The mule inherits from its sire the traits of intelligence, sure-footedness, toughness, endurance, disposition, and natural cautiousness. From its dam it inherits speed, conformation, and agility. Mules are reputed to exhibit a higher cognitive intelligence than their parent species. That said, there is a lack of robust scientific evidence to back up these claims. There is preliminary data from at least two evidence based studies, but they rely on a limited set of specialized cognitive tests and a small number of subjects. Mules are generally taller at the shoulder than donkeys and have better endurance than horses, although a lower top speed.