Fight tactics in wood ants: individuals in smaller groups fight harder but die faster

Abstract

When social animals engage in inter-group contests, the outcome is determined by group sizes and individual masses, which together determine group resource-holding potential ('group RHP'). Individuals that perceive themselves as being in a group with high RHP may receive a motivational increase and increase their aggression levels. Alternatively, individuals in lower RHP groups may increase their aggression levels in an attempt to overcome the RHP deficit. We investigate how 'group RHP' influences agonistic tactics in red wood ants Formica rufa. Larger groups had higher total agonistic indices, but per capita agonistic indices were highest in the smallest groups, indicating that individuals in smaller groups fought harder. Agonistic indices were influenced by relative mean mass, focal group size, opponent group size and opponent group agonistic index. Focal group attrition rates decreased as focal group relative agonistic indices increased and there was a strong negative influence of relative mean mass. The highest focal attrition rates were received when opponent groups were numerically large and composed of large individuals. Thus, fight tactics in F. rufa seem to vary with both aspects of group RHP, group size and the individual attributes of group members, indicating that information on these are available to fighting ants.

Figure 1.

An interaction between focal group size and opponent group size influenced focal group TAI. The estimates (+s.e.) were obtained from separate analysis, having established that this interaction was significant in the minimum adequate model. Opponent group size, open bars, 5; grey bars, 10; black bars, 20.

Figure 2.

Focal group PCAI as focal group size increased. The estimates (+s.e.) for each factor were obtained from a separate generalized linear mixed model analysis, having established that this term was significant in the minimum adequate model.

Figure 3.

The influence of the interaction between body mass difference (mg; focal-opponent group) and opponent group size on focal group attrition rate. The fitted lines show the predicted focal group attrition rate and markers display the observed attrition rates as body mass difference varied when opponent groups contained 5 (dashed lines and open squares), 10 (grey line and grey squares) or 20 (black line and black squares) individuals. Lines were estimated from generalized linear mixed modelling.