there are many doctrinal authorities on record in every large strain of western monotheism against cruelty to animals
Which means that many doctrinal authorities are capable of making stuff up.
While most religions' tenets require some interpretation of their holy books, there are degrees of this. Some claims made by religions come from their holy books in a fairly direct and straightforward way. Others are claimed to come from their holy books but in fact are the result of contrived interpretation. Religious animal cruelty laws fall in the second category. The holy books do not support laws about animal cruelty in the same way that they support "thou shalt not commit adultery".
Furthermore, even those contrived laws don't generally claim it's cruel to eat animals. Bringing up the fact that religions oppose animal cruelty is like pointing out that every religion and culture has rules about sexual immorality, and therefore we should oppose some particular type of sexual immorality that you don't like.
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.
During much of history, most cultures that knew Jews attached zero or negative utility to them, but pogroms only happened every so often. They didn't just kill all the Jews until the Nazi era.
What we know from the psych side is that empathy appears to be basic in humans.
Anthromorphizing is also pretty basic to humans; that's why the Eliza program convinces people.
But it wouldn't surprise me terribly if the "expanding circle of concern" eventually encompassed or re-encompassed things like trees and rivers.
But you're not following the implications of this. The idea that primitive cultures respect the spirit of animals was brought up to show that taking the well-being of animals into account is normal. If the same primitive people respect the spirit of things whose well-being we clearly should not take into account, such as vegetables, it doesn't support the point you brought it up to support.
Furthermore, even those contrived laws don't generally claim it's cruel to eat animals. Bringing up the fact that religions oppose animal cruelty is like pointing out that every religion and culture has rules about sexual immorality, and therefore we should oppose some particular type of sexual immorality that you don't like.
Actually, he's responding to PG, who claimed that no major religion is against cruelty to animals ... presumably implying that this is a modern aberration? Or something? Regardless, it was he who claimed (in your analogy) that since...
When someone complains that utilitarianism1 leads to the dust speck paradox or the trolley-car problem, I tell them that's a feature, not a bug. I'm not ready to say that respecting the utility monster is also a feature of utilitarianism, but it is what most people everywhere have always done. A model that doesn't allow for utility monsters can't model human behavior, and certainly shouldn't provoke indignant responses from philosophers who keep right on respecting their own utility monsters.
The utility monster is a creature that is somehow more capable of experiencing pleasure (or positive utility) than all others combined. Most people consider sacrificing everyone else's small utilities for the benefits of this monster to be repugnant.
Let's suppose the utility monster is a utility monster because it has a more highly-developed brain capable of making finer discriminations, higher-level abstractions, and more associations than all the lesser minds around it. Does that make it less repugnant? (If so, I lose you here. I invite you to post a comment explaining why utility-monster-by-smartness is an exception.) Suppose we have one utility monster and one million others. Everything we do, we do for the one utility monster. Repugnant?
Multiply by nine billion. We now have nine billion utility monsters and 9x1015 others. Still repugnant?
Yet these same enlightened, democratic societies whose philosophers decry the utility monster give approximately zero weight to the well-being of non-humans. We might try not to drive a species extinct, but when contemplating a new hydroelectric dam, nobody adds up the disutility to all the squirrels in the valley to be flooded.
If you believe the utility monster is a problem with utilitarianism, how do you take into account the well-being of squirrels? How about ants? Worms? Bacteria? You've gone to 1015 others just with ants.2 Maybe 1020 with nematodes.
"But humans are different!" our anti-utilitarian complains. "They're so much more intelligent and emotionally complex than nematodes that it would be repugnant to wipe out all humans to save any number of nematodes."
Well, that's what a real utility monster looks like.
The same people who believe this then turn around and say there's a problem with utilitarianism because (when unpacked into a plausible real-life example) it might kill all the nematodes to save one human. Given their beliefs, they should complain about the opposite "problem": For a sufficient number of nematodes, an instantiation of utilitarianism might say not to kill all the nematodes to save one human.
1. I use the term in a very general way, meaning any action selection system that uses a utility function—which in practice means any rational, deterministic action selection system in which action preferences are well-ordered.
2. This recent attempt to estimate the number of different living beings of different kinds gives some numbers. The web has many pages claiming there are 1015 ants, but I haven't found a citation of any original source.