In the classic cartoon posted on my office door, little Calvin refuses to take a phone message for his father, saying "people always assume you're some kind of altruist." Two papers in the latest PLoS Biology show that some altruistic behaviors can be found in chimps and rats, as well as humans. Should we be surprised?
W.D. Hamilton explained how altruism towards relatives can evolve. An altruistic behavior will be favored by kin selection if the cost to the altruist is less than the benefit to the recipient, times their relatedness. Brothers have a 50:50 chance of sharing a given gene, so Haldane supposedly joked that he would give his life to save two brothers (or eight cousins). Strictly speaking, this "relatedness" is the extent to which the two are more related to each other than to their usual competitors.
Experimental tests of Hamilton's Rule are difficult. It is often impossible to measure fitness costs and benefits because "cost" and "benefit" are measured as decreased or increased fitness: survival and reproduction. It's usually impossible to tell how much a given behavior affected lifetime reproductive success. This week's papers may seem to bypass that problem, however. Both papers studied interactions between individuals whose (extra) relatedness was zero. This would seem to predict zero altruism, even if the cost was very low.
Nonetheless, chimps were found to pull a chain twice as often when it opened a door an unrelated chimp was trying to open to get food, relative to another door. In a somewhat similar experiment, rats were more likely to pull a lever that gave another rat access to food, if the helping rat had been helped similarly in the recent past. In both experiments, the individuals were unrelated. Given zero relatedness, these results seem inconsistent with Hamilton's Rule.
1) the fitness cost to the altruist from pulling a chain must be very low, both in absolute terms and relative to the fitness benefit to the recipient of getting food, and
2) evolutionary theory doesn't predict that behavior shaped by past evolution will be optimum under novel conditions. "To fly in a straight line, keep the light on your right" was a good rule for moths navigating using the moon, but it makes them circle (and sometimes fly into) candle flames.
Over much of their evolutionary history, chimps and rats were surrounded by relatives. "Help those around you, if you can do so at very little cost" would have been favored by kin selection under those circumstances, especially if those helped tended to reciprocate. Even though the experiments used pairs of unrelated individuals who were prevented from reciprocating, behavior that would have been favored by past natural selection is exactly what we should expect. Altruism involving a significant cost (sharing limited food) or apparent risk (snakes?) would be harder to explain.
Until recently, humans also spent most of their lives surrounded by close kin. We also interacted repeatedly with people who would remember, and might return, a favor. Even today, we seem to have some tendency to treat those we see most often (members of the same church or military unit, for example) as "honorary family." But as more people move away from home and change cities and jobs every few years, will genetic or cultural evolution tend to undermine altruism? Or will Calvin finally grow up some day?