As others in this thread have said (in other ways) other organisms are black boxes to each of us. We cannot know whether they intent to do things, feel, have emotions, etc. We can only try and guess on basis of analogies (some philosophers will argue we cannot even know other organisms exist outside our own mind)

If it walks and quacks like a sentient being, why not call it a sentient being? One argument is the lack of complexity in reactions. That may be valid, but if so, where do you put the border between simple mechanical response and pain/warns others/…?

If that’s a gradual border, you can’t say the least complex of them aren’t feeling/having emotions/….

I think the distinction is gradual, with no clear border. That’s why I’m fine with calling this kind of responses ‘feeling’. On the other hand, I also don’t think cutting of a branch of an acacia is torture, but maybe I should call it torture on the extreme end of the scale of torture.