Body temperature and torpor
The platypus normally maintains a body temperature close to 32°C. This is a bit lower than the body temperature of most other mammals – for example, the temperature of a healthy human is usually about 37°C. The platypus’s relatively low body temperature is believed to be an adaptation to conserve energy, particularly when an animal is swimming in cold water.
To further reduce heat loss, platypus fur is made up of two layers: an extremely dense undercoat (including up to 900 individual hairs per square millimetre of skin surface) and coarser overlying guard hairs. These layers work together to trap air next to the platypus’s skin when an animal enters the water, so most of the body surface actually remains dry. The combined insulation value of the fur and air layer has been estimated to be similar to a three millimetre layer of neoprene wetsuit material.
Secondly, the platypus has a special network of small intertwined veins and arteries in the pelvic region (known to scientists as a rete mirabile or literally ‘miraculous network’). This network serves as a counter current heat exchange system: cooled blood returning to the heart from the animal’s legs and tail absorbs some warmth from blood being pumped from the chest, reducing the overall loss of body heat to the environment.
One disadvantage of being so well adapted to surviving cold conditions is that the platypus has a propensity to overheat: in captivity, animals become noticeably lethargic when the water in display tanks exceeds 29°C, and a platypus has reportedly lost consciousness after being exposed to an air temperature of 35°C for 17 minutes. Overheating is not normally a problem for platypus in the wild, as they prefer to spend their time either immersed in substantial bodies of water or resting in burrows, where average air temperatures typically do not exceed 18-20°C even in summer. However, it does mean that platypus is likely to overheat badly if they try to travel long distances across land in summer, for example to find new feeding sites during a drought.
Observations in both captivity and along a small stream in Victoria suggest that platypus may periodically enter a state of torpor in which the animals allow their body temperature to drop, remaining inactive for up to about six days. This behaviour has only been recorded in the colder months of the year (late May to early September). Interestingly, no records of inactivity have been recorded in the course of platypus radio-tracking studies undertaken in winter along two rivers in New South Wales or a sub-alpine lake in Tasmania, suggesting that low ambient temperatures are necessary but not sufficient to trigger torpid behaviour in this species.
Grant, T.R. and Dawson, T.J. (1978). Temperature regulation in the platypus, Ornithorhynchus anatinus: maintenance of body temperature in air and water. Physiological Zoology 51: 1-6.
Grant, T.R. and Dawson, T.J. (1978). Temperature regulation in the platypus, Ornithorhynchus anatinus: production and loss of metabolic heat in air and water. Physiological Zoology 51: 315-332.
Grigg, G., Beard, L., Grant, T. and Augee, M. (1992). Body temperature and diurnal activity patterns in the platypus (Ornithorhynchus anatinus) during winter. Australian Journal of Zoology 40: 135-142.