ECOLOGY:

Ants are remarkable insects. Tapinoma sessile ants have adapted multiple times to both urban and natural environments, a feat that many species can't even achieve once.

Acromyrmex ants cut up leaves to grow a Leucoagaricus fungus for food, and secrete candicidin molecules to keep out unwanted fungi. Atta sexdens rubropilosa ants can even solve problems, utilizing atypical stimuli (vision) to reinitiate disrupted chemical-based foraging.

Depending on the ant species, chemical and/or vocal cues establish colony heirarchy, enable them to keep out invaders, and other functions critical to survival. Given that ant colonies are rich sources of food, this communication is commonly exploited in nature.

In the case of Myrmika schencki ants, the caterpillar parasite Maculinea rebeli utilizes chemical and vocal trickery to bring themselves into ant society and establish a high societal status. Despite a wealth of knowledge regarding ant ecology, the chemical basis underlying their interpretation of chemical cues, a complex mixture of similar chemicals on the ants' exoskeleton, remains largely unknown.

Further complicating this issue is that many chemicals on ants' exoskeletons seem to have no function in communication, e.g. serving to prevent water loss. Do ants recognize one chemical among many, or is a complex chemical mixture critical for communication purposes?

What specific features of these chemicals are important? How similar can the chemicals be before they're indistinguishable?

Ellen van Wilgenburg (University of Melbourne, Australia) and coworkers have begun to unravel the molecular-level basis of ant chemical communication. They have tested ants' recognition of similar carbon-based molecules that differ in molecular chain length and/or branching unit position.

Testing ant communication.

The scientists studied Linepithema humile ants, otherwise known as Argentine ants, a common invasive species. These are the ants reported last year to have formed a colony several thousand miles long in Mediterranean Europe.

Specifically, they studied ants from three supercolonies in California. They applied a range of synthetic chemicals to filter paper (augmented a bit with ant chemical solution), and tested ant aggression to the filter paper.

Ant aggression was defined as mandible flaring or either grabbing or biting the filter paper. Tests were for 3 minutes each.

Communication results.

The scientists found that the ants were generally less aggressive to a chemical mixture derived from nestmates than to a synthetic chemical solution comprised of one component. However, they were generally more aggressive to chemical mixtures derived from non-colony members.

These results are reasonable. Evolutionarily speaking, the ants should "know" to not attack nestmates, to attack non-nestmates, and shouldn't necessarily exhibit any strong tendencies towards unknown chemicals.

Regarding synthetic chemicals, ant aggression results depended on the architecture of the molecules in question. When the molecules were of similar molecular mass, but possessed one branching unit that differed in position along the molecule, branching unit position affected aggression, but molecular mass did not.

When a methyl unit was on carbon 15 of a 35 carbon chain, a little over 30% of the ants exhibited aggression, increasing to a little under 45% when the methyl unit was on carbon 17. When the methyl unit was on carbon 19, aggression dropped to under 20%.

In contrast, when the molecules were of similar molecular mass, but possessed multiple branching units, there was an effect of molecular mass. Here, less aggression was observed with increasing molecular mass.

Aggression was 55% when the methyl units were on carbons 5, 13, and 17 of a 33 carbon chain. Aggression decreased to roughly 40% on 35 and 37 carbon chains.

Ant synonyms.

What this shows is that ants have "chemical synonyms." In other words, some molecules induce the same response, yet others do not, depending on small differences in molecular architecture.

Chain length has little or no effect for monomethylated chemicals. Here, there is an effect of methyl unit position on aggression, but the trend is unclear.

Chain length has a clear effect for multimethylated chemicals. Here, increasing chain length appears to decrease ant aggression.

In other words, the molecules of methyl position 15 and chain lengths 35 and 37 are synonyms, within a colony and across multiple colonies. Ants "read" the same information within these chemicals.

In contrast, the molecules of multi-methyl positions 5, 13, and 17 and chain lengths 33 and 37 are not synonyms. Ants read different information within these chemicals, i.e. the chemicals influence their tendency to fight.

Implications.

These findings are of biological significance. Previous research suggests that similar molecules which differ slightly in chain length are synthesized by similar biochemical mechanisms, and elicit similar behavioral responses.

In contrast, in this research, multimethylated molecules that differ in chain length by only two carbon units elicit aggression responses that differ by roughly one-third. This possibly argues against the hypothesis that highly similar molecules are likely to be synthesized by similar biochemical mechanisms.

Ant chemical communication is clearly far more complex than is commonly realized; ants can sometimes distinguish even small differences in branching unit position or molecular chain length. Further research may be useful for confusing chemical communication, and thus for ant-specific pest control, possibly on the level of individual colonies of the same species.

for more information:
van Wilgenburg, E., Sulc, R., Shea, K. J., & Tsutsui, N. D. (2010). Deciphering the Chemical Basis of Nestmate Recognition Journal of Chemical Ecology, 36 (7), 751-758 DOI: 10.1007/s10886-010-9812-4