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Alternatives to “and/or”?

Is it okay to say "and/or"? How else might I phrase a sentence like the following?

The amount of happiness displayed by the bunny increased when either the bunny was well fed and/or when the bunny was watching a sunset.

I am writing a scientific paper, and it is important that the events have the same effect when they occur separately or together; I have written a trivial example replacing esoteric words with more familiar ones.