Trait Plasticity among Invasive Populations of the Ant Technomyrmex brunneus in Japan

Simple Summary

Invasive ants are a global economic and ecological problem. Understanding what makes them tick is thus an active area of scientific research. Many invasive ant species form large networks of inter-connected colonies (‘supercolonies’) which can span many hundreds of kilometres. Supercolonies are typically a characteristic of invasive populations, and studies have also identified shifts in other traits including diet in invasive populations compared to populations in the native range. This suggests that the ability to plastically change behaviour in this way may facilitate invasiveness. In this study, we assess trait plasticity in the invasive range of the ant Technomyrmex brunneus. This species is native to southern Asia and has invaded many islands of the southern Japanese archipelago, allowing us to compare traits among different invasive populations. We find a supercolony in one of the three invasive populations studied, though interestingly, this population did not show the expected pattern of lower genetic diversity. The supercolony population did differ in diet compared to other populations. While it is unclear if variation in these traits is linked, our findings add weight to arguments that plasticity is important in facilitating invasiveness.

Abstract

Characters in invasive populations often differ from those in the native range, and the ability to express different characters may enhance invasive potential. Ants are among the most pervasive and damaging invasive species, by virtue of their transportability and broad-ranging ecological interactions. Their success is often attributed to the ability to exhibit different characteristics in invasive populations, including the formation of large, unicolonial associations (‘supercolonies’). It remains unclear, however, if such characteristics are a product or cause of the ecological dominance of invasive ants, and the advancement of our understanding has likely been restrained by the fact that studies to date have focused on a few globally important species with well-established invasions. In this study, we take advantage of an ongoing invasion of the tramp ant Technomyrmex brunneus in Japan to assess trait plasticity in the invasive range of this species. We find evidence for plasticity in social structure among island populations, with a supercolony evident on one of the three islands studied. Interestingly, we found no evidence of lower genetic diversity in this population, though natural isotope data indicate it was operating at a lower trophic level than other populations. These findings add weight to arguments that invasive species may benefit from the capacity to adaptively mould themselves to new ecological contexts.

Comparing ant behaviour indices for fine-scale analyses

Animal behaviour often is characterised by standardised assays. In social insects such as ants, behaviour assays are for example used to characterise aggressive and peaceful behaviour. Such assays differ in the number of individuals, the duration and place of assays, and the scoring scales. Also the behaviour indices used to summarise the results differ. Here, we compared five behaviour indices (Aggression Index, Mean Maximum Aggression Index; and the newly introduced Mean Maximum Peace Index, Mean Behaviour Index aggressive, and Mean Behaviour Index peaceful) using a scoring scale that comprises peaceful and aggressive behaviour. The indices were applied on eight simulations and three observed data sets. The five indices were correlated but frequently differed in their means. Multiple indices were needed to capture the complete behaviour range. Furthermore, subtle differences in workers' behaviour, that is, differences that go beyond the presence/absence of aggression, were only identified when considering multiple indices. We infer that the indices applied are differently suited for different analyses. Fine-scale analyses of behavioural variation profit from using more than one index. The particular choice of index or indices likely influences the interpretation of behaviour and should be carefully done in the light of study species and research question.

An Alpine ant's behavioural polymorphism: monogyny with and without internest aggression in Tetramorium alpestre

Social structure influences animal societies on various levels (e.g., relatedness, behaviour). In ants, both the number of matings per queen and the number of queens per colony can vary strongly. While workers from both monogynous and polygynous colonies often fight fiercely, in supercolonies (an extreme form of polygyny comprising thousands of queens in spatially separated but interconnected nests), non-nestmates interact peacefully. Studies on social and behavioural polymorphism within ant species can help elucidate their influence on genetic diversity and behaviour and the factors triggering variation in social structure and behaviour. Here, we reveal a behavioural and social polymorphism comprising monogyny with and without internest aggression in Tetramorium alpestre sampled in Tyrol, Austria. The social polymorphism is based on genetic and behavioural evidence and contrasts with the supercolonial organisation known from another location in Austria (Carinthia), 150 km away. Microsatellite genotyping using eight polymorphic loci revealed monogyny-monandry and high intranest pairwise relatedness. Interestingly, various experimental one-on-one worker encounters revealed only occasional aggressive behaviour between monogynous colonies, and thus a behavioural polymorphism. Mantel tests revealed a significant negative correlation between spatial distance and relatedness, while worker behaviour was not correlated with relatedness or spatial distance. These results indicate that behaviour might be influenced by other factors - for example, the experience of workers, ecological, chemical, and/or genetic factors not characterised in this study. However, workers distinguished nestmates from non-nestmates also when aggression was lacking. We hypothesise an adaptive value of reduced aggression. We speculate that the non-aggressive and partly aggressive encounters observed represent different options in the social structure of T. alpestre, the non-aggressiveness possibly also promoting supercolony development. The social and behavioural polymorphisms observed offer opportunities to identify the factors triggering these changes and thus further explore the behavioural and social polymorphism of this ant species.

Polydomy: the organisation and adaptive function of complex nest systems in ants

Many ant species spread their colonies between multiple spatially separated but socially connected nests, a phenomenon known as polydomy. Polydomous species are ecologically and phylogenetically diverse, and often economically significant as invasive pests. Benefits of polydomy may include risk spreading, efficient resource exploitation and ergonomic factors. Very little is known about the costs of polydomy; facultatively polydomous species are good candidates for identifying costs. Analysing polydomous colony structure provides insights into which costs and benefits are driving the colony organisation; for example, a cross-species analysis of inter-nest trail networks shows structural features related to long-distance transport efficiency. Deeper understanding of polydomy will shed light on key issues in evolutionary and behavioural ecology, and also benefit both conservation and pest control.

Are ant supercolonies crucibles of a new major transition in evolution?

The biological hierarchy of genes, cells, organisms and societies is a fundamental reality in the living world. This hierarchy of entities did not arise ex nihilo at the origin of life, but rather has been serially generated by a succession of critical events known as 'evolutionary transitions in individuality' (ETIs). Given the sequential nature of ETIs, it is natural to look for candidates to form the next hierarchical tier. We analyse claims that these candidates are found among 'supercolonies', ant populations in which discrete nests cooperate as part of a wider collective, in ways redolent of cells in a multicellular organism. Examining earlier empirical work and new data within the recently proposed 'Darwinian space' framework, we offer a novel analysis of the evolutionary status of supercolonies and show how certain key conditions might be satisfied in any future process transforming these collaborative networks into true Darwinian individuals.