Is the supercolony fragmenting? Morphological and behavioural differences between insular and continental populations of the Argentine ant (Linepithema humile)

Abstract

The introduction of the Argentine ant (Linepithema humile) has created an unparalleled ecological and evolutionary scenario that has allowed this species to colonize five continents in just over a hundred years. It has been assumed that the invasive success of this species lies in the absence of intraspecific competition resulting from the founder effect and bottleneck that has allowed its expansion as a single social identity. However, as more studies have been conducted, it has been observed that the interaction between introduced colonies is not as homogeneous as assumed, with different degrees of competition being discovered among them. To determine whether this competition increases in geographically isolated environments, we analysed the distribution and variability of island and mainland colonies. We detected the presence of Argentine ants on three of the 11 islands visited. To analyse the variation in phenotypic traits related to invasive potential and adaptive processes, we compared the morphology and behaviour (locomotion, thigmotaxis) of island worker ants (N=3 colonies) with those of nearby (N=3) and distant continental colonies and with those of another genetic group (south, N=2). As an index of intraspecific competition, the mortality rate in confrontations between workers from the sampled colonies was compared. The island and mainland colonies showed differences in distance between eyes and behaviour, but the differences did not follow a general pattern. Low mortality was detected in confrontations between workers from colonies on different islands or between island ants and those belonging to distant continental colonies, but mortality of up to 64% was recorded when island workers came into contact with those from the nearest continental colonies. Mortality among these ants exceeded that found in the other combinations except in the confrontation between island ants and those from the southern genetic group. We explain the observed results as the combined product of divergent evolution among populations of the northern genetic group after colonization of the islands and adaptation to local conditions, and the effect of nasty neighbours.