The ancient Greek Olympics had chariot and mounted horse racing sports. In the Roman Empire, the sport was also immensely popular. Modern racing dates back to the 12th century, when English knights returned from the Crusades on speedy Arab horses. For centuries, European riders dominated the sport, but in the late 19th century, American riders started to compete more aggressively, which led to the development of new races such as the Kentucky Derby and Preakness Stakes. Today's horses are still improved through selective breeding, but many other factors can also influence their speed including size, shape, and makeup of their bones. In addition to human influences, there are animal influences too. For example, fast horses tend to be born faster and have shorter legs than slower horses. This is because they reach skeletal maturity earlier. So although humans can manipulate this process by using growth hormones, it is limited by the genetic make-up of the horse. Another animal influence on racing comes in the form of ticks. Ticks carry bacteria that cause diseases such as Lyme disease and Rocky Mountain Spotted Fever, so avoiding these creatures is important for horse owners' health. Ticks like warm-blooded animals and will not bite cold-blooded ones. That's why dogs are used to check for ticks before going into forests or fields where horses may be present.