A Smart, Social Ant

Scientists have assumed that some ant species are smart, while others are social--never both at the same time. Now one group has turned this conventional wisdom on its head: They describe in the current issue of the journal Animal Behavior an ant species that has the brains to navigate by the sun and an advanced social structure to boot.

When most ants forage for food, they scurry about in a seemingly random path until they find a cache, then plot a direct course back to the nest to deliver the goods. Most researchers assumed that ants with complex social structures--like cogs in a big machine--lacked the neural faculties to do any fancy navigation, and so found their way by following the distant scent of chemical pheromones back home. One less-social species however, uses the sun to find direction: the Cataglyphus somehow monitors the angle between sunlight and the nest, using it to set their bearings.

To see if other species might also use the sun, Jun-Xian Shen and Zhi-Min Xu of the Chinese Academy of Sciences in Beijing tested the navigation abilities of the common ant Tetramorium caespitum, known for its complex social interactions, such as intercolony raids to recruit workers. The researchers compared the speed and aim of the return trips of ants from a food stash to the nest under direct sunlight, indirect daylight, artificial light, and darkness. The ants couldn't find their way home in the dark, suggesting that light was crucial. Furthermore, when the researchers moved an ant to one side--say a meter to the left--it returned to a spot that was a meter away from its home.

"To find this navigation system is surprising," says James Traniello, an ant researcher at Boston University. "It was largely thought to be an inverse relationship between the degree of social complexity and individual learning ability," he says. Ants in complex societies would only be performing simple tasks, and so wouldn't need to evolve much brain power. Yet T. caespitum's ability to decipher its direction would require more sophisticated neural mechanisms, he says. The behaviors of more of the world's ant species need to be studied to sort things out, he says.