human in New York City and one from someone in London and you can tell them apart based on their carbon isotopes and the corn in their diet,” he says. His team wondered if the same thing would work for ants.

The scientists were most interested in Tetramorium sp. E, the species commonly known as the pavement or picnic ant. Little is known about pavement ants other than their incredible adaptability, which has allowed them to set up shop in cities worldwide. “They’re like pigeons or rats,” says Dr. Penick.

The results were unsurprising, but quite clear: Pavement ants and most other species had stopped scavenging dead insects and had started scavenging our fallen french fries. “The chemical makeup of their bodies changed,” Dr. Penick says. “They looked more like humans because they were eating the same foods.”

This is probably not great for the ants, but it could be great news for us. In a paper published today in Proceedings of the Royal Society B, Penick and his colleagues write, “The average person living in a city will produce nearly half a metric ton of garbage this year, and of that, 15 percent will be food waste.” By gobbling up what we leave behind, ants are doing us a service. Just how much are they actually eating?

Naturally, scientists can tell us. In 2014, another team of researchers left weighed samples of hot dogs, cookies and potato chips on NYC sidewalks. After 24 hours, they weighed what was left, and the results were staggering: “We calculate that the arthropods on medians down the Broadway/West St. Corridor alone could consume more than 2100 pounds of discarded junk food, the equivalent of 60,000 hot dogs, every year—assuming they take a break in the winter.”

On the other hand, says Penick, the volume of our delicious trash is keeping the ants and their garbage-eating colleagues around. “If we weren’t dropping this food,” he said, “how many ants would be in our cities? How many pigeons? How many rats?”

“We don’t really think too much about what happens when we drop part of our lunch on the sidewalk,” he says, “but the cumulative effects of all of these actions of thousands of people every day can have a pretty significant impact on the species that live alongside us. When we consider a city as an ecosystem, it’s important to include humans and our actions.”