06/26/2010 05:12 am ET Updated Dec 06, 2017

The Kardashian Sisters Go To A Pool Party (PHOTO)

The Kardashian sisters celebrated Kourtney's 31st birthday on Saturday at Wet Republic in Las Vegas. They stayed uncharacteristically covered up for the cameras... maybe those tabloid bikini covers pay more that way?

Kim told Us Weekly that Kourtney, four months after giving birth, already weighs less than she does. Kim is 5'3" and Kourtney is 5'1." Kourtney attributed her speedy weight loss to breastfeeding--a strategy Khloe would like to try.

"If that's all it takes, breastfeeding?" she told Us. "Then someone breastfeed off of me! I don't care!"