Here’s how magnetic fields shape desert ants’ brains

For desert ants, Earth’s magnetic field isn’t just a compass: It may also sculpt their brains.

Stepping outside their nest for the first time, young ants need to learn how to forage. The ants train partly by walking a loop near their nests for the first three days. During this stroll, they repeatedly pause and then pirouette to gaze back at the nest entrance, learning how to find their way back home.

But when the magnetic field around the nest entrance was disturbed, ant apprentices couldn’t figure out where to look, often gazing in random directions, researchers report in the Feb. 20 Proceedings of the National Academy of Sciences. What’s more, the altered magnetic field seemed to affect connections between neurons in the learning and memory centers in the young ants’ brains.

The finding “may make it easier to better understand how magnetic fields are sensed [in animals]” as scientists now know one way that magnetic fields can influence brain development, says Robin Grob, a biologist at the Norwegian University of Science and Technology in Trondheim.

For years, scientists have known that some species of birds, fishes, turtles, moths and butterflies rely on Earth’s magnetic field to navigate (SN: 4/3/18). In 2018, Grob and other scientists added desert ants to that list. Young ants first appeared to use the magnetic field as a reference while learning how to use landmarks and the sun as guides to orient themselves in the right direction to gaze back toward the nest with its small, hard-to-see entrance.

However, knowing where in the brain magnetic cues are processed has proved challenging.

To help crack the case, Grob and colleagues turned to one species of desert ant (Cataglyphis nodus) living in a southern Greece dry pine forest. The team marked ants with a colored dot to distinguish between experienced foragers and young ones. Identifying the stage of ants was easy, says Pauline Fleischmann, a behavioral biologist who did the work while at the University of Würzburg in Germany. “The experienced foragers [would] just run out of the nest and disappear [searching for food], whereas the new ones walk in little circles.”

Just outside the nest entrance, the researchers also set up a device that messes with magnetic fields. In one experiment, as the team used the device to nullify the part of Earth’s magnetic field that runs parallel to the ground, the young ants struggled to orient themselves to face toward the nest entrance during their learning walks.

The researchers then collected the ants, extracted their brains and examined a pair of structures called the mushroom bodies that are involved in learning and orientation. Ants exposed to a disturbed magnetic field had smaller mushroom bodies with fewer number of connections between neurons compared with ants that ran about in a normal magnetic environment.

“That makes a lot of sense,” says Charalambos Kyriacou, a behavioral geneticist at the University of Leicester in England who was not involved in the study. The ants’ learning behaviors, he notes, seem to be directly associated with anatomical changes in their brain. “When the ants were learning, you see the changes also in their mushroom bodies. But when you disrupt the magnetic field, the ants were not learning, and you don’t see [those] changes.”

Yet, weirdly, when researchers completely eliminated the magnetic field around a nest entrance — not just that part parallel to the surface — the ants could orient themselves just fine and their brains did not display the same effects as seen in ants exposed to the altered magnetic field.

That might be because ants can prioritize navigational cues based on what information is available, the researchers suggest. When the magnetic field is disrupted but available, the ant may still try to make sense of that. Whereas “if there is no magnetic information at all, they might realize that they cannot use it and start using other cues as the backup,” says Fleischmann, who is now at Carl von Ossietzky University of Oldenburg in Germany.

It could be interesting to learn whether these magnetic signals have an impact only early in life or throughout the life span of the ants, says Susanne Åkesson, a sensory ornithologist at Lund University in Sweden. Such signals could be especially valuable for organisms living in more arid deserts like the Sahara, where the shape of landmarks keep changing. “It could be a sandstorm overnight and the environment is completely different when you go out from the nest in the morning.”