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Ants Build Landmarks for Navigation

Photo credit: Markus Knaden, Max Planck Institute for Chemical Ecology.

Picture being alone on a featureless planet. You’ve just left your underground station and are seeking minerals necessary to supply the base. Soon, the station entrance vanishes from sight as you continue searching. When you find the minerals, you turn around and have forgotten the way home! The horizon all around you is a monotonous waste. You wish you had left breadcrumbs like Hansel and Gretel, or better yet, had built a tall visible tower where the base is. 

This is the plight faced by Cataglyphis fortis ants that live on a salt pan in Tunisia. Tree ants and rock ants do not have this problem; they have plenty of landmarks. But the ones that live on the salt pan are surrounded by a white, flat horizon in all directions. Along the shoreline there are visual cues, but those out in the middle of the playa have few to none. The ants face death from heat exhaustion as they forage under the hot sun alone, walking fast to keep their feet from overheating. Scientists from Germany were intrigued how they can almost always navigate their way back to the nest. 

Eric Cassell discussed ant navigation in Animal Algorithms, noting that with brains only a quarter the size of a honeybee brain, ants “produce exquisitely efficient, robust navigation in complex environments” with their 250,000 neurons. As mentioned for the salt pan, simple environments without landmarks can be no less challenging. The situation calls for exceptional methods of path integration, especially for solitary foragers like C. fortis. For path integration to work, the ants need a neural compass and neural odometer and memory to store the global vector, Cassell says. The global vector can also be informed by odor trails, a polarized light compass and sun compass, and by landmarks.

Landmark Foresight

The successful strategy of C. fortis ants was revisited recently in Current Biology by Freire, Bollig, and Knaden. Their observations and experiments showed that the ants build landmarks on the journey out — but only when needed. Commenting on the research in the same issue, Cornelia Buehlmann rhapsodized about the architectural abilities of ants and many other creatures.

When we look at structures built by animals, we instantly appreciate that animals are naturally better architects than most humans. For example, beavers build fascinating constructions containing dams and dome-shaped lodges; birds construct elaborated nests; badgers form setts consisting of extensive underground networks of tunnels and chambers; termites build enormous mounds that can be a few metres in height; and ants build amazingly complex nest structures in many sizes and shapes. The key purpose of most homes is to provide a safe shelter for its inhabitants. Animals need to be safe from harsh weather conditions, hide from prey or house their offspring. Most animals have optimised the way of building their homes to get the best possible protection. Termites and ants, for example, have the ability to build nest mounds that allow perfect thermoregulation and ventilation and also protect from occasional flooding. Of course, nest structures can also have other functions: bird nests, for example, can play a role in sexual selection. A new study reported in this issue of Current Biology by Freire et al. now shows that nest hills from desert ant colonies not only provide a safe home but that they are built as visual landmarks and are crucial for successful navigation. [Emphasis added.]

The height of the nest hill is one strategy used by C. fortis ants for navigation. The entrance hill can be up to 10 cm high, visible tens of meters away. Some foraging ants, though, travel up to 1 km away, beyond line-of-sight visibility. This new measurement, longer than previously recorded for this species, shows that ants must be able to use multiple cues for path integration.

Experimental Setup

To see if the ants use their tall nest hills for navigation, the research team removed 16 nest hills at sites deep in the salt pan too far out for visual cues from the shoreline. They placed cookie crumbs at various distances to tempt foragers to venture out. Adjacent to half of the nest entrances, they placed 50-cm black cylinders as artificial landmarks, then followed individual foraging ants at a safe distance. They found that the ants were able to use the artificial landmarks to get back, but navigation was impaired without the cylinders. At those sites, they observed the ants inside the colony busily rebuilding their nest hills. This proved that the height of the nest hill is important as a visual landmark, not just for flood protection or thermoregulation. Buehlmann comments,

These results show that ants are less likely to build these hills when other landmarks are available. This is a fascinating finding that suggests that ants sensibly decide whether it is necessary to build a nest hillthat facilitates the accurate localisation of the nest entrance.

To Buehlmann, this implies that “Small-brained animals have the cognitive ability to control the colonies’ navigational success.” As a result, fewer individuals die of heat exhaustion or are killed by a predator.

Foraging in the salt pan is a race against time. About 20 percent of the foraging ants that were displaced by the researchers died in the desert sun while trying to find their way back. Other arthropods routinely die in the heat, too, providing a main source of food for the C. fortis ants. The cookie crumbs were undoubtedly a treat. One ant got as far as 2 km out but didn’t make it back. The record for distance and successful return with its cookie crumb was 1.1 km — probably near the limit of its physical ability. So far from its visual cue, it must have relied on other cues to successfully integrate its path back home. 

Investment Wisdom and Information Flow

The importance of having a tall nest was a surprise to the research team:

We were surprised that Cataglyphis ants not only build their own nest-associated landmarks but also do so readily when deprived of other visual cues necessary for navigation. A colony’s investment into building a nest hill is justified when other guiding visual cues are absent, as fast and efficient homing is evidently paramount for survival in the harsh habitat of the salt pan. However, as soon as other visual cues are present, the investment does not seem justified anymore and no nest hill is rebuilt. 

The whole colony must be involved in the investment of having a tall landmark.

Foraging is usually the last task in the life of a Cataglyphis worker, while the digging involved in building the nest hill is often performed by younger ants. This calls for some kind of information flow between the older foraging ants that face the lack of visual cues surrounding the nest and their younger nestmates responsible for building the nest-defining landmark.

How Do They Know?

If the younger ants do the work of rebuilding the nest hill, Buehlmann wonders how they know to get to work on the project.

If so, how do they know whether there is a lack of nest-defining cues around the nest? At the start of an ant’s foraging career, ants do well-choreographed learning walks around the nest. Their general purpose is the acquisition of information about the visual surrounds of the nest with relation to celestial compass cues. Future experiments will need to reveal the mechanisms of the described nest building behaviour and show who is triggering the hill re-building. It will be exciting to learn how individual foraging and navigation relates to colony-level nest building activities and if and how information is shared between nestmates about the need to build a nest hill for navigational purposes.

Beyond Machinery

In this extreme case of life-or-death navigaton, the ants show themselves remarkably well equipped. They are born into the world with sensors, algorithms, and integrated systems of moving parts supplied by nutrients and machines that enable them to walk fast under the hot sun and perform sophisticated navigation. But more is required.

Of interest in the context of the design debate, information flow is once again shown to be central to the story. Having neurons packed into a tiny brain is important, but neurons are useless without information. Information is an intangible, nonphysical, conceptual reality that makes life work. Information is the substrate on which life operates, and wisdom is the effective use of information. It can be programmed into code, but wisdom is the bequest of a beneficent and capable mind.