Butt Sniffing?

Okay, I've had cats many years, but I am unsure about this aspect of cat behaviour. When I got my newest cat, Lexi, she was about 1.5 yrs old -
the vet figured. My Diablo boy and Sheba girl, didnt take well at all to her.

There was hissing and swatting, LOL. I believe Diablo even *bit* Lexi at one point!!

However, no butt sniffing or nose touching.

But... my newest foster girl, a 6-8 month old kitty - well, Diablo AND Lexi have both butt sniffed her AND nose touched her. Lexi will play with her (chase round house in a semi aggressive way, no biting and blood drawn tho, mostly swats). Diablo generally ignores her. even when she gets on the bed (which is HIS spot, LOL).

What is this all about?

Esp. with my old Diablo sniffing butt... does it mean "acceptance" or "curiousity without acceptance"???

Its interesting since Diablo seems to object less to her than he does to Lexi (he and Lexi will NEVER be friends I'm afraid!) And Lexi isn't a dominant
cat I don't think. So I cant' figure why Diablo doesn't like!!

Any thoughts?

It could mean many things. It could be either them trying to get to know her, or them reacquaintencing themselves with her smell again, or just saying hello. Cats and dogs...both butt sniffers. It's gross, I know...

My kitties do it with each other every now and then, too, and they're litter siblings! They've been around each other their entire lives (they're almost two), but they still reaffirm each other of their scents every now and then.