Credit: CNRS PHOTOTHÈQUE

J. Exp. Biol. 212, 499–505 (2009)

Although leaf-cutter ants leaving their nest often encounter leaf-laden colleagues coming the other way, the ant traffic never becomes gridlocked. Using an experimental nest of Atta colombica, Audrey Dussutour of the Paul Sabatier University in Toulouse, France, and her colleagues have determined the rules of the insect road.

When the ants were forced to cross a narrow bridge, they tended to form clusters of inbound and outbound foragers. Inbound clusters were headed by leaf-carrying ants and were almost always given priority; outbound ants tended to cross when there were no inbound ants.

The few inbound ants without loads followed behind slower, load-bearing members of their cluster. Had they raced ahead, head-to-head encounters would have caused twice the delay imposed by tailgating behaviour.