When a puppy is born, his body temperature is the same as his mother’s. Immediately thereafter, his core temperature drops several degrees (how much depends upon the temperature of the room). Within 30 minutes, if the puppy is dry and snuggles close to his mother, his temperature begins to climb and reaches 94°F (34.4°C). Twenty-four hours later, his core or rectal temperature is 95° to 97°F (35°C to (36°C). It steadily increases, until at 3 weeks of age the rectal temperature is 98° to 100°F (36.6°C to (37.8°C).
During the first week of life, puppies do not have the capacity to constrict the blood vessels at the surface of their skin to retain heat. A newborn is able to maintain a body temperature 10°F to 12°F (3.8°C to 4.9°C) above his immediate surroundings for only short periods. When his mother is away for 30 minutes in a 72°F (22.2°C) room (well below the recommended level), the pup’s core temperature quickly falls to 94°F (34.4°C) or below. This gravely reduces his metabolism.
Chilling is the single greatest danger to infant puppies. Low temperatures for these first few weeks are also one of the biggest reasons herpesvirus is able to successfully attack a litter. The temperature in the whelping box and surrounding area should be kept at 85° to 90°F (29.4°C to 32.2°C) for the first week. During the second week, reduce the temperature to 80°F (26.7°C). Then reduce the temperature gradually so that it is 70°F (21°C) when the litter is 6 weeks old. (Remember that the puppies still get heat from the mother dog, one another, blankets, and, often, a heat lamp over one corner of the whelping box.) Keep a constant check on the temperature using a thermometer placed on the floor of the whelping box.