In such cases it is basically the writer's choice whether to use "who" or "which/that", but "who" seems more likely with higher animals that can display something like the human attributes, as in the "lion" and "donkey" sentences, and less likely with lower organisms or inanimate things, especially if the relative clause can't literally apply to a person, as in the "chocolate cake" and "cucumber" examples.
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In such cases it is basically the writer's choice whether to use "who" or "which/that", but "who" seems more likely with higher animals that can display something like the human attributes, as in the "lion" and "donkey" sentences, and less likely with lower organisms or inanimate things, especially if the relative clause can't literally apply to a person, as in the "chocolate cake" and "cucumb