Polyethism and the importance of context in the alarm reaction of the grass-cutting ant, Atta capiguara

Abstract

Leaf-cutting ants exhibit an aggressive alarm response. Yet in most alarm reactions, not all of the ants encountering a disturbance will respond. This variability in behaviour was investigated using field colonies of Atta capiguara, a grass-cutting species. Crushed ant heads were applied near foraging trails to stimulate alarm reactions. We found that minor workers were disproportionately likely to respond. Only 34.7-2.8% of ants travelling along foraging trails were minor workers, but 82.1-6.1% of ants that responded were minors. Workers transporting grass did not respond at all. The alarm response was strongest at the position and time where minors were most abundant. Ants were more likely to respond when they were travelling along trails with low rather than high traffic. Minor workers followed a meandering route along the trail, compared with the direct route taken by foragers. We argue that an important function of minor workers on foraging trails is to patrol the trail area for threats, and that they then play the key role in the alarm reaction.