Lean animals with fine bone are often more nervous compared to large-muscled animals with heavy bone. In the fox experiments discussed earlier, the tame foxes were stockier with heavier bone than wild foxes. Nervousness associated with a lean build and fine bone is also noted in cattle and pigs. The conformation-temperament relationship also applies to horses. As a general rule, Arabians are lean, fine-boned, reactive, and spirited.
Draft horses are muscular, heavy boned, and have mostly calm temperaments.
This is a general principle that also applies to animals within a breed. At a horse show I recently attended, the conformation-temperament relationship was clear both between horses of a breed and between different breeds. Over-selecting for performance traits, such as speed in Thoroughbreds, or cutting ability in Quarter Horses, can have unwanted effects on other traits. The lean, fine-boned conformation of some racehorses may be inadvertently linked to a highly reactive nervous system and extreme fearfulness. In a study of the effects of fear on race performance, researchers found that three behavioral measures of fear shown by horses just before a race can often predict certain losers.