The kinds of things relative pronouns refer to in modern English can be divided roughly as follows:
that – things and people
which – things, but not normally people
who – normally people, not things, sometimes animals or human-like entities (“animate but not human”, says Robert Burchfield; “having an implication of personality”, says the OED)
When it comes to relative pronouns, animals often aren’t accorded the same grammatical status as people. We’re more likely to say The crow that was here than The crow who was here, though of course it varies with the speaker, type of animal, and context.
So I was struck by a line in last week’s Galway Advertiser reporting the recent entry of the dormouse to Ireland’s ecology (we already have the wood mouse and house mouse):
Dormice are woodland animals, who nest in shrubs and hedgerows, particularly those containing hazel (as their name suggests) or brambles.
I haven’t looked into it, but I’d bet that of references to dormice in equivalent contexts, at least 95% would use that or which rather than who.
Not everyone supports this extended use of who, but it is defensible; the Merriam-Webster Dictionary of English Usage quotes lines by John Updike (“the hamster who had died”) and Stanley Kauffman (“Tonto is his cat, whom he walks on a leash”) showing its literary acceptability.
Dormice of the world, welcome to Ireland – and to the Grammatical Who Club.