Acid, silk and grooming: alternative strategies in social immunity in ants?

Abstract

Parasites are an important force in evolution, driving the need for costly resistance mechanisms. The threat from disease is potentially high in group-living species such as social insects, which have accordingly evolved behavioural and chemical defences that vary between species depending on their life histories. Several ant genera have lost a key exocrine antimicrobial defence, the metapleural gland, and yet are still able to thrive in environments abundant with parasites. We investigate, in species lacking the metapleural gland, how the production of antimicrobial venom, grooming behaviours and the use of potentially antimicrobial larval silk may have evolved as alternative antiparasite defences. We focus on the Australasian weaver ant Oecophylla smaragdina and compare this to Polyrhachis weaver ants. We show that the production of venom by O. smaragdina workers is important for disease resistance but that the presence of larval silk is not, and that workers use their acidic venom to maintain nest hygiene. The grooming defences of O. smaragdina differ between castes, with minor workers allogrooming more and major workers showing greater upregulation of grooming in response to parasites. Chemical and behavioural defences differ interspecifically between O. smaragdina and Polyrhachis, with O. smaragdina appearing to rely primarily on its venom while Polyrhachis use higher rates of grooming. The results show how alternative investment strategies can evolve for disease defence, notably the highly effective application of acidic venom by O. smaragdina, and highlight the need for targeted comparative studies to understand how organisms respond to the ubiquitous threat from parasites.