Animals have evolved behavioral and physiological responses, known as 'sickness behaviors,' that increase survival from infection. These behaviors, which include lethargy, somnolence and anorexia, are not caused by the infectious agent itself but are induced by the central and peripheral release of cytokines in response to a disease agent. Such behaviors are exhibited throughout the animal kingdom and have been observed in invertebrates as well as vertebrates including amphibians, reptiles, birds and mammals.

Sickness behaviors are believed to be adaptive. When sick, animals reorganize their priorities and adjust their behaviors in order to improve their chances of recovering from infection: an overall reduction in activity preserves metabolic resources that can be reallocated to fighting the infection, whereas anorexia reduces the availability of nutrients essential for bacterial growth. But sick animals must also weigh the benefits of such behavioral adjustments with the potential costs to their reproductive fitness.

In a recent issue of Proceedings of the Royal Society B, Patricia Lopes (University of Zurich, Switzerland) reviews studies of rats, mice, guinea pigs, zebra finches, sparrows, domestic pigs and rhesus monkeys that show how a diversity of social contexts can act as modulators of the sickness behaviors exhibited by these animals (281, 20140218; 2014). Across these species, animals modulate the expression of these behaviors when in the presence of mates, offspring, mothers or intruders, as well as in group-housing contexts, in order to minimize the costs to their reproductive success.

Lopes suggests that the ability to modulate symptoms of illness according to the social context appears to be adaptive. Appearing less sick might allow the animals to keep their social position in the group, preserve mating opportunities and increase the survival of the offspring that depend on them. On the other hand, not engaging in behaviors that allow the body to more easily fight infection could have damaging effects on health.

Lopes also notes that although engaging in appropriate social interactions while sick can increase individual fitness, it also creates increased opportunities for disease transmission. Furthermore, because the expression of sickness behavior can be modified in certain social circumstances, it may not always be possible for veterinarians to identify sick animals in captive animal populations. She suggests that veterinarians and animal facility managers could benefit from improved understanding of the modulation of sickness behaviors in order to improve the health and management of animals in captivity and control the spread of disease in captive populations.