> This is also true in nature - animals will viciously defend as much territory as they can, and not limit themselves to what they "need".
This is very much not true. It doesn't even make sense from an evolutionary perspective; why would an animal risk their life and waste energy to protect something they don't need? Some animals do take a sizable territory. Typically carnivores, who need a crazy amount of space to get enough meat. Most animals aren't even territorial.
> Furthermore, having secured the best territory, food, and mating opportunities, they (whether as individuals or in groups) will spend their free time harassing their competitors.
Again, this isn't true. Why would they do that? It's a risk and an energy expenditure for no gain. They'll fight over territory if they have to, but I have never heard of an animal going out of it's way to harass each other for the sake of it.
> It doesn't change the rules of the game, as it were. We've not suddenly become a colony of ants, selflessly behaving as a single organism. Nor do I think it reasonable to carry the expectation that we suddenly will. Nor do I consider it particularly virtuous to compel us by force to emulate the ants.
I think you're basing this on a flawed understanding of nature and how we fit into it. The only way humans are a dominant species is through cooperation. Not every person all the time, but in aggregate.
Humans are not particularly strong. Humans are not particularly fast. We are smart, but that's only helpful insofar as it improves our fitness, and can be a weakness due to increased caloric needs. As individuals, we are not particularly high on the food chain. What puts us high on the food chain is the ability to collaborate. We invented farming, giving us a caloric surplus. Then we created societies, allowing us to use that surplus to have people dedicated to research. We then use that research and some caloric surplus to have people create things like guns and concrete and ships that make us highly adaptable and deadly to the rest of the food chain.
Without that collaboration, we go back to being the middle of the foodchain and jumping at every shadow at night.
> We live in a world where optimal behaviour is self-interested and the cost of failure is total.
Self-interested behavior is too local of a maxima to be useful as a species. Evolution encourages the survival of the species, not the survival of the individual. A purely self-interested view would see the optimal solution being to take from others, leading to a collapse of the species. It takes a lot less energy to kill the farmers and take their crop than it does to actually grow the crop.
> Authority should be used to ensure our coexistence is peaceful, and that we resolve disputes via due process.
And this is the logical (rather than moral) reason why we take care of the less fortunate. Starving animals tend to become very aggressive, and humans are no exception. During the Soviet famines, people killed and ate each other. You can't tell someone starving to death that they need to peacefully coexist; it won't happen.
It is dramatically cheaper and easier to prevent that by just preventing starvation.