Ant workers carry over friends to help forage for food
A common species of stinging ant has a simple way to handle food items that are too large for one ant to carry alone. It brings in a fellow worker to help.
When worker Pachycondyla chinensis ants find very cumbersome snacks, they return to their nest and literally grab another ant in their jaws, carry it over and drop it next to the food.
Researchers described this novel foraging strategy in the journal Naturwissenschaften.
The ants were of particular interest to the North Carolina State University team because they are an invasive species, first introduced to North America from Asia in the 1930s.
"I was doing some observation on their foraging behaviour and how they compete with other ants, when I noticed something very unusual," recalled lead researcher Benoit Guenard.
"When a worker ant found a food source that was too large to carry on its own - like a cockroach - it would return to the nest, pick up another worker in its [jaws] and carry it to the food."
The two ants would then set about dismantling and carrying the food back to the nest.
To find out more about how the ants decided whether or not they required help, Mr Guenard placed dead cockroaches in two boxes, or "foraging cells", close to the nest. One of the cells contained lots of small cockroaches that could be carried individually and the other contained one very large cockroach.
"They quickly learned to bring another worker to the cell that contained the large cockroach," explained Mr Guenard.
- An estimated 20,000 species of ants exist in the world
- Only half this number has been documented by scientists
"But then we tricked them by switching the cells around."
Within five minutes, the ants had learned to carry another ant to the correct cell.
"So it seems that the first worker acts as a scout," the scientist explained.
"It reaches the large food item and makes the decision that it's too big to carry alone and returns to the nest to recruit help."
The ant that is carried is "totally passive" in the whole process. And the researchers do not yet understand how it orientates itself once it is dropped next to the food.
Mr Guenard concluded: "This foraging behaviour is novel in ants. No other species are known to recruit workers this way."
Brian Fisher, an ant expert from the California Academy of Sciences, said the study demonstrated that ants were "great problem solvers".
He told BBC Nature: "The authors show that tandem recruitment provides a flexible approach to carrying food that is just too heavy for one ant."