Effects of group size on disease transmission risk and immune strategies in ants

Abstract

Social insects employ collective disease defence strategies that provide social immunity to mitigate increased disease risks associated with dense populations and high genetic relatedness. However, little is known about the influence of group size on individual immunity, social organization, and active responses to pathogens. In this thesis, black garden ants, Lasius niger, were used as model system to investigate the effect of group size on (i) prophylactic organisational immunity, (ii) active social immunity responses, and (iii) investment in individual immunity. For this purpose, I used individual tracking, gene expression assays, social network analysis, as well as epidemiological simulations, and developed a pipeline for automated behaviour classification using supervised machine learning. The findings reveal increased division of labour and spatial heterogeneity in larger colonies, with stronger segregation between nurses and foragers. Baseline properties known to inhibit pathogen transmission were more pronounced in large colonies. Moreover, simulations showed that larger networks more effectively inhibited disease spread, but only when transmission originated from foragers and not from nurses. Colony size also affected behavioural responses elicited when exposed to fungal pathogens. Allogrooming of treated ants was increased more in large colonies, while still achieving their isolation by reducing contacts. Still, in small colonies, disease-carrying individuals showed a tendency to self-isolate outside of the nest. Despite more grooming, pathogen loads did not differ between colony sizes, highlighting how nurse-originating transmission can overwhelm organizational benefits of larger groups. Furthermore, the gene expression assays provide partial support for the hypothesis of lower individual immune investment with increasing group size. Overall, this research contributes to our understanding of disease defences in social insects and provides evidence for group size dependent collective disease defence behaviours and immune investment, which could have implications not only for entomology but also for broader studies in collective immunity and social organization.

Date of Award	19 Mar 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Nathalie I K A Stroeymeyt (Supervisor)