Desert ants cover great distances in search of food, and yet they always manage to find their way home. How do such small creatures accomplish such a difficult feat?
Imagine you are a desert ant. You get out of your nest to search for food in the white sands of Tunisia. You don't know where to find food and thus proceed randomly around the desert. Moving in a widening course away from your nest, you keep searching until you find food. Let us say that you do, eventually, find some food. Well, how will you return to the nest now?
In their observations, scientists noticed that ants do not follow the same long, meandering track they walked in search of food, but walk directly back to their nest. Given that ants do not have cell phones with navigation apps, how can an ant know the shortest way back to its nest?
It is known that red forest ants leave chemical signs which serve as landmarks for them. These ants find their way home by leaving behind smells or visible signs as reminders of where they passed. In a desert, however, this sort of marking is nearly impossible. The structure of desert sands, and other circumstances, do not allow such chemical marks to survive. If such marks were left in the desert, the situation would resemble that of Hansel and Gretel, who saw that the breadcrumbs they had left behind were nowhere to be found.
Accordingly, desert ants must be equipped with some other cognitive mechanism to help them get home. Black desert ants (Cataglyphis fortis) leave their nests and cross sands whose temperature reach 70 degrees Celsius. They can only remain on the hot sands and under the scorching sun for up to an hour. In short, they have to find their food within an hour and return to their nest – all without getting lost. If things go even a little wrong, it will cost their lives.
Scientists conducted research on a colony of ants in the hot deserts of Tunisia. The first thing the researchers did was study the nest in the early hours of the morning, before the heat strikes, and marked the area in squares. This would enable them to observe and mark the movements of the ants.
They started observing the ants. One ant came out of the nest in the morning. After some time, it found the bait placed by the researchers. But then the ant met a surprise: it was detained for a time by the researchers. The ant was eventually released in a different spot. Had it used a system of leaving clues, it would first try to find the location where it was caught. The ant, however, directly headed for the nest. The ant determined its own location with respect to the nest and headed back immediately.
The researchers repeated this experiment with many ants and observed the same result. As soon as it was left on the ground, the ant began moving toward the nest. By means of certain neurons coded in its nervous system, it immediately located the nest, even from a new position.
Moreover, only 10% of the ants missed the nest from a distance of 500 meters – and they only did so by 2 degrees. More interestingly, the ants took the possibility of error into consideration. If the exact location was not found, the ant reached its nest by moving back and forth in parallel lines.
The researchers concluded that while the human eye has one lens element, the eye of the desert ant has a thousand. Scientists discovered that in each eye of an ant, there are 80 lenses which can perceive polarized light coming from different spots in the sky. Polarized light forms when sunlight entering the atmosphere hits air molecules and other particles and is thus dispersed in every direction. This dispersion causes polarization, and light vibrating across many planes begins to vibrate on a single plane. Thus, an explicit polarization results, the strongest of which always makes a 90-degree angle to the sun. The lens system in the eye of the desert ant allowed it to map the sky by taking advantage of this polarization. When the ant stopped, it moved its head to locate the polarization. Thus, it discerned the direction for returning home. It kept repeating the movement along the way. If it failed to find the nest, it employed a series of patterned circular moves and usually got home. Each ant knew, at every step of the journey, how far it was from the nest and in which direction the nest was. (Incidentally, let us add that the ant did not know at what time or location it would be able to find food.)
In order to test this hypothesis, a number of mirrors were positioned over the ants, so that they would perceive the sun to be elsewhere in the sky. It was observed that the ants changed their direction in accordance with the new position of the sun.
This created a new problem: the sun moves in the sky throughout the day. To examine this problem, the researchers trapped the ants by placing a box over them immediately after they found the bait. The ants waited under the box a few hours, during which they could not see the sun and its movements. It was expected that the ants would have difficulty finding their way home when the box was removed. If they proceeded merely by considering the sun, they would make a systematic error and walk in the wrong direction.
Yet, in this experiment the ants also returned home successfully. They were able to find the optimum solution on the way back. In this case, researchers understood that desert ants do consider the factor of time and are aware of its passing – even if they do not see the sun.
Ants benefit from the sun’s movement in the sky to calibrate the inner clock with which they are created. But how did they know how much distance they were supposed to cover? Mathematically, even if they had the opportunity to make out the angle and direction, how were they able to calculate the distance?
Researchers developed three hypotheses. The first was the hypothesis of energy. Accordingly, ants calculated the energy they spent reaching their food and thus they knew how much energy they needed for the return; they then covered the distance that much energy allowed. Coming to the end of their energy marked the end of their journey.
In order to test this hypothesis, researchers put an extra load on the ant after it reached the food. They thought that since their body weight had increased, they would spend more energy and thus fail to reach their nest. The extra load did not impact the ants: they still took the shortest way home.
The second hypothesis was the “optical-flow” hypothesis. Researchers deemed it possible that the ants had “visual memory”; if they saw somewhere similar to their nest, they would mistake it for their home. For this experiment, the researchers blindfolded the ants that reached the food. They would not benefit from any visual memory on their way back to the nest.
Yet, the ants still reached the nest.
The researchers extended this second experiment by placing a wide TV screen in front of the ants. The motive was to generate a simulation as if the ants have crossed an immense desert. They did even change the time settings of the simulation, and yet, the ants still found their way home.
The last thing researchers tried was the step-counting hypothesis. The ant needed some measurement to know where it was going. Counting its steps could provide that measurement.
When certain ants reached their food, tiny legs made from hair were stuck to their legs, making them longer. This made the ants take longer steps. A separate group of ants had their legs shortened.
Researchers observed both categories of experimental ants to see how they returned home. Those with short legs thought they had arrived home before reaching the nest and those with long legs walked farther than the nest. So it turned out that ants counted their steps and thus knew the distance they had covered.
As for another group of ants in a control group, they were released from the nest, some with shortened and some with extended legs. All of them successfully returned to the nest with their food.
The research showed that ants are equipped with an internal system allowing them to calculate the steps they take and make the relevant adjustments for returning home. People make these calculations with tools and by knowing the rules of trigonometry. These tiny creatures, however, have been given all they need to solve a complex problem. They do not use their perception or any other method; they just remember the direction of and distance to home.
When ruminating over the fascinating qualities of such creatures, one seeks a satisfactory truth, which may still be plain and simple, but makes better sense of this splendid universe.