Call it the ant version of Facebook. A new study finds that, whereas most red harvester ants share information with a small number of nestmates, a few convey news to a wide network of others. The results help explain how ant colonies quickly respond to predators and natural disasters.
Red harvester ants (Pogonomyrmex barbatus) are native to the deserts in the American Southwest and live in large colonies of several thousand individuals. Most social interactions occur in the colony's entrance chamber. At first light, patroller ants emerge from the colony to ensure that the surrounding area is free from predators and natural hazards. If most of the patroller ants return, they signal forager ants that it's safe to gather seeds, their primary food.
Like all ants, red harvester ants use chemical signals to send information. The ants secrete small molecules on their exoskeletons, and their nestmates rub the exoskeletons with their antennae—the ant equivalent of "Hi, how are you?"—to read these signals. The particular combination of chemicals on an ant's exoskeleton can provide information on what task an ant performs (patroller versus forager), where it has been, and what food it has found.
Researchers at Stanford University in Palo Alto, California, measured information exchange in red harvester ants by counting the number of antennae meet and greets each ant experienced in a mock entrance chamber in the lab. The scientists videotaped the entire scene and then used a sophisticated computer program to identify each ant and count how many interactions it had during the experiment (see video). The researchers measured 4628 interactions during a trial with red harvester ants from each of two different colonies.
Most of the ants had about 40 interactions, but about 10% made more than 100 contacts with their nestmates, the team reports online today in the Journal of the Royal Society Interface. The researchers are currently investigating what sets these highly connected ants apart from other members of the colony.
This pattern of interactions matches how humans share information on social networking sites like Facebook, says the study's lead author, biologist Noa Pinter-Wollman. Most Facebook users are connected to a relatively small number of friends. A handful of users, however, have thousands of friends and act as information hubs. "As soon as they post something, a lot of people will see it."
Indeed, computer simulations of the ants' social networks showed that information flows fastest when a small number of individuals act as information hubs. Fast-flowing information allows ant colonies to respond faster to threats such as predators and weather hazards, Pinter-Wollman says. "If a lizard comes into the nest, you want to make sure everyone knows about it and goes into hiding.”
But there are also trade-offs. Previous studies in honeybees, which have a similar social structure, found that infectious diseases travel through the same networks as information. These well-connected ants might have an advantage in responding to threats, but they are also more vulnerable to infectious diseases, which can spread quickly through the colony.
Understanding these social networks is the key to understanding ant behavior, Pinter-Wollman says. Ants will decide to forage for food, tend to larvae, or hide from predators based on how frequently they encounter other ants and the chemicals they use. "These interactions are the main things that determine how a colony behaves," she says.
"These results mirror what we know about human social contact and technological networks," says Melanie Moses, a network scientist at the University of New Mexico, Albuquerque. "[These ants] are another empirical system we can look to for answers about how natural selection has been able to build these complex networks."