The last two mornings, I have noticed my 3 week old doeling mounting her sister. This looks different than when they're playing and jumping on and over each other ... in fact, looks exactly like what I've seen the 4-week old baby boy trying on his sister in the next stall. What's going on? I actually picked her up and checked things out - could I have made a wrong "call" when she was born? Nope. All girl stuff.