But there's a whole spectrum of responses between the two, from "I don't like that noise" to "I don't like seeing that other rat in pain" to "I know if I don't do something about this, I'm going to feel bad later" or "It's wrong to let another animal suffer, therefore I should help him".

A small mammal doesn't have to have the exact same nuanced understanding as a human would in order to still be 'feeling empathy'.

>But there's a whole spectrum of responses between the two

What seems to be going on in recent decades is, through experimentation and arguments, the scientists are getting clearer on what are the various points on the spectrum.