No evidence of “social immune memory” in Acorn ants (Temnothoax nylanderi)

ABSTRACT Social organisms are predicted to experience higher rates of infectious disease, as the likelihood of transmission increases with higher spatial and temporal proximity of potential hosts. In social insects, this risk is counteracted through a social immune system, where altruistic and collective anti-pathogen behaviours reduce or eliminate the impact of disease at the colony, or ‘superorganism’, level. Superorganismal immune strategies are often analogous to immune responses in metazoan organisms, suggesting that disease may have played a crucial role in shaping multiple major evolutionary transitions. In this study, we investigated whether acorn ants use prior experience with a pathogen to perform more efficient social immunity behaviours upon pathogen reencounter, a potential parallel to metazoan immune memory that we term ‘social immune memory’. Using Temnothorax nylanderi ants, we created experience with the fungal pathogen Metarhizium brunneum both individually and in groups, to investigate the effect of prior experience when presented with the same pathogen. We further examined the effect of prior pathogen experience when groups of ants differ in their pathogen-experience levels. We found no evidence for social immune memory in our experiments, suggesting positive results in previous studies may be due to sublethal infections, rather than cognitive effects. Alternatively, ants may employ different immune defence strategies depending on context or species, with potential differences in immune system flexibility across major evolutionary scales.