Location-specific cuticular hydrocarbon signals in a social insect

Abstract

Social insects use cuticular hydrocarbons (CHCs) to convey different social signals, including colony or nest identity. Despite extensive investigations, the exact source and identity of CHCs that act as nest-specific identification signals remain largely unknown. Perhaps this is because studies that identify CHC signals typically use organic solvents to extract a single sample from the entire animal, thereby analysing a cocktail of chemicals that may serve several signal functions. We took a novel approach by first identifying CHC profiles from different body parts of ants (Iridomyrmex purpureus), then used behavioural bioassays to reveal the location of specific social signals. The CHC profiles of both workers and alates varied between different body parts, and workers paid more attention to the antennae of non-nest-mate and the legs of nest-mate workers. Workers responded less aggressively to non-nest-mate workers if the CHCs on the antennae of their opponents were removed with a solvent. These data indicate that CHCs located on the antennae reveal nest-mate identity and, remarkably, that antennae both convey and receive social signals. Our approach and findings could be valuably applied to chemical signalling in other behavioural contexts, and provide insights that were otherwise obscured by including chemicals that either have no signal function or may be used in other contexts.

Keywords: antennae; cuticular hydrocarbons; nest-mate recognition.

Figure 1.

Discriminant analysis using chemical signals. (a) Chromatogram (GC-FID) of the CHCs from different ant body parts and from the entire ant, all from the same colony. (b) Discriminant analysis plot of pooled body parts extractions (20 individuals from each of five colonies). Signals on each body part are clustered, and antennal signals are well separated from the rest of the body parts. This model correctly classifies 87.5% body parts, despite the colony difference (cross indicates group centroid). (c) Discriminant analysis plots of (i) antennae (correctly classifies 90.0% individuals), (ii) legs (correctly classifies 93.1% individuals) and (iii) abdomens (correctly classifies 88.89% individuals). Each point represents the body part profile from a single ant from five different colonies distinguished by colours. (For statistics see the electronic supplementary material. Online version in colour.)

Figure 2.

Worker antennation interest in different body parts of nest-mates and non-nest-mates. (a) Display behaviour of I. purpureus. (b) The variation in antennation frequency was explained by a significant colony identity × body part interaction (error bars indicate s.e.m.), with differences between nest-mates and non-nest-mates when the behaviour is directed to the antennae only (F_2,862 = 269.5, p < 0.001). (Online version in colour.)

Figure 3.

Workers fail to distinguish non-nest-mates after manipulation of their opponents' antennae. (a,b) The duration of display behaviour and number of antennations, when intact ants encounter nest-mates and non-nest-mates with antennae amputated. The duration of display behaviour and antennation frequency were compared by ANOVA (significant differences, *p < 0.001; error bars indicate s.e.m.). (c,d) Duration of display behaviour and antennation frequency, when intact ants encounter nest-mates and non-nest-mates with antennal signals washed (the duration of display behaviour, and antennation frequency were compared by ANOVA (significant differences, *p < 0.001; error bars indicate s.e.m.). (e) Box plot of quantity of total CHCs on intact and washed antennae (line, median; box, 25th and 75th percentiles; whiskers, data range). (Online version in colour.)