ponies and horse breeds that evolved in relatively harsh environments with only sparse grass, the proverbial "easy keeper," tend to be more prone to ems and insulin resistance. this possibly occurred as a survival mechanism, where the animal would lay down fat during plentiful times, such as the spring and summer, and maintain their weight more easily during the harsh, cold seasons. ems appears to be more common in welsh, dartmoor, and shetland ponies, morgans, paso finos, saddlebreds, spanish mustangs, and warmbloods; and may also been seen in quarter horses and tennessee walking horses, although is rarer in breeds such as the thoroughbred and standardbred. most horses are 5–15 years of age when they develop laminitis that can be attributed to ems.