Fòram na Gàidhlig

I've been confused about how zoo is used: sù/sutha.
Colin Mark has "sutha" as feminine, but Am Faclair Beag has both "sù" and "sutha" as masculine.
So I'd expect it to decline as: an sù, anns an t-sù, geata an t-sù (and the same for sutha).
But I've seen "Rathan an Sutha" (BBC Alba), "dhan sù" (in a song), etc. The prefix t- is not being used.
Google confirms that "anns an t-sù" is essentially unused but "anns an sù" is common.
Even though "sù/sutha" is a loan word, how can it behave like any word beginning with s+vowel?
I note that another loan word "sòfa" behaves grammatically: air an t-sòfa, etc.
What's going on with "sù/sutha"?

I can only assume it's the pronunciation.

sù is essentially an attempt to render "zoo" in Gaelic spelling, though it's pronounced with a Z, which Gaelic doesn't have. The S in sù is therefore not a typical Gaelic S sound, and doesn't feel natural with spelling / pron. rules that belong to the sound S.

Sofa, however, does have an S sound, so sounds more natural with the t- in the usual places.

It's also worth pointing out that loan words do wander a bit between users, both in gender and in the extent to which they obey the rules.

e.g. I would say I'd heard "air a' fòn" more often than "air an fhòn".

No doubt our linguist friends on here will have more scientific explanations.

Yes, good point about Z, which is a voiced S sound.

Regarding "fòn", there is also the verb form fòn/fònadh.
What do you think of the following:
* Tha mi a' fònadh gu mo bhràthair
* Fònaidh mi a-màireach = I'll phone tomorrow
* Dh'fhòn mi thuige = I phoned him (do you think this sounds right?)
* Tha mi air a dh'fhònadh mu a dheidhinn = I've phoned about it

You can certainly use fòn, a' fònadh as a verb, but there is an alternative form that is often used (and my own preferred one), cuir fòn gu. That to me sounds more idiomatic, on the analogy of cuir litir / post-d / fios gu.
It also keeps the word fòn unchanged, so no confusion about meaning through lenition etc, though I think that's not a problem in context.

BTW, that last example should just be the verbal noun " Tha mi air fònadh ...".
If there was an object, then the inversion structure would kick in: Tha mi dìreach air Calum fhònadh./ Tha mi dìreach air fòn a chur gu Calum.
The a dh'fhònadh form would only occur after a verb of motion: Chaidh mi a dh'fhònadh Calum. / Chaidh mi a chur fòn gu C.

Sgoinneil. 'S toil leam "cuir fòn gu" cuideachd.

Mòran taing airson a' cheartachaidh!