Here's How Ants Walk Backward to Find Home

Scientists used to believe the insects needed to see something from the front to know a familiar location. However, a research team has shown that the ants use several ways to find their path and recognize familiar scenery even when they're walking behind.

The study, according to neuroethologist Pauline Fleischmann of the Julius Maximilian University of Würzburg, is a "fantastic behavioral experiment."

"When you [observe] how the ants [travel] in their natural habitat, [you are] deeply impressed by their skills," the neuroethologist - who wasn't involved in the present work - said.

Spanish desert ants (Cataglyphis velox), when walking forward, use a technique called "path integration." These ants remember the feeling of the twists and turn they took and how many steps they're from the nest, which they use to compute the fastest route back home. 

They also believe the angle of the Sun to urge their bearings and that they shop around at the passing scenery and remember specific landmarks that will help them on their return journey.

But how they know where they're going while walking backward is a smaller amount evident. Sometimes the ants drop their food and switch around to ascertain the trail ahead-a behavior called peeking-before learning the crumb again and trudging along on their backward way. 

"We [needed] to [achieve] if they [remember] anything visually while they're walking [backward]," says Sebastian Schwarz, an ethologist at Paul Sabatier University who is one of the authors of the study.

Schwarz and colleagues selected ants that walked to a feeder from their nest in the desert so that they knew where they were. The researchers also ant-napped some insects from outside the nest who believed they were home. They deposited the ants a long way faraway from the den with an outsized crumb of ant-approved cookie. 

As the ants began to tug the cookie back to the nest, the researchers would sometimes change the scenery around them, mimicking strange mountains by adding black plastic bags and tarps alongside the trail.

The ants, when faced with such new landmarks, glimpsed after walking only 3.2 meters along the 8-meter path, while ants who familiarized the route could go nearly 6 meters without turning around. The observations reveal that the insects were taking in their surroundings as they walked backward and using them to navigate and choose when to peek.

As expected, ants that already knew where they were doing far better no matter the scenery, the team reports this month on the preprint server bioRxiv. They might walk for extended distances before they peeked behind them, and more of them made it home with their cookie. 

A couple of "clueless" ants got lost. Others surprisingly found their way back to the nest even once they hadn't previously tracked where they walked using path integration. The results suggest they need to have only been using their visual memories of their surroundings and possibly the angle of the Sun.

The ants' eyes have appropriate view angles-they have nearly 360° vision, whereas humans can only see about one-third of their surroundings without turning their heads. Schwarz says the insects are likely taking in information from beside and behind them as they walk off from the nest, then using it to guide them back as they're dragging food. 

Schwarz, after examining the behavior of the ants, prepared to make a model showing the circumstances during which the insects believe their visual surroundings versus other sources of data, just like the angle of the Sun or their internal step-counter to seek out their way home backward.

Fleischmann says the findings of the study are "exciting," although there's still more to find out about ant navigation. The ants within the survey were walking back and forth on straight paths, she notes, so Schwarz's model of how they navigate doesn't take into consideration that "under natural conditions, the ants might enter all directions and might flexibly combine different views."

Schwarz says future experiments will involve covering one among an ant's eyes with paint to ascertain how its navigational strategies change. He and collaborators also are creating a small "ant cinema," where an ant walks on a treadmill through a virtual world. Therefore the researchers can better manipulate the scenery. Within the future, the setup may provide even more insights about how ants absorb their visual surroundings-and save researchers from hours spent watching ants within the desert.