Venom alkaloids in Monomorium "rothsteini" Forel repel other ants: is this the secret to success by Monomorium in Australian ant communities?

Species of the cosmopolitan ant genus Monomorium are small, slow-moving and non-aggressive ants that are extremely successful members of diverse Australian ant communities. Unlike other abundant taxa, they have no obvious specializations enabling them to co-exist with the highly aggressive species of Iridomyrmex that dominate these communities. Here we report results which suggest that Monomorium species owe their success to the topical application of venom alkaloids which repel other ants. The venom alkaloids (trans-2-ethyl-5-undecylpyrrolidine and trans-2-ethyl-5-tridecylpyrrolidine) of Monomorium "rothsteini" were identified and synthesized, and the repellency to other ants of the synthetic alkaloids were tested using bioassays involving the attraction to honey baits of three native species of Iridomyrmex, three tropical "tramp" species, and M. "rothsteini" itself. Repellency to all other ant species was total or nearly so, but only partial to M. "rothsteini". Defensive alkaloids produced from a variety of glands are found in other ant genera, and may be a potent yet poorly appreciated force in interference competition between ant species and thereby the structure of ant communities.