I'm a bit fuzzy about what counts as signalling and what doesn't, but I think it covers more cases than those involving conscious planning.

But anyway, I'd say you care about about animals because you're a kind person, but that humans tend to be kind mostly because evolutionarily it's been a benefit by facilitating cooperation and reciprocation. I don't know whether evolution just implemented "be kind to everything" instead of just humans because it took less lines of code (kindness to animals as spandrel), or whether kindness to animals was deliberately implemented because of it's signaling value (it may not be hard-coded, but just learnt as children).

(For what it's worth, I tend to save small bugs and throw them out of the window instead of killing them, which my wife would prefer. This device is convenient for safely and easily catching bugs, and observing them!)