(Post made at local midnight in the assumption you know some LW jargon, if that is false, explanations will be provided tomorrow morning GMT)
One of the more obvious things that come to mind is appearing honorable to outsiders. Depending on their intelligence it might even be required within the tribe. Explaining cooperating in PD with other conditional cooperators to outsiders via TDT would be doomed to failure but saying "it's our honor" is something even non-x-rational people can understand and thus cooperate with. Similarly one could expect there to be strict penalizing of self-serving mistakes, e.g. if you make a promise you keep it or get ostracized by everyone; the only way out is if the person you made it to concedes or the whole tribe decides you shouldn't keep it.
This helps avoid problems in situations like Parfit's Hitchhiker or whatever-that-problem-with-incentives-regarding-the-eccentric-millionaire-and-the-poison-is-called-again.
One could also be expected to pull their weight and sacrifice themselves for the good of their group if the sacrifice would make the tribe better off with the expection that others would do the same in other situations and thus overall damage is minimized, bailing out would result in harsh punishments.
For example let's say that a hunting party encounters a monster. They can either try to escape together and probably lose many with a minuscule chance of all surviving or leave one (chosen randomly) to certain death and have the rest escape unscathed. If the person chosen tries to escape they could do it because the monster would follow the bigger group; only attacking it (and thus losing any chance of escape) would save the others. The members of the hunting party wouldn't even think of choosing the first option because doing so would cause a much worse outcome. On the other hand, if monsters are rare they might be expected to either die all if bringing back the body/ies of lost hunters would be impossible because otherwise murdering unpopular individuals would be too easy: "Oh yeah, we totally encoutered a ravenous bugblatter beast and Durk sacrificed himself to save us. No foul play at all sirr..."
There would probably be quite a jarring contrast between an otherwise humane and reasonable society and the extremes taken in extreme situations. During a famine the weakest might kill themselves and if healthy, be eaten by the rest instead of consuming already scarce resources with little hope of surviving anyway.
Having some kind of ritual self-punishment for failures to be honorable (in most extreme cases seppuku) would be very likely. Situations such as the "nuke in NY; torture or not?" scenario would be recognized and people who broke the rules to get a good outcome would be punished anyway because otherwise the rules would be broken even when there isn't such an imminent and serious need. Accepting this would be expected as the bad "moral luck" of one having to choose between bad and worse would save others from more suffering.
When fighting (if there are other tribes to fight) they might be similar to Mongols; cultivating a brutal reputation as deterrent, perhaps focused on the enemy leaders as normally groups tended to treat others' leaders well (at least in the middle ages although my knowledge doesn't extend to whether "leader-mercy" existed before pre-modern civilization and can't be arsed to research it at 1:00 AM) even if the mooks were slaughtered in expection of similar treatment themselves; if the Bayes!tribe tortured and then executed enemy leaders who had chosen to attack them (if they weren't killed outright in battle which they probably would avoid) those who actually affect the chance of getting attacked would have a good incentive not to do so. On the other hand, engaging in positive-sum trading would be held in very high esteem and the "sanctity" of trade would be another SRS BSNES GAIS.
TL;DR: what Oh So Into Cats said; a strict code of honor in relevant things but probably relaxed where business isn't so serious; take some inspiration from Mongols, medieval Japanese and most importantly "Bayesians versus Barbarians"
edited 3rd Oct '11 3:10:26 PM by ChurchillSalmon