Sickness behavior

Sickness behavior is a coordinated set of

History

Sick animals have long been recognized by farmers as having different behavior. Initially it was thought that this was due to physical weakness that resulted from diverting energy to the body processes needed to fight infection. However, in the 1960s, it was shown that animals produced a blood-carried factor X that acted upon the brain to cause sickness behavior.

In 2015, Shakhar and Shakhar[13] suggested instead that sickness behavior developed primarily because it protected the kin of infected animals from transmissible diseases. According to this theory, termed the Eyam hypothesis, after the English Parish of Eyam, sickness behavior protects the social group of infected individuals by limiting their direct contacts, preventing them from contaminating the environment, and broadcasting their health status. Kin selection would help promote such behaviors through evolution. In a highly prosocial species like humans, however, sickness behavior may act as a signal to motivate others to help and care for the sick individual.^[14]

Advantages

General advantage

Sickness behavior in its different aspects causes an animal to limit its movement; the metabolic energy not expended in activity is diverted to the fever responses, which involves raising body temperature.^[1] This also limits an animal's exposure to predators while it is cognitively and physically impaired.^[1]

Specific advantages

The individual components of sickness behavior have specific individual advantages. Anorexia limits food ingestion and therefore reduces the availability of

According to the 'Eyam hypothesis',^[13] sickness behavior, by promoting immobility and social disinterest, limits the direct contacts of individuals with their relatives. By reducing eating and drinking, it limits diarrhea and defecation, reducing environmental contamination. By reducing self-grooming and changing stance, gait and vocalization, it also signals poor health to kin. All in all, sickness behavior reduces the rate of further infection, a trait that is likely propagated by kin selection.^{[citation needed]}

Social advantage

Humans helped each other in case of sickness or injury throughout their hunter-gatherer past and afterwards. Convincing others of being badly in need of relief, assistance, and care heightened the chance of survival of the sick individual. High direct costs, such as energy spent on fever and potential harm caused by high body temperatures, and high opportunity costs, as caused by inactivity, social disinterest, and lack of appetite, make sickness behavior a highly costly and therefore credible signal of need.^[14]

Immune control

Lipopolysaccharides trigger the

The components of sickness behavior can be learned by conditional association. For example, if a saccharin solution is given with a chemical that triggers a particular aspect of sickness behavior, on later occasions the saccharin solution will trigger it by itself.^[26][27]

Medical conditions

Depression

It has been proposed that

• Evolutionary medicine
• Proinflammatory cytokines

References