Parasitism and the tradeoffs of social grouping: The role of parasite transmission mode

Animals use social grouping for numerous fitness-enhancing processes, such as foraging, social learning, defense, and energy expenditure. One broadly referenced negative consequence of social grouping is the increased risk of exposure to parasites, which are defined broadly here as organisms with obligate, persistent, and harmful consumer associations with a host. However, there is growing evidence that group living can also act as a defensive mechanism against parasites. Here, we present a conceptual framework that explores host sociability in the context of parasite life history, arguing that the positive or negative impact of a social lifestyle on infection risk is strongly linked to the parasite’s transmission mode. We discuss the link between host sociability and infection risk with respect to common, non-mutually exclusive differences in transmission: direct vs. indirect, density- vs. frequency-dependent, and simple vs. complex life cycles. We then use our framework to discuss the mechanisms for active parasite avoidance, passive effects of infection-induced phenotypes, and their impacts on host social networks. Further, we highlight additional important factors that can modulate these dynamics (e.g., parasite virulence, infection intensity, co-infection by multiple parasites, and environmental factors). The goal of this broad, comparative approach is to provide researchers from multiple disciplines with a unified framework to better understand the relationship between social grouping and host-parasite interactions across diverse systems.