Dogs are domesticated mammals that are descended from the wolf. Dogs are omnivores and in the wild usually hunt in packs. There are large numbers of breeds of dogs as well as crossbred animals (mongrels). Because of this, there are a tremendous variety of physical features to be seen across the breeds. Size can vary from the small Chihuahua (approx. 15cm tall) to the huge Irish Wolfhound (up to 100cm tall). Correspondingly, weight too can vary enormously, with the St. Bernard being one of the heaviest (approx. 75kg). Ears may be pointed and erect or drooping. As most dogs are covered in fur, they only have sweat glands on their nose and feet, allowing only a small amount of heat to be lost in this way. Consequently they pant to increase the heat loss from their throat and tongue. Dogs' coats may be of various lengths and textures and most have two coats, an outercoat and undercoat. However, all dogs in the wild shed their coat (moult) once a year, triggered by daylight hours and temperature. Animals that live indoors may moult all year, as these two factors are more constant. Dogs are one of the most intelligent of domestic animals and can be readily trained. Different breeds have different natural instincts e.g. herding, hunting, and guarding. Dogs also have very acute senses of hearing and smell. All these specific traits are used to train working dogs. The breed of a dog is most important when choosing one for a pet. Feeding and exercise requirements vary along with temperament and a lot of thought is needed to find one to suit the individual's needs.