all 3 of our Basenjis do this funny sit. My theory is because Basenjis originated in countries where the ground is hot they have evolved to sit like this to protect their delicate rear ends :D . Now that they live in different climates i believe they do it to avoid, heat, cold, mud. I also think there is something in the fight/flight theory.
Lisa your description of Xander made me laugh. Malaika hovers about an inch from the ground when doing her down stays in class, in cold weather she is allowed to do it on a rug, however she still likes to hover if we don't insist she goes down completely.
