A comparison between sexes, castes, and life-histories supports the adaptive role of metal enrichment in the mandibles of Lasius ants

Ant mandibles perform different functions (e.g. feeding, hunting, and nest building), and previous studies on workers of some species revealed that they are enriched in transition metals (Zn and Mn), which improve cuticle hardness. Whether this trait is adaptive in all ant species is still unclear. One way to test for such adaptation is to compare sexes and castes within species as well as species with different life-histories. Here, we evaluated the transition metal enrichment in workers, queens, and males of 12 species of the Holarctic genus Lasius Fabricius 1804. The genus is adequate to test for the evolution of this trait because of two reasons. First, males show small, often toothless mandibles and do not contribute to nest activities, hence leading to the prediction of lower metal-enrichment compared with workers and queens. Second, this genus includes both socially parasitic (of other Lasius species) and non–parasitic species, leading to the prediction that females of the former, because they engage in fights with the host, have higher metal-enrichment compared with females of the latter. Our Scanning Electron Microscopy/Energy Dispersive X-ray analysis provided evidence for an adaptive role of Zn and Mn in Lasius mandibles. At intra-specific-level, males had lower amount of Zn and Mn in their mandibles compared with workers and queens (which did not differ). Although parasitic and non-parasitic queens had similar metal amounts, parasitic workers showed higher Mn content and marginally higher Zn content than workers of non-parasitic species. Overall, our results strongly suggest that both colony activities and parasitic life-style promoted greater metal-enrichment in Lasius females. Given the huge biological diversity of ants, large comparative studies are needed to assess the generality of our findings.