Yeah, I mean it's not like anthropologists and the oral histories of the tribes themselves have ever recorded the practice of head hunting on multiple continents or anything...
It is peculiar that you have suddenly chosen to emphasize head hunting in relation to forager lifestyles, because head hunting is not a practice typically associated with hunter-gatherer bands. Its a practice typically associated with horticulturalists (e.g. Jivaroan peoples, various Austronesian groups, e.g. in New Guinea and Melanesia) organized into tribes or chiefdoms. There is endemic inter-group conflict in tribes and chiefdoms (understood as types of social organization), and that inter-group conflict may be a consequence of populations approaching the threshold of ecological sustainability (for given socio-technological capabilities), and may periodically relieve such pressures; such conflicts might even be regulated by a ritual calendar of sorts: indeed, that was more or less the thesis of Roy Rappaport's Pigs for the Ancestors: Ritual in the Ecology of a New Guinea People.
But, and I repeat, these are horticulturalists, not foragers. And so, again, I say, "No, its not "pretty well established" that the sustainability of a foraging lifestyle was maintained by inter-band conflict." In almost every case, the kinds of conflict people mention, are referring to horticulturalists.
It's true that the same population pressures associated with headhunting and ritualistic warfare tend to also result in hunter-gatherer tribes developing some form of horticulture as a less-easily-depleted food source, but that supports rather than detracts from my observation of the necessity of adopting extreme measures for population control to ensure the long term sustainability of pure hunter-gathering as a way of life for the masses
Either way, if one is to draw the distinction between city states with monocrop agriculture and the "the forager/garden-farm way of life" the poster I was originally replying to did, the Jivarons and arguably even the Dayak Iban belong more to the latter than the former. And ultimately it was the former intensive agricultural form of social organization which proved more ecologically sustainable in the long term without artificially keeping population densities down, and the latter hunter/forager form of organization that is suspected of wiping out most of the world's large mammals to hasten the transition to agriculture in many parts of the world.
