Platypus body temperature is normally close to 32oC, both when active in the water or resting in a burrow (Grant 1983; Grigg et al. 1992). This is lower than the body temperature of most other mammals – for example, the temperature of a healthy human is usually about 37oC. The platypus’s relatively low body temperature helps it to conserve energy, especially when swimming in cold water.
To further reduce heat loss, platypus fur consists of an extremely dense undercoat (with up to 900 individual hairs occupying a square millimetre of skin) and coarser overlying guard hairs. These layers work together to trap air next to the platypus’s skin, so most of the platypus’s body actually remains dry in the water. The insulation provided by the fur and trapped air layer is comparable to that of a 3-millimetre layer of neoprene wetsuit material (Grant and Dawson 1978).
The platypus also has a special network of small intertwined veins and arteries in the pelvic region (known to scientists as a rete mirabile or “miraculous network”). This network provides an internal heat exchange system: cooled blood returning to the heart from the legs and tail absorbs warmth from blood being pumped to the legs and tail through arteries, thereby helping to retain heat inside the body and maintain platypus body temperature (Grant and Dawson 1978).
Because the platypus is so well adapted to surviving cold conditions, it can easily overheat. In captivity, animals become “noticeably lethargic” when water temperature exceeds 29oC, with ambient temperatures above 34oC considered to be life-threatening (Booth and Connolly 2008). This normally isn’t a problem in the wild when animals either spend time in the water (mainly at night) or rest in an underground burrow. However, heat stress may become a real concern if a platypus is forced to walk a long distance in warm to hot conditions, for example to find a suitable place to feed during a drought.
Observations made in captivity and along a small Victorian stream suggest that a platypus may sometimes enter a state of torpor in which body temperature drops low enough that it can remain in a burrow without needing to feed for up to around six days. This behaviour has only been recorded in the colder months of the year (late May to early September) (Serena 1994). However, no evidence of torpor was recorded in radio-tracking studies undertaken in winter along rivers in New South Wales (Grant 1983; Grigg et al. 1992) or a subalpine lake in Tasmania (Bethge et al. 2009), implying that cold weather is necessary but not sufficient to trigger torpor in this species.
Photo: APC
LITERATURE CITED
Bethge P, Munks S, Otley H and Nicol S (2009) Activity patterns and sharing of time and space of platypuses, Ornithorhynchus anatinus, in a subalpine Tasmanian lake. Journal of Mammalogy 90, 1350-1356.
Booth R and Connolly J (2008) Platypuses. Pp. 103-132 in Medicine of Australian Mammals (L Vogelnest and R Woods, eds) CSIRO Publishing: Collingwood VIC.
Grant TR (1983) Body temperatures of free-ranging platypuses, Ornithorhynchus anatinus (Monotremata), with observations on their use of burrows. Australian Journal of Zoology 31, 117-122.
Grant TR and Dawson TJ (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.
Serena M (1994) Use of time and space by platypus (Ornithorhynchus anatinus: Monotremata) along a Victorian stream. Journal of Zoology 232, 117-131.