If you were in New York City recently and you saw a man sucking stuff off of the sidewalk with a weird contraption that resembled a water bong, you may have inadvertently witnessed serious biological fieldwork in action.
To study the diet of urban ants, Clint Penick, a postdoctoral researcher at North Carolina State University, went to Broadway, aspirator in hand, to collect specimens.
"Nobody ever talked to me," Penick said. "I basically was walking around and then crouching down on the ground and huffing something off the sidewalk through this ridiculous-looking device, and not a single New Yorker was like, 'Hey, what are you doing?' I guess I was not the weirdest person on the street." [Image Gallery: Ants of the World]
What Penick was doing was trying to find out if city ants, like their human counterparts, are eating a junk-food-heavy diet. It turns out that yes, some of them are, according to his results, published today (March 31) in the journal Proceedings of the Royal Society B.
That conclusion might seem obvious to anyone who's ever watched an army of ants attack a spilled can of soda or an ill-fated ice cream cone on the sidewalk. But like New Yorkers, ants can have quite nuanced eating habits, the study found.
For humans and ants alike, a diet heavy in grasses such as corn and sugarcane will leave a chemical signature in the body in the form of carbon-13, a carbon isotope. So Penick and his colleagues looked at the levels of carbon-13 present in 21 ant species collected from dozens of sites on New York City's sidewalks, traffic islands and parks.
They found that the species that lived in areas with a greater human presence, like the medians of the city's broad avenues, generally had higher levels of carbon-13 than the species that stuck to parks.
"Medians actually make a pretty good place for ants to nest because there's a little bit of dirt there, there's some trees and some other plants, and there's a trash can at either end — so there's some food sources there that humans are providing," Penick said.
The pavement ant, or Tetramorium Sp. E. — the most common on sidewalks and medians — had the highest levels of carbon-13 of any species, the study determined.
"In the case of pavement ants, the degree to which they're shifting their diets to human foods is actually changing the chemical makeup of their bodies to look more and more like humans," Penick said. It's still not clear what this means for pavement ants. They probably won't become overweight or get diabetes like humans, since they seem to do well on a sugary diet, Penick said, but it's unclear if they're missing out on some nutrients that they would get from eating insects and their sweet honeydew secretions.
Not all ant species living close to people were so keen on eating garbage. One species, Lasius cf. emarginatus, which has only been found in New York in the past five years, was abundant in traffic medians but didn't seem to have a taste for human foods, the researchers found.
"Anecdotally, they're also one of the only ants you find on the sidewalk, but whenever we found them on the sidewalk, they were always going up the trees and always foraging up there, and not on the ground," Penick said.
Terry McGlynn, an associate professor of biology at California State University, Dominguez Hills who wasn't involved in the study, said the research is "useful in constructing a world view if we're thinking more broadly about energy flow, urban planning and urban ecosystems."
McGlynn said he thinks it's significant that Tetramorium workers were not only found on sidewalks and traffic medians, but also in parks, and that those populations had relatively low levels of carbon-13, meaning they ate more natural foods.
"It is possible that they are unique in their ability to both rely heavily on human-based food resources as well as naturally occurring resources, which allows this overall ecological dominance," McGlynn said.
"We still don't know why Tetramorium is such an urban-adapted species compared to all other species," he added. "This study helps get us there."