A minimal investment of time would convince anybody willing to be convinced that at the very least there are many doctrinal authorities on record in every large strain of western monotheism against cruelty to animals, and that these authorities adduce evidence from ancient holy texts to support their pronouncements. Feel free to disagree with Aquinas, eastern patriarchs, a large body of hadiths, and many rabbinical rulings about the faiths they represent. There is a hermeneutical constellation of belief systems that posits texts speaking for themselves without any interpretation and announces that meanings are clear to the newcomer, or outsider, or even the barely literate, in ways they were never clear to bodies of scholars who gave their lives to the study of the same texts. I'm not sure you want to be in that constellation. That is Constellation Fundamentalism, though to be fair to the actual fundamentalists, they don't seem to be amenable to animal bloodsports at all.

I grant zero weight to the well-being of clothes, but that doesn't mean I go around destroying my clothes

Clothes aren't a threat to ambush you, and aren't eating tapirs you could eat. I assume you would burn them if you feared ambush or starvation.

doesn't make something else worse that you do care about (such as risking death to your own tribesmen in war)

Total war doesn't mean you can't be tactical in your approach, obviously. Dissembling and biding time are smart.

What I mean about the tribes being in constant total war is that since, as was pointed out, they are in competition for resources with neighboring tribes, they would kill neighbors whenever they thought they could get away with it if they attached zero utility to these people's survival. And we see that's not the case, not at all. Hunter-gatherers trade, they intermarry, they feast together, they form friendships and alliances between tribes, they do a bunch of things that would be socially impossible if there were not any empathy at all. Sometimes they betray and murder. But by no means all the time. Napoleon Chagnon's accounts of the Yanomamo, where most of this stuff about violent stone-agers comes from recently, are quite clear that elders intervene to stop axe fights some times, and that the Yanomamo are mostly just terrified by the violence around them.

What we know from the psych side is that empathy appears to be basic in humans. Our researchers would have to be pretty consistently wrong about something very large if Stone Age people, just because they were Stone Age, were incapable of empathy with people outside their immediate kin group.

I wonder how many of the cultures who pray to the spirit of the animal also pray to the spirit of the plants, rocks, the sun, or other things that even vegetarians don't think have any rights.

Yeah, this is pretty interesting to me, too. I suspect, though, that a lot of people into deep ecology and Christian environmentalism and similar forms of environmentalism have...analogous?...attitudes toward the parts of nature that lack nervous systems. Not inside the rationalist/hedonic calculus/Peter Singer/utilitarian communities, probably, because there's so much emphasis on pleasure and pain there. But it wouldn't surprise me terribly if the "expanding circle of concern" eventually encompassed or re-encompassed things like trees and rivers.