Numerical assessment affects aggression and competitive ability: a team-fighting strategy for the ant Formica xerophila

Abstract

The relationship between numerical advantage and competitive ability is a fundamental component in contests between groups of social animals. An individual's ability to correctly assess the numerical state of its group is of vital importance. In addition to numerical dominance, the group's fighting ability also plays an important role in competitive interactions. By staging experimental fights between two Formica ant species, I show that Formica xerophila are able to assess their own group's strength prior to any competitive encounter. Ants that perceive themselves as part of a large group act more aggressively toward a competitor than ants that perceive themselves as isolated individuals. This increase in aggression improves F. xerophila's competitive ability. Furthermore, the number of individuals in a contest was found to affect competitive ability. In contests with equal number of competitors, groups of F. xerophila were more successful than individual F. xerophila. Contrary to previous predictions using Lanchester's laws of fighting, F. xerophila's ability to kill competitors increased nonlinearly with group size. This nonlinearity was due to the collective fighting strategy of an F. xerophila group isolating and engaging a single Formica integroides competitors.

Figure 1

(a) Mean proportion (±95% confidence intervals) of F. xerophila that act aggressively in contests for a resource in (i) the 1 versus 1 and (ii) 5 versus 5 contestant number trials. Sparse (filled up triangle) and dense (filled down triangle) refer to storage container treatment conditions. Aggressive F. xerophila were given a score of ‘1’ and F. xerophila that retreated were given a score ‘0’. (b) Mean proportion (±95% confidence intervals) of F. xerophila deaths in (i) the 1 versus 1 and (ii) 5 versus 5 contestant number trials. (c) Mean proportion (±95% confidence intervals) of F. integroides deaths in (i) the 1 versus 1 and (ii) 5 versus 5 contestant number trials.

Figure 2

Score of contest outcomes (measure of resource-holding potential) for F. xerophila versus F. integroides. A value of +1 signifies F. xerophila victory, and a score of −1 signifies F. integroides victory. Each bar represents a different group of 20 contests. Grey bars represent sparse storage treatment conditions for F. xerophila. Black bars represent dense F. xerophila storage treatment conditions. 1 versus 1 and 5 versus 5 refer to contestant number.