i) Atheist William Rowe famously cited a fawn dying in a forest fire as a paradigm-case of gratuitous evil. On the face of it, that's not gratuitous evil, because forest fires are necessary to maintain the balance of nature. In fact, animal death is necessary to maintain the balance of nature. So that's purposeful suffering.
ii) Now perhaps Rowe would say it's gratuitous in the sense that an omnipotent God could create a world without predation, forest fires, &c. But there are problems with that response:
a) Yes, God could, but that would be so unrecognizably different from the actual world that we can't even begin to do a comparative axiological analysis. We can't say which is better and which is worse because a world without our type of ecosystem is so far removed from human experience that it's hard to even conceive of what that would be like. Atheists have a bad habit of artificially deleting "bad things" from the world, then leaving everything else intact. But, of course, that requires corresponding adjustments. It's unclear what's left after the dust settles.
b) Moreover, a world without animal suffering might be better in some respects, but worse in others. Even if it's better overall, the goods might not be as good as a world that's worse overall. You could have a world that's worse overall, but the peaks of goodness are higher. So there's no single criterion of goodness.
iii) Furthermore, animals lack our human viewpoint on suffering. Take the somewhat amorphous distinction between lower and higher animals. A continuum of sentience.
A few years ago I read about some men who discovered a rattlesnake pit right by the playground of an elementary school. A communal rattlesnake pit. That posed an obvious danger to the school kids.
So the men poured gasoline down the snakepit and set it afire. End of problem!
I'm sure the snakes writhed about as they were roasting alive. Does that mean they were in pain, or is that a reflexive reaction? For instance, decapitated snakes continue to writhe.
How does a snake brain process or interpret that stimulus? I doubt what it's like for a man or German Shepherd to burn alive is the same for a snake. For one thing, it has a much simpler brain. Does the same stimulus mean the same thing to reptile? Seems unlikely.
Same thing with cooks who put live crabs directly in boiling water. Seems cruel, but isn't that just an anthropormophic projection on our part?
iv) I don't necessarily mean that's reducible to neurological structures. It's possible that animals have souls. But if they have souls, they have animals souls. If a wolf has a soul, it has a soul specific to the nature of a wolf; a soul with a lupine viewpoint. An outlook in many respects alien to a human viewpoint.
I think it would be cruel to set a dog on fire. But I don't think it was cruel to incinerate the rattlesnakes.
v) To take another comparison, during the Vietnam War, some Buddhist monks protested the war by setting themselves on fire. There are Youtube videos of that horrific scene. Yet, as I recall, they were very stoic about it. They didn't scream or flail about.
If we were just judging by body language, we'd infer that a snake is in greater pain than a man. But, of course, because we're human, we know that's not the case. So body language can be deceptive.
vi) From the standpoint of Christian ethics, given borderline cases, it's best, all things being equal, to allow ourselves a wide margin of error in the avoidance of possible animal cruelty.
Another factor concerns intent. To set a dog on fire is an act of malice. That is done with the intent to inflict pain on the dog. The person who does it takes depraved pleasure in cruelty. Even if, unbeknownst to him, the effect is painless, his motivation is heinous.