Have you ever felt too sick to go to work, but not too sick to go out later that night? Or perhaps you’ve felt fatigued all day due to a cold, but then get a second wind after you start a conversation with someone cute at the grocery store.

A new study says that there’s often a difference between feeling the effects of illness, or "sickness behaviors" — fatigue, lack of interest in daily activities, lack of appetite — and actually being sick.

Published in Proceedings of the Royal Society Biological Sciences, the study was led by Patricia C. Lopes from the University of Zurich. Lopes examined the implications behind why certain animals suffering from an actual infection exhibited fewer “sickness behaviors” in particular situations, such as when a mother was protecting her child or males were attempting to lure a mate. This suggested that the level at which animals felt sick changed and oscillated depending on context, even if they actually were sick. She hoped to answer the question, “When should animals experience symptoms of sickness and when should they suppress them?”

Lopes described sickness behavior as changes that included “reduced food and water intake, reduced activity, reduced engagement in social activities, decreased exploratory behavior, inability to experience pleasure, decreased libido and increased somnolence.” These behaviors are not often caused by the infection itself, but rather by the immune system’s response to it.  

The research team graphed a curve to depict the modulations of sickness behaviors as they were linked to survival and fecundity, or the ability to reproduce. Generally speaking, scientists believe that sickness behaviors are an immune response of sorts that actually help fight the infection by forcing animals to rest and recover. Thus, Lopes argues, sickness behaviors would impact survival positively — but only to a certain extent. Likewise, if more time and energy is invested into feeling sick, reproduction would decrease. But if an animal were in a situation with plenty of potential mates around, their sickness behaviors would hypothetically decrease — they would experience that "second wind" or rush of adrenaline and forget, at least momentarily, the physical symptoms of being sick.

It’s the social environment that plays a large role in determining the modulations, or ebb and flow, of sickness behaviors, Lopes concludes. That includes mating, parental care, as well as territorial disputes and social status.

However, even though animals are capable of suppressing sickness behaviors in dire circumstances — as humans often are when wanting to party — this might not be all too good for the body’s recovery. “The ability to modulate sickness symptoms according to the social context could prove adaptive in the sense that it might allow the animals to keep their social position in the group, preserve mating opportunities and increase the survival of offspring,” Lopes said. “But on the other hand, not giving the body the opportunity to fight the infection could have damaging effects on health.”

 

Source: Lopes P. “When is it socially acceptable to feel sick?” Proceedings of the Royal Society Biological Sciences. 2014.