Ants typically forage between their nests and surroundings using two-way trails. Navigation is usually via the same route by which others are coming and going, and is guided by chemical or visual cues.
If typical navigational cues are unavailable, and the ants are unable to return home by the same route from which they came, can they still find their way home, without disrupting the success of foraging? Pedro Ribeiro (University of São Paulo, Brazil) and coworkers have shown that they can, demonstrating basic problem-solving behavior.
The ants.
The scientists chose to study Atta sexdens rubropilosa ants, a leaf-cutting, fungus-growing species. There are two advantages of studying these ants.
One is that the ants' foraging can be easily tracked by the leaf cuttings taken from their surroundings back to their nests. The other is that the success of foraging can be evaluated based on the health of their fungus farms.
Preventing two-way foraging.
The scientists wanted to disrupt two-way foraging. Their goal was to determine if the ants could learn to find an alternate route home, while still maintaining successful foraging.
They allowed ants to develop a nest, physically connected to a foraging area. After the nest had established itself, a 4.5 centimeter gap was placed between the connector and the foraging area.
A second connector was placed between the nest and the foraging area. Here, a 4.5 cm gap was placed between the connector and the nest.
Thus, the ants could not directly return to their nests via the first connector, nor could they directly enter the foraging area via the second connector. In order to return to their nests, they had to learn to use the second connector.
Demonstrating problem-solving.
The scientists found that the ants can solve the problem of returning to their nests via an alternate route. Additionally, they maintained successful foraging.
Twenty-two of the twenty-five nests studied were thriving within 20 to 90 days. Three nests were unable to maintain a steady supply of food, and their experiments were ended after six days.
These ants were not observed to jump, or pile on top of one another to cross obstacles. They crossed over the gap between the nest and the foraging area by falling over each other at the ant pileup at the gap.
The return trip was not random. After at least 20 days of using the unidirectional route home, 80% of the ants walked towards the viable second connector immediately after cutting a leaf fragment, instead of walking towards the unviable first connector.
Speed of adaptation.
These results bring up the question of how quickly the ants learned to forage unidirectionally. Adaptation was not immediate.
On day one, there was little leaf cutting and leaf transport back to the nest. On the second day, the situation improved; there was more leaf cutting, but still only a little more than 20% of the leaf cuttings were returned to the nest.
On day three, more than 80% of the leaf cuttings were returned to the nest; on the fourth day, approximately 95%. This is almost as successful as the 100% success rate of control nests (in which bidirectional foraging was maintained).
Mechanism of adaptation.
How do the ants learn to adapt? These particular ants have tiny eyes, and forage at night.
The scientists subjected the experimental ants, who had learned to forage unidirectionally, to darkness. They displayed little leaf cutting, and little success on returning to their nests with leaf cuttings.
When the light was turned back on, they adapted again. These results contrast with control ants.
Control ants, who were allowed to forage bidirectionally, were perfectly able to forage as usual in darkness. This suggests that this ant species typically uses chemical cues for normal foraging behavior, but requires the use of their eyes when faced with adapting to an abnormal foraging routine.
Problem-solving ants.
These experiments are strongly suggestive that these ants possess problem-solving abilities. Clearly, in the field, the ants would face additional challenges in overcoming obstacles to normal foraging behavior, such as topographical and weather challenges.
However, in the semi-ideal environment here, ants can learn to use atypical stimuli to re-establish successful foraging. It will be interesting to see if ants can overcome more challenging obstacles, and to establish their problem-solving abilities in different contexts.
for more information:
Ribeiro, P. L.; Helene, A. F.; Xavier, G.; Navas, C.; Ribeiro, F. L.
Ants can learn to forage on one-way trails.
PLoS ONE 2009, 4, e5024.