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In battles between rival groups, aggression levels, and ultimately the outcome, may be influenced by the number of individuals and the resource-holding potential (RHP) of the individuals in each group. Since fighting in social animals can involve intragroup cooperation, the distribution of individual RHPs within the group could influence overall group ability. Thus, the total group fighting prowess might not simply equate to the sum of individual RHPs. We investigated the effects of within-group variance in individual RHP during battles in wood ants, Formica rufa. Rival groups were matched in number of individuals and mean body size of individuals but differed in the variance among body sizes. Larger individuals were most aggressive and small individuals were least aggressive. Overall, small individuals had the highest attrition rate and large individuals the lowest, with medium-sized individuals experiencing moderate attrition rates. Consequently, aggression and attrition rates in low-variance groups did not differ significantly from rates in high-variance groups, indicating that when group size and mean body mass are equal, the distribution of individual RHPs does not influence the overall fighting ability of the group. Rather, it appears that large individuals are able to compensate for poor performance by weaker group members.

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.

Animal interactions often involve chemical exchange but simultaneous evaluation of chemistry and behaviour has been problematical. Here we report findings from a novel method, atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) coupled with manipulation of molecular-mass achieved by rearing organisms on deuterium-enhanced nutrients. This allows real-time monitoring of the occurrence and quantity of volatile chemicals released by each of two interacting individuals, in tandem with behavioural observations. We apply these methods to female–female contests in the parasitoid waspGoniozus legneri. We show that this species emits the spiroacetal 2-methyl-1,7-dioxaspiro[5.5]undecane. Chemical release is most common in more behaviourally aggressive contests, which occur when prior resource owners successfully resist take-over by similar-sized intruder females. Volatiles released during contests are always emitted by the loser. Aggression in contests is reduced after spiroacetal release. We suggest that the spiroacetal functions as a weapon of rearguard action. We anticipate that APCI-MS, which is rapid, non-intrusive and relatively inexpensive to operate, will be widely applied in studies linking chemistry and behaviour.