Although echidnas are pretty common, I’ve rarely seen seen them in Berowra.  So when I saw this one flopped out in the neighbours’ front yard on my sweaty walk home from work yesterday – ambient temperature in the mid 20s even at 7pm – I was worried.  Were echidnas, like fruit bats and stingless bees, troubled by temperature extremes?  Had this one been driven from its usual secluded haunts by this horrid heat wave?  Should I be on the blower to WIRES?

A cursory google did not inspire confidence.  Wikipedia notes that echidnas don’t sweat (in fact, they don’t pant, lick, or even wee on themselves and flap their wings to cool off either) and suggests that they “cannot deal with heat well“.  WIRES agrees that echidnas “cannot tolerate temperatures above 30 degrees“.

Notwithstanding, this seemed to be a very sprightly monotreme.  Most of the echidnas I’ve met have more or less fixed this position:

This beastie, however, was very busy, ignoring me and rummaging around in the flowerbeds.  Could it really be in trouble?  I decided, for the moment, that human intervention wasn’t necessary and then went home to check whether I’d just made a stupid decision.  Should I trust Wikipedia or my instincts?

I hit pay dirt when I found “Thermoregulation by Monotremes” by Queensland’s “Mr Hot Echidna”, Peter Brice.  As a researcher who has devoted time to  inserting “calibrated temperature sensitive radio-transmitters … coated with a smooth layer of inert wax… into the abdominal cavities of echidnas” (Brice et al 2002), he’s your go-to guy on this topic.

Niche subject as it might seem, the way echidnas and platypuses manage their body temperature has played an interesting role in propping up a hierarchical, human-centred view of evolution.  You’ve heard the story before, I’m sure. From this viewpoint, animals whose bodies are set up differently to us fancy-pants “classic mammals” are viewed as “primitive”.  Early twentieth century researchers decided that, for instance, that playpus had “an inadequate regulating mechanism” when after only 17 minutes at 35 degree temperatures, their research subject  “turned onto its back and fainted” (Brice, 2009, 260).  A poignant tale and the sort of thing that led one early twentieth century researcher to conclude that monotremes were “‘the lowest in the scale of warm-blooded animals’” (Brice, 2009, 256)

Don’t get me wrong, echidnas are very weird animals.  And that’s leaving aside the egg laying mammal business and the once-venomous spurs in their backwards facing hind legs.

Echidnas are cool. 31 degrees counts as a “normal” body temperature for them, though only females incubating eggs really keep their temperatures steady for long.  They seem to occupy a half-way house between warm blooded and “cold blooded” animals.  Echidnas don’t entirely rely on their surrounding environment to warm themselves as reptiles – ectotherms – do.  But unlike us humans – homeothermic endotherms – with our tedious need for a stable and predictable body temperature – echidnas can run hot or cold.

Just when you thought we were all “thermed”-out, you find that echidnas are also constitutional eurytherms – they can handle wide range of temperatures – and facultative endotherms – they warm their bodies up, in part, by doing stuff.

Echidnas also hibernate – well, some of them do, if they’ve eaten enough by the time the cold weather rolls in. But their hibernation, or its timing at least is, according to Brice (2009, 257) “distinctly odd”.  They often kip out during the late summer and then wake up to mate at coldest part of the year.  And they can deal with fluctuating temperatures in a way I can only envy – for instance, after a chilly night, they can dig their way out of shelter with their blood temperature of only twenty degrees.  With no bedsocks.

And they can also deal with a day spent in a hollow log at 40 degrees C without expiring from the heat, though no one knows exactly how.

We shouldn’t seeing echidnas and their wildly fluctuating temperatures as primitive, I reckon. Instead we should admire the way these critters harbour resources with their furry but weirdly chilly bodies, helping them live above the snowline in the Alps, in the desert and even on the sizzling streets of deepest surburbia.

References

Brice, Peter H. (2009) “Thermoregulation in monotremes: riddles in a mosaic” Australian Journal of Zoology, 2009, 57, 255–263

Brice, Peter H., Gordon C. Grigg, Lyn A. Beard, Janette A. Donovan (2002) “Heat tolerance of short-beaked echidnas (Tachyglossus aculeatus) in the field” Journal of Thermal Biology 27(6), 449-57