How Do Ants Get Their Magnetic Compasses?

By Ker Than

Tropical leafcutter ants don’t need to stop and ask for directions—they have internal magnetic compasses that help them navigate. 

Now, scientists have figured out just how the insects get their “sixth sense,” which is also found in an increasing number of animals, including birds, bats, and rodents. (See “5 Amazing Animal Navigators.”)

leafcutter ant picture
A leafcutter ant carrying a leaf in Costa Rica. Photograph by Paul Souders, Corbis

Leafcutter ants are famous for traveling far from their home nests in search of leaves; the ants don’t eat the leaves themselves, but rather use them as fodder for a fungus that they grow in their colonies as food.

A few years ago, biologists Robert Srygley and Andre Riveros accidentally discovered that the ants were also using an internal compass to navigate. When the researchers used a powerful magnetic pulse to disrupt the local magnetic field, foraging leafcutters lost their way. When the magnetic field was reversed, some of the ants marched off in the opposite direction—making them the only insect known to use such a built-in compass.

If that weren’t remarkable enough, the ants are also known to use landmarks and even stars to help keep track of their positions while they’re out and about. (See “Dung Beetles Navigate Via the Milky Way, First Known in Animal Kingdom.”)

Herding Ants

In the new experiments, the scientists tested the responses of two groups of leafcutters to a change in the local magnetic field.

One group consisted of wild leafcutters taken from a natural ant colony. Using flakes of barley, the scientists lured these ants to an outdoor feeder that they set up. After two days, the ants had forged a new trail to the feeder. (Also see “How Leafcutter Ants Evolved From Farmers Into Cows.”)

The other group consisted of ants reared in a lab, in plastic boxes that lacked soil, where they stayed for about a year, explained Srygley, who is with the USDA’s Agricultural Research Service.
Srygley and his colleagues at the Centro Brasileiro de Pesquisas Físicas (CBPF) and the Smithsonian Tropical Research Institute then set up an outdoor, circular arena, which they filled with sand and surrounded with a coil made of copper wire that they could use to manipulate the local magnetic field.

Next, the team took ants from both groups and plunked them down separately inside the arena. They then shifted the local magnetic field by 90 degrees in the horizontal direction and watched what happened, according to the study, published in the August issue of the journal Behavioral Ecology and Sociobiology. (Watch a video of leafcutter ants.)

The ants raised in a soil-free environment managed to find their way home, but the soil-exposed ants seemed confused. They marched in a direction that faced neither home nor the direction of the shifted magnetic field.

Magnetic Mineral

What was going on? The scientists think that the indoor ants navigated using a less precise sense, called proprioception, that allows them to keep track of how far they go and in which direction they turn.

This “proprioception compass” appears to work well for short distances, but over longer distances small errors can add up, making this compass less reliable. (Also see New Theory on How Homing Pigeons Find Home.”)

For longer distances, the scientists think leafcutters rely on their internal magnetic compasses. Other studies have linked this magnetic sense to specks of magnetite (a magnetic mineral found in soil) in their antennae. This explains why the wild ants seemed to lose their way when the magnetic field was shifted.

The indoor ants were raised in a soil-free environment, and thus lacked access to magnetite, so they didn’t have magnetic compasses that could be confused by the magnetic field change (the sand in the arena lacked magnetite).

Getting Magnetized

While the new findings strengthen the link between magnetite found in soil and the leafcutter’s magnetic compass, questions remain about how the particles are incorporated into the ants’ bodies.

Leafcutters aren’t known to ingest soil directly, and they primarily feed on the fungus they raise in their colonies.

One idea, Srygley said, is that the leafcutters are inadvertently feeding magnetite-packed soil to their fungus, in addition to leaves.

“The [magnetite] would then be fed to the larvae, and the larvae would incorporate it into their antennae when they get close to adulthood,” Srygley said.

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