Most dog breeds, with the exception of the hairless Chinese Crested and Xoloitzcuintle, are covered in a layer of fur. This fur inhibits the dog's ability to cool down by sweating through the skin as other mammals do. Dogs do have other means to control their body temperature, although overheating is still a major health concern in hot weather.
The modern domesticated dog evolved from wolves in the northern regions of the world. Wolves developed a thick double coat to combat bone-chilling temperatures during the winter, which made sweating through the skin impractical. Small sweat glands developed in the nose, tongue and pads of the feet developed to help the wolves regulate their body temperature. These features were passed on through thousands of years of breeding to modern domestic dogs.
Dogs are unique in the fact that they are one of the few mammal species that do not sweat through the skin. The sweat glands in dogs are restricted to a much smaller area of the body and make it more difficult for dogs to cool themselves. A hot dog will lie in cool, shady spots and pant heavily in an attempt to release excess heat from the body. A dog with a low body temperature will shiver in an attempt to stimulate muscles and produce extra body heat.
Dogs have two types of sweat glands: apocrine glands and eccrine glands. Apocrine glands, which secrete sweat in other mammals, are minuscule in dogs and produce pheromones rather than sweat. Eccrine glands secrete moisture and sweat to help the dog stay cool. Eccrine glands are located on the nose and pads of the dog's feet, which is what makes them feel damp to the touch.
The normal body temperature for a dog ranges between 37.8 to 38.9 degrees Celsius, and when the dog's core temperature remains above 38.9 degrees C, serious health risks arise. Hyperthermia, or heatstroke, can be deadly occurs when the dog's temperature rises above 40 degrees C. Low body temperature, also known as hypothermia, sets in when a dog's temperature drops below 37.8 degrees C and can also be fatal if left untreated.
Regulating body temperature is essential for optimal health, and it is important that dog owners recognise the signs of an abnormal temperature. A dog suffering from hyperthermia will pant heavily, feel hot to the touch and have dry, red tongue and gums. Hypothermic dogs will be extremely lethargic, shiver excessively and appear lost and confused. A dog displaying signs of either hyperthermia or hypothermia should be treated immediately by a veterinarian.