Do you have a co-worker, friend, or roommate? If you do, have you ever been in a position to “borrow” that person’s food? Did you? Ethical dilemmas aside, did you consider whether said person was sick?
Let’s say I have a co-worker named Gunter. If I take Gunter’s lunch, I will benefit from acquiring a “free lunch.” However, I might suffer costs if Gunter discovers who took his lunch. When I decide whether to purloin Gunter’s food, therefore, I am conducting a cost-benefit analysis. An additional factor I might include in this analysis could be whether Gunter is sick. If Gunter is sick, for example, I might risk acquiring his infection by eating his food. Such infection would likely more than negate the benefits I obtained from eating Gunter’s food. Gunter’s sickness would consequently dissuade me from “borrowing” his food. But, what if from past experience I know that Gunter gets incredibly angry when anyone borrows his food – except when he is sick. Naturally, he is more displeased about losing his lunch when he is sick than when he is healthy, but when he is sick he doesn’t have the energy to confront anyone about the loss. And now I have a conundrum. I am likely to suffer negative consequences from a healthy Gunter if I am discovered with his lunch. I may also acquire an infection from Gunter if I take his lunch when he is sick. But I am less likely to suffer negative consequences from Gunter at that time, even if I am discovered. So should I steal Gunter’s lunch, and when?
Unfortunately, no simple answer exists for this situation. The answer all depends on the cost:benefit ratio – the benefits I get from Gunter’s food relative to the costs I may suffer from stealing. If benefits are high enough, it would be optimal for me to steal all the time, regardless of Gunter’s state. If benefits are high, but costs due to Gunter’s behavioral response are high, it might only be optimal for me to steal when Gunter is sick.
If you think this example is a little contrived, well, you’re right. But not so much as you might think. For instance, Bouwman and Hawley (2010) discovered that male house finches preferentially fed near males who were infected with a bacterial parasite (mycoplasma), even though feeding sites enabled high transmission rates of the parasite. Bouwman and Hawley also noted that infected males were less aggressive than uninfected males. The authors therefore concluded that males who fed near an infected male benefited from encountering less aggressive males (meaning the former males were more likely to be able to feed without incident). This benefit might or might not outweigh the costs of infection for the population as a whole, but at least at the individual level, benefits appeared to override the consequences.
This house finch result has interesting implications for the evolution of host-parasite interactions. We often think of hosts evolving to maximize fitness, which means reducing parasite costs to an absolute minimum, but this does not have to come from reducing parasite infection to a minimum. If you engage in behaviors that lead to infection, but you get more benefits from those behaviors than the costs of infection, selection will favor that behavior, even though it increases infection.
Bouwman, K. M., and D. M. Hawley. 2010. Sickness behaviour acting as an evolutionary trap? Male house finches preferentially feed near diseased conspecifics. Biology Letters 6:462-465.