Ants recognize foes and not friends

Abstract

Discriminating among individuals and rejecting non-group members is essential for the evolution and stability of animal societies. Ants are good models for studying recognition mechanisms, because they are typically very efficient in discriminating 'friends' (nest-mates) from 'foes' (non-nest-mates). Recognition in ants involves multicomponent cues encoded in cuticular hydrocarbon profiles. Here, we tested whether workers of the carpenter ant Camponotus herculeanus use the presence and/or absence of cuticular hydrocarbons to discriminate between nest-mates and non-nest-mates. We supplemented the cuticular profile with synthetic hydrocarbons mixed to liquid food and then assessed behavioural responses using two different bioassays. Our results show that (i) the presence, but not the absence, of an additional hydrocarbon elicited aggression and that (ii) among the three classes of hydrocarbons tested (unbranched, mono-methylated and dimethylated alkanes; for mono-methylated alkanes, we present a new synthetic pathway), only the dimethylated alkane was effective in eliciting aggression. Our results suggest that carpenter ants use a fundamentally different mechanism for nest-mate recognition than previously thought. They do not specifically recognize nest-mates, but rather recognize and reject non-nest-mates bearing odour cues that are novel to their own colony cuticular hydrocarbon profile. This begs for a reappraisal of the mechanisms underlying recognition systems in social insects.

Figure 1

Synthesis of 11-meC₂₆ (11-methylhexacosane).

Figure 2

Aggression level of resident ants (either supplemented with a hydrocarbon or deficient of it) towards alien ants (either deficient or supplemented, respectively). White bars show the aggression level of ‘supplemented resident’ towards ‘deficient alien’ ants; black bars show the aggression level of ‘deficient resident’ towards ‘supplemented alien’ ants. (a) ‘nm (d⁺)’ are alien ants supplemented with 3,11-dimeC₂₇, ‘nm (d⁻)’ the respective deficient aliens, lacking the extra hydrocarbon. (b) ‘m’ stands for 11-meC₂₆ and (c) ‘l’ for n-C₂₀. Aggression differed significantly among all six assayed resident–alien combinations (F_5,164=7.71, p<0.0001). Bars depict mean and s.e.m.; different letters indicate significantly different aggression indices (Scheffe post hoc comparisons between all six combinations p<0.01); numbers insides bars refer to sample sizes.

Figure 3

MORs displayed by six groups of test ants (‘residents’) to five different test stimuli. (a,c,e) MOR level of resident-supplemented ants; (b,d,f) MOR level of resident-deficient ants. The hydrocarbons used for supplementation were (a,b) 3,11-dimeC₂₇ (group d), (c,d) 11-meC₂₆ (group m) and (e,f) n-C₂₀ (group l). The stimuli were solvent (sol); extracts of alien-deficient ants (nm(d⁻), nm(m⁻) and nm(l⁻)) and extract of the respective alien-supplemented ants (nm(d⁺), nm(m⁺) and nm(l⁺)); pure hydrocarbons (groups d, m and l); extracts of control non-nest-mate ants (nnm) coming from another colony than the test ants. Each of the residents (n=30 per subset) was stimulated with each of the five stimuli. Bars show mean and s.e.m.; different letters indicate significant differences (Scheffe post hoc tests; p<0.05, in all cases). White and black bars indicate those tests that correspond, in their logic, to the situation in figure 2.