While Miley Cyrus got her grind and her tongue on some of Britney Spears' female dancers at the opening of BritBrit's Las Vegas residency, Katy Perry got her butt on her seat ... and barely moved.

TMZ cameras caught the 'Dark Horse' singer, joined by an entire gaggle of pals, watching the show intently, but not dancing along to Spears' hits whatsoever. Perry, who's written and even sung backup for Spears (on 'Ooh La La'), didn't appear nearly as into the show as their mutual pal Cyrus, who can be seen in the above video dancing her twerkaholic little butt off.

Perry, hair in a high braid, just ... sits there. Eventually, she stands up, but she doesn't even register a head bop or a foot tap. Maybe she's just taking in all of Spears' heavy choreography to practice for her own someday. Despite Spears' knee surgeries, she's still able to kill it on the floor, while Perry, who has never reported any leg issues, often comes off as awkward and stiff when she dances.

John Mayer