When is it socially acceptable to feel sick?

Abstract

Disease is a ubiquitous and powerful evolutionary force. Hosts have evolved behavioural and physiological responses to disease that are associated with increased survival. Behavioural modifications, known as 'sickness behaviours', frequently involve symptoms such as lethargy, somnolence and anorexia. Current research has demonstrated that the social environment is a potent modulator of these behaviours: when conflicting social opportunities arise, animals can decrease or entirely forgo experiencing sickness symptoms. Here, I review how different social contexts, such as the presence of mates, caring for offspring, competing for territories or maintaining social status, affect the expression of sickness behaviours. Exploiting the circumstances that promote this behavioural plasticity will provide new insights into the evolutionary ecology of social behaviours. A deeper understanding of when and how this modulation takes place may lead to better tools to treat symptoms of infection and be relevant for the development of more efficient disease control programmes.

Keywords: disease spread; mating; parental care; social hierarchy; social modulation; territoriality.

Figure 1.

Graphical representation of how modulation of sickness behaviours can occur due to the social environment. Survival (blue curve): during disease, adopting sickness behaviours should increase survival to a certain extent. Prolonged/extreme sickness behaviours should then impact survival negatively, by increasing the likelihood of animals suffering from predation, further parasitism or, eventually, death by starvation. The shape of the survival curve will vary with how critical the expression of sickness behaviours is towards fighting the specific infection (not represented). Fecundity (orange curve): as more time and energy is being invested into sickness behaviours, the ability to reproduce should decrease. If the social circumstances were so that animals had no chance of reproducing (for example, no available mates), they should invest in the amount of sickness behaviours that maximizes survival (arrow no. 1). When social circumstances are such that investment in reproduction is possible, the point at which the survival and fecundity curves intersect should dictate the optimal amount of investment in sickness behaviours (arrow no. 2). The behavioural change from arrow 1 to arrow 2 represents the socially induced modulation of sickness behaviours.