In the face of dragging yourself out of bed for yet another day of expectations and responsibilities, illness on top of it all can seem at times an insurmountable obstacle. How can you be expected to work? You’re sick. It’s not fair.
But if you haven’t shaken off the cold by the time the weekend rolls around, sickness might seem more manageable in the face of a party you were really looking forward to. You suddenly feel like you can power through.
Many of the symptoms we associate with sickness—tiredness, lack of appetite, decreased ability to feel pleasure—are not caused by infection itself, but rather by the immune system’s response to it. And while doctors have traditionally viewed these “sickness behaviors” as unfortunate side effects of disease, some researchers have proposed that they are actually an adaptive response. Reducing activity gives the body more energy to fight infection, so the thinking goes, and when you eat less, that gives fewer nutrients to the bacteria for growth. A 1964 study also suggested that sickness behaviors are motivational—you’re incentivized to rest, so you can recover.
A new study published in Proceedings of the Royal Society Biological Sciences expands on this research, positing that animals are more or less motivated to engage in these sickness behaviors, depending on their social context. Author Patricia C. Lopes of the University of Zurich explains that “because social context is a main determinant of the costs and benefits associated with investment into survival versus reproduction, it should greatly affect the amount of investment in sickness behaviors.” In some contexts, it may make more sense to prioritize a group, or the species as a whole, over your individual health.
One of the obvious contexts is parenting. Studies in mice have shown that mothers show less sickness behavior in situations where they need to protect their offspring. In birds, who are apparently more egalitarian with childcare, the male will step up his parenting if the female is sick and needs to take a backseat. When it comes to reproduction, Lopes says males are more likely than females to stop acting sick in the face of a potential mate. From an evolutionary perspective, “females that decrease mating behaviour when ill are reducing the risk of spontaneous abortion of the fetus during infection, thereby minimizing fitness losses.”
Social status also seems to be a factor—male mice who were dominant in their social hierarchy, when infected, took the time to rest and exhibited more sickness behaviors, whereas more submissive male mice actually showed more “defensive and social exploratory behaviors,” in one study. If your social status is shaky, or you’re looking to climb the coolness ladder, missing a day to sickness just might not be acceptable.
Most of this evidence comes from non-human animals, so we can’t extrapolate too much, but it is interesting to consider how “acting sick” and “being sick” are two different things, and what we might prioritize over our health. Going to work or to a party sick could mean diverting resources that your body needs to heal.