[attenborough mode]
Leg muscles need to be kept warm in case you have to start running at short notice.
Recumbent quadrupeds tend to sit with their legs folded underneath them.
Bipeds, with little hair on their limbs, attempt to retain heat by pressing legs together or crossing them and wrapping the arms round their torso.
[/attenborough mode]
Unless there are sabre-toothed tigers or the like in your area, you could probably start the evolutionary process of sitting with legs akimbo (cue Ethiopian sprinter joke)
