Animal compass: Magnetic fields shape ant brains

Cataglyphis ants memorize the location of their nest entrance using their sense of the Earth’s magnetic field. © Robin Grob

On the trail of the mysterious magnetic sense: desert ants calibrate their navigation system using an internal compass, a study illustrates by looking into the insects' brains. The associated learning process leaves behind neuronal traces that change noticeably when the ants are confused with artificial magnetic fields. Thanks to their relatively small nervous system, the insects offer particular potential for gaining further insights into the basis of the mysterious ability in animals, say the scientists.

Migratory birds are the best-known example, but the “sixth sense” has also been demonstrated in other animal groups: Some mammals, reptiles and also some insect species have an ability to orientate themselves that is at least partially based on the perception of the earth's magnetic field. However, the mechanisms of these animal compass systems still pose a number of mysteries to scientists. In addition to the sensory organ, the neural processing systems involved in the perception and memorization of geomagnetic information are also unclear.

A research team from the Julius Maximilian University of Würzburg has now gained initial insights into these processes using Cataglyphis ants. These are also called desert ants - but some species are also found in southern Europe. Compared to other ants, they are characterized by a particularly complex ability to orientate themselves: while searching for food, they sometimes move several hundred meters away from their nest on winding paths, but can then walk back to the entrance in a direct line. This ability has been the focus of research for some time. It was shown that the Cataglyphis ants can orientate themselves based on the Earth's magnetic field, along with other clues.

On the trail of neural signatures

The Würzburg researchers were able to show that the ability is linked to characteristic behavior: “Before an ant leaves its underground nest for the first time and goes looking for food, it has to calibrate its navigation system,” says senior author Pauline Fleischmann, explaining the background to the work. During so-called learning runs, the insects explore the surrounding area around the nest entrance and rotate repeatedly around their own body axis. They take breaks during which they always align themselves exactly with the position of the nest entrance. Apparently they calibrate their navigation system using their magnetic sense, according to preliminary investigations.

The researchers have now investigated the question of whether neuronal traces of this system can be found in the ants' brains. They carried out studies on Cataglyphis ants in southern Greece. The scientists analyzed the brain structures of young workers before and after they took their first learning runs.

Using a mobile Helmholtz coil, the research team manipulated the magnetic field at the entrances to ant nests. © Robin Grob

In some cases, however, they manipulated the natural magnetic field around the nest entrance using a Helmholtz coil, which provided chaotic directional information. This fundamentally confirmed once again: Due to the confusing information, the ants were unable to correctly align themselves with the nest entrance during the learning runs.

Significant changes in the nervous system

By examining the nerve structures in the brains of the experimental animals, the researchers were able to show that in the ants that made their first excursions under natural magnetic field conditions, there was a significant increase in synaptic connections in brain areas whose importance for orientation skills is known.

A confocal micrograph shows the nerve structures in a thin section of an ant brain. © Wolfgang Rössler

This reflects how the insects memorize a combination of information about the magnetic field, the path of the sun and the visual environment during the learning process, the researchers explain. However, a different picture emerged in the ants that were exposed to the confusing magnetic fields: the researchers found a smaller volume and fewer synaptic complexes in the relevant brain area.

As the team explains, these findings indicate that magnetic information is crucial for the formation of spatial memory in Cataglyphis ants: the results demonstrate that they need a functional magnetic compass during their learning runs, to calibrate their navigation system and in connection with further information to be stored in long-term memory. According to the researchers, what is remarkable is that the insect brain, which has a comparatively few neurons, can achieve these complex tasks.

This is exactly where there is potential for further research: compared to the complicated structures in other animals with a magnetic sense, the basics of the ability in insects can be easily researched. That's why the scientists now want to stay on the ball: They hope to be able to track down the magnetic sensory organ and its connections in the Cataglyphis ants.

Source: Julius Maximilians University of Würzburg, specialist article: PNAS, doi: 10.1073/pnas.2320764121

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