Able navigators: How desert ants know which way to go when walking backward

The ants go marching … backward.

Desert ants forage alone, each carrying the snacks they find back to their nest. But sometimes the little insects find meals too massive to lift up in their jaws, and they have to drag their prize home, backward. 

Now researchers have an idea how the ants figure out where to go without looking where they're going. The little insects might combine visual memories with cues from the sky to follow the right path, even if they can't see it every step of the way. And these findings, published Thursday in the journal Current Biology, suggest that individual ants are much more sophisticated than often assumed.

"I think we've underestimated the individual ant in terms of what it can do," Brian Fisher, an entomologist and curator at the California Academy of Sciences who was not involved in this research, tells The Christian Science Monitor.

Researchers have long studied the forward navigational abilities of desert ants. "We know that these ants rely on vision," says study co-author Antoine Wystrach of the French National Center for Scientific Research, Toulouse III University, and the University of Edinburgh. "Each individual has its own visual memory that can be used to navigate back and forth between the food and the nest."

But research has suggested that this visual memory is egocentric, meaning that the ants see snapshots of their environment just from their perspective, Dr. Wystrach explains in an interview with the Monitor. "They learn by basically seeing the visual scene as projected on their retina, which means that their body orientation is very important for navigating."

In other words, the ants learn the route home based on how it looks when they're walking forward. But if they're facing another direction, such as backward, while walking, the ants get an entirely different view. 

Wystrach and his colleagues, and others, had previously observed desert ants walking backward and successfully navigating back to their nests, so they wondered how they could do it without their view of the environment matching their visual memories of the route. 

To find out, the researchers built a walled path for the ants to navigate in their natural habitat. They first trained individuals of the desert ant species Cataglyphis velox to follow that one path, and then the ants were presented with either a small morsel of food that they could carry normally or a larger one that had to be dragged backward.

While the ants with the smaller cookies took off down in the correct direction right away, the backward-walking ants had a little more trouble. The researchers saw that some of these ants missed a sharp right turn built into the route. But others periodically stopped, put down their cookie, turned around to face forward – presumably to reorient themselves – and then turned back around to resume dragging their cookie, continuing along the right route.

Because of the way the researchers think the ants' visual memories work, such a simple peek forward wouldn't have been enough for the backward-traveling ants to find the accurate path again. Instead, the scientists suspect that the ants were combining multiple cues to keep themselves on track. 

When they peeked forward, Wystrach explains, they needed to memorize the vision from the forward-facing body orientation but then apply it to the backward-facing body orientation. So what, then, were the ants using as a frame of reference?

"We suspected they use an external directional cue," he says. "We suspected celestial cues because ants, and insects in general, are famous for using celestial cues."

The idea is that the ants would note the location of the sun in the sky, for example, and use that to reframe their visual memory of the route so they know which way to walk.

"To me it shows that the ants use different navigational strategies, and they can integrate those strategies" as needed, Adrian Smith a myrmecologist at the North Carolina Museum of Natural Sciences and North Carolina State University who was not involved in the study, tells the Monitor.

These ants "have a representation of direction and of space that is much more sophisticated than we thought," Wystrach says. "It's not just, 'I see a stimulus and I have a typical response,' " he says speaking from an ant's perspective. " 'No, I can store a lot of different information, some long-term for my life, like the view of the scenery, some short-term, like I just dropped my cookie behind but I keep that in mind.' And all that can interact to make a very flexible behavior … rather than just a little automaton, a little machine."

And this could help inform engineers actually building little intelligent machines, Wystrach and Dr. Fisher both suggest. Studying these ants could give those engineers ideas as to how to build neural networks for small computers that allow robots to navigate environments like the ants'. Perhaps this could yield better search-and-rescue robots, Dr. Smith suggests.

Not sniffing their way around in groups?

The classic image of ants is of them marching, two-by-two or otherwise, along pheromone trails as they go back and forth from a food source to their nest. But, for these desert ants, pheromones are not a factor.

That's because of the insects' foraging strategy, Wystrach explains. In the hot, dry desert, ants forage for insects that have roasted in the sun, he says. As such, the food isn't all in a pile for a bunch of worker ants to retrieve. Because the food is scattered across the sand and an individual can typically carry or drag it back in one trip, they don't need pheromone trails to signal to the other workers where to go.

And, Wystrach adds, even the ants that use pheromone trails have also been found to rely on visual memories when they lose the pheromone trail or if it gets disrupted.

Ants have been known to accomplish incredible feats of engineering and agriculture, but that's with an entire colony acting as one superorganism. This research shows individual worker ants in a different light, all three scientists say.

"It's true that if you look at the collective behavior, ... what can emerge at the colony level is more than the sum of what the individual ants can do," Wystrach says. "But that doesn't mean the individual ants are stupid."

"We have our assumptions about how these animals work, how intelligent they might be, how sophisticated they might be," Smith says. "But when we do these experiments, it makes us reconsider just how sophisticated the tiniest parts of life all around us are."