Host colony integration: Megalomyrmex guest ant parasites maintain peace with their host using weaponry

Abstract

Social parasites exploit resources of other social species, to the detriment of their host. In order to enter and integrate in a host colony, social parasites must avoid being detected as a non-nestmate. The parasites, therefore, use one or a combination of chemical strategies: (1) producing recognition cues that match host's (mimicry), (2) acquiring recognition cues from the hosts or its nest (camouflage), (3) not producing recognition cues (insignificance) and/or (4) using substances for confusing, suppressing or appeasing the host (weaponry). In this study, we investigate the integration strategy of Megalomyrmex symmetochus ants into colonies of the fungus-growing ant Sericomyrmex amabilis. We compared the chemical odour profiles of parasitized and nonparasitized S. amabilis colonies with the profiles of the parasites. Additionally, we conducted behavioural assays, where we introduced a single ant, being either a nestmate, a conspecific non-nestmate or a parasite into an arena with five S. amabilis workers and scored the behaviour of the latter ants. The chemical analysis revealed that the social parasites have distinct odour profiles and share only one hydrocarbon with its host, have a low overall abundance of cuticular hydrocarbons and have high concentrations of venom-derived alkaloids. In behavioural experiments, we found that workers of nonparasitized colonies fight against parasite intruders, whereas workers of parasitized colonies treat introduced parasites (from their own and from another parasitized colony) similar to their conspecific nestmates. All workers (parasitized or not) show more submissive behaviour towards parasitized workers and parasites than towards nonparasitized workers. The chemical analysis of odour profiles suggests that the parasites use a chemical insignificance strategy. Furthermore, the chemical and behavioural data suggest that the parasites use weaponry to maintain an amiable association with their host ants. We discuss the biological significance of the lack of aggression in S. amabilis workers from parasitized colonies.