I have several times commented on the problem of altruistic punishment (see links here), so I was interested to see a report by John Hawks (Jan. 30) on punishment among pigtailed macaques (monkeys).

Altruistic punishment by definition occurs where one member of a society takes action against another, with a resulting benefit to the society, but at a net cost to himself. The problem is to explain such behaviour, since it does not seem prima facie to be consistent with natural selection at individual level.

I am sceptical about whether altruistic punishment in this sense is often found among non-human animals, but the research by Flack et al. (see especially the paper in American Naturalist) shows that it does sometimes occur. Among pigtailed macaques individual monkeys will frequently intervene to break up disputes among other, unrelated, monkeys. Such intervention carries a cost (risk of being bitten, etc), which seems greater than any immediate benefit to the intervener. It is significant that the intervention is nearly always by individuals high in the dominance hierarchy. Since these are the most powerful animals, they incur relatively little risk of injury. But it is still necessary to explain why they bother to intervene at all.

Several possible explanations have been suggested:

a. multi-level selection. The intervention is selected against at individual level, but it is beneficial to the group, so those groups with a high proportion of interveners will flourish, which may be sufficient to offset the cost to interveners within the group.

b. indirect benefits to interveners. The idea here is that dominant individuals stand to gain disproportionate benefits from the prosperity of the group as a whole, in terms of territory, number of females, etc., so it is in their individual interest to promote group success.

c. the Handicap Principle. By intervening in disputes, at a risk to themselves, dominant individuals are demonstrating their fitness, with resulting benefits in attracting females, discouraging rivals, etc.

I rather like the Handicap suggestion, but I wonder if there is not another explanation? In a dominance hierarchy any dominant individual has a direct interest in preventing subordinate individuals from rising in the hierarchy, and potentially challenging his own position. Individuals rise in the hierarchy mainly by defeating other individuals in contests. Dominant individuals therefore have an interest in preventing subordinate individuals from winning contests. Imagine a Mafia boss who finds there is a feud between two of his subordinates. He will try to stop it, not only because it disrupts the smooth operation of the family, but because the winner of such a feud will have enhanced power and status, and will be a natural rival to the boss himself.

It would probably be difficult to find a way of deciding among these possible explanations, but I suggest that one implication of my own hypothesis is that dominant individuals will be especially likely to intervene if one of the contestants appears to be winning the contest. This is not obviously predicted by the other hypotheses. Another prediction is that males will intervene disproportionately in disputes among males, and females among females, since these have separate dominance hierarchies. I am not clear from Flack et al.'s paper whether this is the case.