My oldest sow licks, and a one of her sow pups (7 weeks old) licks routinely as well. Both sows (mom and pup) seem to do it mostly when being held with one hand, though the mom sometimes does it while petting near her face but otherwise on my stomach. I've yet to see them licking each other (or any of my gp licking each other with exception of eldest boar and ear tugs and mom with newborn pups) so it might be an argument it's something they like on the skin. I always wash hands with Irish Springs bar soap before handling, but have noticed other times when hands are clammy, they lick more so. Since they lick themselves to groom, they might actually be licking skin to either add or remove a smell. If they have some dirt on their bellies and it's transferred to your hand, they might inadvertently think "you" are dirty.
My eldest boar (roughly 6 months old) will sometimes gently do a partial "bite" which I'm pretty sure is some sort of affection given the context and timing i.e. not talking about after 15 minute lap time which could mean "I want to go back." I know gp (my boar does it a lot, and my youngest boar has been seen trying to do it) will do gentle "ear tugs" to each other, so I started doing it to them as well by gently pinching an ear and very gently tugging on it as if to emulate their behavior. I do it mainly with the boars, and the eldest boar will eventually do the gentle "bite", which is inline with the ear tug he does to others, especially his cage mate.
