Communication is essential for any efficiently functioning community, and social ants are no different. ‘Division of reproductive labour – with a queen who specialises in reproduction and female workers who do not reproduce – is a defining characteristic of insect societies’, says Adrian Smith from the North Carolina Museum of Natural Sciences and North Carolina State University, USA. And this division of labour only works if the female workers can distinguish queens from fellow labourers. ‘This is an essential communication task that all ants have to figure out’, he says. But little is known about how most of the 14,000 ant species communicate their position in the hierarchy, although recent evidence had suggested that all social insects use the same specialised odour (pheromone) molecules. ‘The problem was, this was based on comparing single, distantly related species’, says Smith. So, he and Andrew Suarez from the University of Illinois at Urbana-Champaign, USA, decided to compare the aroma profiles of three species of closely related trap-jaw ants (Odontomachus) to find out whether all ant queens smell similar to each other.
Having dug ant colonies out of the ground and rotten logs in south Florida and Alabama, Smith had little trouble keeping the insects happy in the lab in artificial nests built from plaster-lined Petri dishes and tubes. ‘They usually hunt live insects for food, so we gave them termites to hunt and feed to their larvae’, Smith says. Then, he began painstakingly collecting samples of the wax odours coating the ants' bodies – from queens, female workers and male drones – ready for chemistry sleuth Jocelyn Millar, from the University of California Riverside, USA, to analyse by gas chromatography–mass spectrometry.
Comparing the chemical profiles of the queens' odours with those of workers of their own species, it was clear that the queens' odours differed significantly from the worker scents, although the monarchs and labourers shared some compounds. However, it was also evident that the profiles of the chemicals that contributed to the odours of the O. ruginodis, O. relcitus and O. haematodus queens were dramatically different too. Most surprisingly, O. ruginodis queens seemed to be sporting scent molecules – 2,5-dialkyltetrahydrofurans – that Smith had never seen before in a waxy ant coating; he recalls that Millar had to resort to some sophisticated chemistry to identify the molecule. However, when the duo analysed the odour profiles of the males from all three species, they found that the odours were much more similar to each other and the males all produced more pentacosadienes, pentacosenes and tricosanes to distinguish themselves from their females.
Puzzled by the intriguing composition of the O. ruginodis queen's aroma, Smith wondered whether the novel dialkyltetrahydrofurans molecules communicated a regal presence. Having first confirmed that worker ants recognised a queen ant and submissively withdrew their antennae when the queen was near, Smith bathed several queen ants in a solvent to collect their odour cocktail. Then, he separated the dialkyltetrahydrofurans (and other similar compounds) from the waxy components and checked the workers' reactions to each mixture individually. This time, neither the waxes nor the dialkyltetrahydrofurans produced a submissive response. So, the workers only recognised that the dialkyltetrahydrofurans were being produced by a queen when the molecules were in the context of the other background scents.
Having found that queens of different species do not use the same odour compounds to communicate their reproductive roles, Smith is keen to learn more about how the remaining 65 species of trap-jaw ants interpret scent communications to maintain their reproductive division of labour. ‘There is a whole lot left to discover’, he chuckles.