Ant defence system: A mechanism organizing individual responses into efficient collective behavior

A colony of social insects is an excellent model for investigating the orga nization of responses of Subunits (i.e. colony members) with limited skills into sophisticated collective behavior. The defence system of Lasius niger ant colonies is well organized in a context-dependent way. The proportion of fighting ants to fleeing ants changes gradually according to the importa nce of the area being defended, and was higher where ants tended honeydew-r ich aphids and on traits for foraging with heavy traffic, than where ants w ere walking alone or on trails with light traffic. Although there were intr insic differences in aggressiveness between individual ants, the difference s in aggressive responses between defended areas was not due to the presenc e of highly aggressive or timid individuals in each area. Instead, it was d ue to a change in aggressiveness of individuals in response to external con ditions. The cue that altered individual aggressiveness was the presence of surrounding nestmates, rather than the presence of aphids. We concluded th at the defence system of this ant species consists of three processes: (i) a recruiting system that allocates more workers to more valuable resources; (ii) individual ants following a simple decision rule to become more aggre ssive in response to increased numbers of nest-mates nearby (hence aggressi ve behavior reflecting the importance of each area to the colony); and (iii ) variability in individual responses causing a gradual change in the propo rtion of fighting ants responding to a threat.