Cool creatures
Echidnas are highly individual in many ways.
At 33°C, echidnas have the lowest active body temperature of all mammals except the platypus. Most mammals daily body temperature only varies by a degree or so, but the echidnas can fluctuate by up to 6–8°C, dropping down to 28°C without any problems.
They're very adaptable in their use of torpor, an energy saving strategy which they can use at any time of the year.
In 1987 Gordon Grigg and Lyn Beard and Michael Augee radio tracked echidnas living in Kosciuszko National Park and were the first to describe hibernation in montremes. The echidnas dropped their body temperature to 4°C and reduced their breathing rate to one breath every three minutes. Every couple of weeks they would warm up over about a 12-hour period to a normal active temperature then soon fall back to near ambient temperatures, the same pattern of hibernation as seen in placental mammals.
The fact that echidnas hibernate has important evolutionary implications. Many scientists see hibernation as 'primitive', that mammalian hibernation in winter is a reversion to reptilian metabolism. Others think it's a very advanced ability.
The echidna is evolutionarily old, having split from other mammals over 120 million years ago but it certainly doesn't fit easily into our ideas of what is primitive and what is evolutionary success.