Unusual Pet Tricks

Scene: Vetrinarian's office, two dogs running around with entusiasm unmatched, my mom and the vet. Time period: a few years ago....

Mom:
So, Doctor, can you explain why Shadow tends to wrap her paws around Raven and occasionally attempt to "hump" her? They're both girls, it doesn't make much sense.

Doctor:
Well, commonly an alpha female will "hump" another female to express and display dominance.

Mom:
Interesting. [dramatic pause]

Doctor:
It's very common, so don't think it's an odd behavioral trait.

Mom:
Okay, that makes sense. Now can you explain why Shadow does it to my son's leg?

Doctor:
[Silence]