Desert-dwelling ants in the featureless salt pans of Tunisia have developed a remarkable strategy to prevent getting lost: They construct taller anthills that act as navigational landmarks.
These particular ants, which inhabit the scorching and level salt pans of Tunisia, dedicate their days to foraging for food. Their successful food-seeking expeditions can lead them as far as 1.1 kilometers away from their nests. To ensure a safe return, some of these resourceful ants build towering hills above their nests, serving as prominent landmarks that help guide them back home. This fascinating behavior was documented and reported by researchers in the July 10th edition of Current Biology.
Desert ants belonging to the Cataglyphis species employ a navigation technique known as path integration, where they rely on the position of the sun and count their steps to maintain awareness of their location relative to their nest. However, as the ants venture farther away from their nest, this system becomes increasingly unreliable. To compensate, desert ants, like other ant species, also rely on their vision and sense of smell. Yet, in the vast and nearly featureless salt pans, where the surroundings appear uniform in all directions, these sensory cues alone are insufficient for effective navigation.
Observing this dilemma, Markus Knaden, a neuroethologist at the Max Planck Institute for Chemical Ecology in Jena, Germany, and his team made an intriguing discovery. They noticed that as the salt pan-dwelling ants approached their nest, they suddenly became highly accurate in locating their nest hill from several meters away. This led the researchers to speculate that these nest hills serve as prominent landmarks, essentially defining the location of the nest.
To investigate further, Knaden and his colleagues captured ants (specifically C. fortis) from nests situated in the middle of the salt pans as well as those from nests along the shorelines. They observed that only nests in the interior of the salt pans had distinct hills, some of which could reach heights of up to 40 centimeters. In contrast, the hills associated with shoreline nests were either shorter or barely noticeable.
To further investigate the significance of the nest hills as navigational aids, the research team conducted an experiment. They removed the hills from 16 nests and relocated the captured ants at a distance from their original nests. The team observed that the ants from the salt pan interiors, where the hills were prominent landmarks, faced more difficulties in finding their way back home compared to the ants from the shoreline nests. The shoreline ants, accustomed to using the shoreline itself as a reference point, were less affected by the absence of the hills.
Curious about whether the ants purposefully built taller hills in the absence of visible landmarks, the researchers conducted another experiment. They removed the hills from 16 salt pan nests and placed two 50-centimeter-tall black cylinders near eight of the nests while leaving the other eight nests without any artificial visual cues.
After three days, the researchers made an intriguing observation. Ants from seven of the nests without visual aids had reconstructed their hills. However, ants from only two of the nests with cylinders bothered to rebuild the hills.
Entomologist John Longino from the University of Utah, who was not involved in the study, expressed his fascination with the ants' behavior. He remarked on the ants' ability to employ path integration and step counting for orientation and found the concept of ants constructing their own visual landmarks incredible. He pondered whether the ants engage in a collective decision-making process to determine the need for a larger landmark or if this behavior has evolved specifically in this particular species of desert ant.
The mechanism behind the ants' decision-making process regarding the construction of nest hills remains uncertain at present. It is noteworthy that the task of nest building is typically performed by younger ants that have not yet become foragers. These younger ants have not experienced the challenges of finding a nest in the absence of a hill. Markus Knaden suggests that there is an exchange of information between the more experienced foraging ants and their inexperienced nest mates.
There are also considerations regarding the potential risks associated with building taller structures. Bronstein raises a valid point, questioning whether the conspicuousness of the nest hills serves as a clear signal to ant predators that food can be found in those locations. The fact that the ants do not build such structures where they are unnecessary suggests an implicit awareness of these risks.
M. Freire, A. Bollig, and M. Knaden. "Absence of visual cues motivates desert ants to build their own landmarks." Current Biology. Vol. 33, July 10, 2023. doi: 10.1016/j.cub.2023.05.019.