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Monotremes are better able to control their body temperature than are reptiles. Except during hibernation, or torpidity of some species, the eutherian, or placental, mammals maintain a high and virtually constant body temperature usually between 36° and 40° C (97° and 104° F). The body temperature of most reptiles varies with that of the surroundings. The body temperature of the monotremes, however, does not quite reach that of the eutherian mammals, being about 31° to 32° C (88° to 90° F). The monotremes are usually described as physiologically primitive in regard to temperature regulation. It has been shown, however, that Tachyglossus is an excellent temperature regulator in some situations but less capable in others. It is probably no poorer in this respect than many eutherian mammals (e.g., anteaters, armadillos, and sloths). In cold surroundings the echidna maintains a well-regulated body temperature. Studies of temperature regulation and torpor in Tachyglossus have shown that this echidna appears to be a true mammalian hibernator, with the ability to arouse from torpor without the aid of heat from external sources. This ability is lost after repeated periods of torpor in the laboratory at 5° C (41° F). The echidna resembles other hibernators in that torpor is not continuous but is interrupted by frequent arousals. Continuous periods of torpor lasting 9.5 days, with intervening periods of wakefulness ranging from 30 hours to 11 days, have been recorded for the echidna.
The platypus has periods of hibernation or semihibernation, at least in southeastern Australia. In captive animals, Fleay found that the hibernation periods were short and irregular during the cooler months, the longest single period being 6.5 days.
Laboratory studies have shown that the platypus is slightly superior to Tachyglossus in temperature regulation. The platypus has sweat glands on the muzzle and over the body surface, whereas Tachyglossus has them only in the pouch area. The heat tolerance of Zaglossus, which has many well-developed sweat glands distributed over the body, has not been studied. In its natural environment it is unlikely that the platypus will be exposed to high heat loads. The highly insulating coat of the platypus would protect it against heat loss in water and allow it to exploit cold environments.
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