Ants, as eusocial insects, are especially prone to infection and disease because they live in confined colonies with high numbers of genetically-related individuals. However, ants achieve social immunity (collective defenses against disease that provide protection for the whole group, often at the cost of the individual) through a variety of strategies, such as grooming behaviors, antimicrobial glandular secretions and pathogen recognition. The extent of social immunity defenses in ant colonies has led some researchers to suggest that higher numbers of individuals in a colony leads to increased immunity, even with an inherently higher risk of disease transmission. Yet, many aspects of ant social immunity remain unresolved. Immune priming (the increase in successful defense against a pathogen upon re-exposure; similar to adaptive immunity in vertebrates) is perhaps the most disputed aspect of social immunity. There is mixed support for the importance of initial exposure to a pathogen. Likewise, how different strategies (such as immune priming and grooming) interact with colony size is understudied. Here, we investigate the effects that immune priming, colony size and grooming have on the immunity of two abundant forest ant species in eastern North America, Aphaenogaster rudis and A. picea.
Results/Conclusions
When exposed to disease agents without prior exposure, small ant colonies showed increased disease resistance, whereas large colonies demonstrated a significant reduction in worker survival. This suggests an increased risk of pathogen transmission in large colonies. We found that prior exposure to a pathogen increases worker survival when colonies are exposed to the same pathogen a second time, supporting the idea of immune priming in the genus Aphaenogaster. However, this defense is dependent on colony size; small colonies did not show evidence of immune priming, suggesting that this defense is only important as ant colonies increase in size. In some instances, allogrooming (grooming of another) behaviors increased when ants were exposed to a pathogen, but self-grooming behaviors decreased. Reduction of grooming behaviors, which varied by level of pathogen exposure, might occur because of reduced overall activity and reduced interactions with nestmates. Reduced interactions suggest that ants display avoidance behaviors when exposed to entomopathogens. We confirm that ant social immunity is complex and assert that many interacting factors affect the ability of ant colonies to defend against disease.