Mighty mules are no dumb donkeys

Hybrid vigour gives mules greater intelligence

source: New Scientist vol 199 no 2667, 2 August 2008  p19

Mules have proved more successful than donkeys and horses at learning which of two buckets to select to obtain food. Mules learnt more symbol pairs and did better in terms of consistency, according to University of Sussex's Leanne Proops. She argues that their better cognitive skills are the result of hybrid vigour, which means that offspring may be fitter than either parent.
HO,BC