I have NO clue either. I saw them do it once or twice while their Mommy was still around - they'd snuggle together and one of them would start nursing from the other one for a little bit, but dude, once MeiMei had found the jackpot and realized she just had to curl up to suck her own belly, she was doing it all the time. I wonder if it's because they kept nursing until the very last day Mommy was there - and there were already 3 mo old then, and so it was harder for them to break the habit once she wasn't there anymore?