Sickness behaviour within cleaning interactions

The benefits of sociality bring a hidden burden as constant social interactions increase the probability of pathogen transmission. When facing diseases, simple strategies such as social distancing have proven effective in preventing disease spread. Cleaning interactions are textbook examples of multispecies sociality, where cleaner organisms remove ectoparasites, diseased and injured tissue from a heterospecific client. These interactions can be potential vectors of infectious diseases in reef fishes, being characterised as potential disease hotspots. Here, we aimed to determine if, in the context of infections, cleaner wrasses ( Labroides dimidiatus ) employ social distancing and other behavioural tactics to prevent disease spread within the context of cleaning interactions. To this end, we used bacterial lipopolysaccharides (LPS) immunostimulation to induce “sickness” behaviour in cleaner wrasses without harming the fish. After LPS immunostimulation, we performed four behavioural trials: the social preference test (from cleaner and client perspectives), the social interaction test, and the bystander cooperativeness test. In the social preference test, LPS-injected cleaners showed lower activity rates but no change in their social preference. However, two tested client species preferred “sick” cleaners. Contrarily, during the social interaction test, cleaners reduced the number of interactions, despite clients’ willingness to interact. For the bystander test, LPS-injected cleaners were unable to adjust their behaviour to be more cooperative. Reduced activity levels revealed lethargy, which may lead to passive self-isolation, allowing for some social distancing of healthy individuals. The findings of these experiments suggest that while feeling sick, immune-stimulated cleaners undergo changes in their behaviour which can be viewed as a passive social distancing-like barrier preventing disease spread. This is especially relevant as some clients prefer and continue to request interactions from “sick” cleaners.