Bulldog,  also known as the British Bulldog or English Bulldog, is a medium-sized dog breed. It is a muscular, hefty dog with a wrinkled face and a distinctive pushed-in nose. The Kennel Club (UK), the American Kennel Club (US), and the United Kennel Club (US) oversee breeding records. Bulldogs are popular pets; they were the fifth most popular purebreed in the US in 2017 according to the American Kennel Club. Although not as physically capable as their ancestors, decreased levels of aggression associated with modern bulldogs have resulted in far calmer temperament while remaining physically capable guards and companions. Bulldogs are very sensitive to heat. Extra caution should be practiced in warmer climates and during summer months. Bulldogs must be given plenty of shade and water, and must be kept out of standing heat.