Wednesday, May 13, 2009

Blood feuding and antagonistic pleiotropy

This week, I wrote a story for Nature’s online news about a PNAS paper on the link between violence and reproductive success in the Waorani, an Amazonian tribe. The basic message is that men who went on more blood-feud-motivated raids had fewer children surviving to adulthood. This is the opposite result to that found by Napoleon Chagnon’s studies of the Yanomano, where men who had killed in raids had more wives and children.

Chagnon’s work and methods have been controversial, and the debate around all that, and what this study says about the possible link between aggression, reproduction and natural selection is too complicated to go into here.

Instead I want to offer up some quarter-baked speculation on a slightly different topic. The paper suggests that one reason that Waorani violence seems counterproductive is that, unlike the Yanomano (and apparently other societies where blood feuding goes on), there’s no gap between rounds of violence. The Yanomano, says the paper, wait a generation between taking up the cudgels, during which time the attacks are sorcery-based. The Waorani were always at it.

This long delay reminded me of what’s called antagonistic pleiotropy. This is the idea that organisms age because natural selection can favour a gene that has a selective benefit in one’s youth but is detrimental later on — i.e., the bad stuff kicks in after you’ve bred and evolution has stopped caring about you. Could something similar, I wondered, allow delayed blood-feuding to persist, or even confer a benefit to such behaviour in some circumstances?

I can imagine how the initial aggressor might benefit. You prove your toughness in a handicap-principle kind of way, and/or reduce the competition, and then get to enjoy the benefits for a couple of decades. But what does he who waits get out of it? Maybe it reduces the cost that continual feuding imposes on everyone — apparently the Waorani were on their way to wiping themselves out. Or maybe the delay has kin-selection benefits — if the feud passes down the generations, maybe the next cohort gets to prove its toughness, reap the benefits etc.

I thought the analogy was interesting, but I don’t know if there’s any validity to this comparison, and I’m not saying blood feuding is, like, cool. And besides, these days we have libel lawyers to settle this sort of thing. Who, I suppose, are the sorcerers de nos jours.

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