An ant is not exactly the picture you see in the dictionary next to “rule-breaker.” Colonial ants work together to collect food and generally act in the best interest of the group. Yet certain enormous ants in South America break a basic rule in biology: as you move up the food chain, you should find a smaller group of organisms at each step. These ants are top predators that take up far more than their fair share of space. To find out what their secret is, scientists staked out the forest floor.
“We’re all ant nerds,” says Chad Tillberg, a biologist at Linfield College in Oregon, of himself and his coauthors. So when they started visiting a park in northeastern Argentina and noticed what seemed like a whole lot of Dinoponera australis ants, they thought it might be an illusion created by their excitement. Plus, Tillberg points out, “Dinoponera are huge.” The seven species in this genus, which can be more than an inch long, are some of the largest ants in the world.
There are many plentiful bugs in the rainforest, of course. But an abundance of this particular ant—which locals call hormiga tigre, the “tiger ant”—demands an explanation. That’s because the species is known as a top predator of the soil. Other champion carnivores—like, say, an actual tiger—are rare, compared to the things they eat.
To see why, imagine that in a given area, you could gather every individual plant or animal of one species and pile it onto a huge scale. Usually, as you moved up a food chain, each group of living things would tip the scales less. It takes a large mass of plants to feed a moderate mass of herbivores, which can satisfy a smaller mass of carnivores. If the animals are bulky, it will take fewer of them to make up their species’ allotted weight on the scale. Dinoponera australis ants are both hefty and high up on the food chain—so something about them must be out of the ordinary.
Maybe, for a start, they’re not as abundant as they seem. To find out, “we started mapping and digging up colonies,” Tillberg says. First the researchers found ant nests by spotting ants on the park trail and following them home. (He notes that this type of research would be harder if the ants weren’t “so enormous.”) Within three plots of land, they marked the location of each nest and calculated how close the ants lived to their neighbors. They also left “pitfall” traps—like buckets for bugs to stumble into—along other trails in the area.
They found that D. australis ants aren’t equally dense everywhere in the rainforest. But within the study plots, there were lots and lots of them—about 180 underground nests per hectare (a hectare is about two and a half acres), holding almost 8,000 ants. Each ant weighs about 320 milligrams. That means the “biomass” of these animals (their total on that huge imaginary scale) is more than 2,500 grams, or 5.5 pounds, per hectare. That’s at least four times the biomass of other predatory rainforest ants.
The ants were as abundant as they’d seemed. But could they be lower on the food chain than scientists thought—not truly the tigers of the soil? To find out, the researchers stole the food from the jaws of worker ants returning to their nests. Almost all of it was the bodies of other insects they’d hunted. “They weren’t secretly collecting lots of nectar or honeydew,” Tillberg says.
Another way to find out where an animal sits on a food chain is to chemically analyze its body. Heavy nitrogen isotopes start out in plants at the bottom of the food chain, then accumulate in the bodies of animals that eat them, and build up even more in animals that eat those animals, and so on. The researchers measured nitrogen isotopes in the ants’ bodies and compared them to other insects and food items around them. This confirmed the status of la hormiga tigre: not only were these ants top predators, but they probably ate other predatory insects as well.
D. australis is just what it seems—a huge, predatory ant that roams the rainforest in huge numbers. How does it break the biomass rule? Taking one more stab at solving the mystery, Tillberg and his coauthors used paint to make distinguishing marks on the backs of ants. Then they staked out the ants’ nests. “We were marking workers and watching nest entrances for hours and hours every day,” Tillberg says. Each time an ant left the nest, the researchers recorded where it went.
They saw that most ants stuck to a single hunting route. Rather than roaming freely, each ant set out on the same path whenever it looked for food.
This behavior may make the whole ant colony more efficient. “Different individuals head in different directions from each other,” Tillberg says, “so on the whole, the entire surrounding habitat of a nest gets searched.”
There may be other factors that let D. australis ants take up so much room in the forest—a lack of competition from other predators, for example, maybe because other species can’t thrive in this disturbed habitat. But Tillberg says he thinks their hunting efficiency is “at least part of the story explaining their abundance.” It seems that if you want to take over the forest floor, it pays to be efficient as well as ruthless.
Image: by Alex Wild (via Wikimedia Commons)
Tillberg, C., Edmonds, B., Freauff, A., Hanisch, P., Paris, C., Smith, C., Tsutsui, N., Wills, B., Wittman, S., & Suarez, A. (2014). Foraging Ecology of the Tropical Giant Hunting Ant (Hymenoptera Formicidae)-Evaluating Mechanisms for High Abundance. Biotropica, 46 (2), 229-237 DOI: 10.1111/btp.12097