Why does an animal behave the way it does? Because it wants to.
Why does it want to? Because it feels the best.
Why does it feel so? Because those feelings evolved to trigger the 'best behaviour'.
Why trigger the 'best behaviour'? Because it leads to the success of genes.
It means an animal acting of it's own free will, will be as healthy and as happy as it's genes and the environment allow it to be.
I think the trouble with this approach is that these behaviors you’re talking about are created by drives, and it’s these drives that are shaped by genetic success, BUT, the drives themselves end up expressed along a continuum from ‘maybe enough’ to ‘the perfect amount’ to ‘way too much.’ It seems to me that this is where both our success AND many of our conflicts come from; directly from the processes you’re talking about.
Say generations of successful genes result in a population of individuals with the trait ‘a drive to protect your siblings,’ on top of the older trait ‘a drive to protect yourself.’ The sibling-protection trait, in individuals, would show up as a bell curve type of distribution, right?
If an individual’s drive to protect siblings is too strong, they may die trying if the circumstances are too dire. But, because sometimes they are lucky and the circumstances are never too dire, that end of the bell curve never goes away completely, and you will continue to have recklessly heroic individuals.
If an individual’s drive to protect siblings is too weak, they may lose siblings they could have protected, and so lose some of the community that could have supported them in other dire times. But, again, sometimes they are lucky and will survive just fine with a little less community support, and sometimes being a coward is the right move, so that end of the bell curve stays too.
The middle of the curve is everybody else: a strong, but cautious, drive to protect siblings. Help them (because they share your genes and may help you in turn) but be reasonably sure you don’t get yourself killed trying (because your genes will spread better if you personally get to spread them, especially if your sibling is getting eaten at the moment).
So, is it these drives that are morally correct? The drive to help the sibling; the drive to be cautious and protect oneself? If so does that mean that all of these behaviors are moral? Or is it extreme expressions of these drives at the ends of the curves that ‘feel’ immoral?
Imagine a situation where one sibling is in a danger the other sibling is not able to help.
An individual with a middle-ground mix of these traits will feel distress at its inability to help a sibling, because it can’t follow both imperatives at once. My instinct is that this individual is behaving morally.
An individual with a strong self-preservation drive and little or no drive to help its siblings will not feel very distressed as it decides not to try to help its sibling escape from a dire threat. My instinct is that this individual is displaying understandable, but not commendable, behavior. Even though its actual actions are the same as the first individual’s actions, it bothers me that it is not distressed.
An individual with a very strong drive to help its siblings will not feel very distressed either as it temporarily ignores its own safety in favor of attempting to help its sibling - but it also will probably just plain get itself killed. My instinct is that this individual is behaving heroically but that it’s going way over the line required for moral behavior.
So I’m not seeing the connection between behaviors that are the result of drives that are shaped by genetic success, and morals as such. Unless the middle of that distribution is what moral behavior is? Even though in this hypothetical, the one that seems most moral to me is the one that does nothing and feels bad about it?
"The thief and the murderer follow nature just as much as the philanthropist. Cosmic evolution may teach us how the good and the evil tendencies of man may have come about; but, in itself, it is incompetent to furnish any better reason why what we call good is preferable to what we call evil than we had before."
"Evolution and Ethics" T.H. Huxley (1893)