You’ve heard about about platypuses, the monotreme egg-laying mammal that lays eggs, a primitive condition inherited from the ancestor of all modern mammals (and their earlier reptilian ancestors). But perhaps you also know of the “echidnas“, or “spiny anteaters” (not very related to regular anteaters), also in the order Monotremata and the only other egg-laying mammal. (These are not marsupials; they diverged from the placental/marsupial mammal group, called therians, between 250 and 160 million years ago.)
The living monotremes comprise four species of echidna and only one species of platypus (Ornithorhynchus anatinus), and these two groups diverged from each other between 57 and 21 million years ago. Further, the monotremes diverged from the “regular” (therian) mammals between 218 and 187 million years ago.
The article at hand is about one of the four echidna species, the short beaked-echidna (Tachyglossus aculeatus). Here’s what it looks like:
Click on the article below to read about how echidnas keep cool in the hot climate of Australia (the pdf is here, the full reference is at at bottom, and there’s a popular article here. But the article below, from Biology Letters published by Britain’s Royal Society, is short and easy to read:
So the issue is this: earlier studies had demonstrated that this species had a low thermal tolerance, with a lethal core body temperature of 38ºC (100.4º F) and a lethal air temperature of just 35ºC (95º F). (From now on I’ll just give the Celsius temperatures, as you should get familiar with the conversion.) Yet the echidna is found in Australian habitats where the air temperature is higher than this, so they must have a way to cool off. The paper reports thermal-imaging studies of wild echidnas to see how they do this.
Clearly, the echidna must have some way to lower the temperature it encounters in the wild, which the authors call a “thermal window” or “regions of the animal’s body surface that vary heat exchange with the environment being ‘opened’ or ‘closed’ by changes in exposure and/or blood flow.” (Below are some cool examples of how other species do this.)
The authors measured the echidnas’ body temperature by thermal imaging, and estimated the ambient temperature as the average of the air temperature and tje ground temperature. They also measured a “wet bulb” temperature, which is the temperature measured by a wetted thermometer bulb. Wet-bulb temperature is cooler than the air temperature because the evaporation of the water from the bulb cools it off.
They found two ways that echidnas cool themselves off at higher temperatures. The temperature comparisons of echidna body parts with environmental temperature was measured by plotting, over a variety of echidnas observed at different temperatures, the wet bulb temperature (x axis) versus the surface temperature of the animal (y axis). That’s shown below, but first one observation.
The first way of cooing the authors found was seeing the animals press their relatively furless (and spineless) inner leg an belly surfaces against the cool soil. This is similar to what kangaroos do; see below. The spines also help keep the sun off their bodies, and there is a subcutaneous fat layer, into which the spines are embedded, that also provides insulation.
The second way of cooling is the swell finding given in the headline. You can see it below in the lower right section of the following graph. It shows body temperatures versus wet bulb temperature for various parts of the echidnas’ bodies (remember, this is done by thermal imaging). The body areas measured are shown in green, and the body temperatures measured at varying wet-bulb temperatures for each body area, are shown as dots, one for each echidna part measured. Measurements were done on 124 echidnas (some may have been duplicates, as they couldn’t identify individuals) at the Dryanra Woodland and Boyagin Nature Reserve in the West Australian wheatbelt, 170 k southwest of Perth, in western Australia:
What you see is that the surface temperature (the height of the dots) is, for six regions of the body, higher than the wet-bulb temperature, which means there’s no evaporative cooling of those warm body surfaces. But look at the “beak tip” at lower right. At all the wetbulb temperatures, the beak tip temperature is the same as the wet-bulb temperature. That means that somehow there is extra cooling going on at the tip of the snout.
How do they do this? They blow snot bubbles out their nose, which, when they burst, keep the nose moist, thus cooling the echidna. In effect, the the beak is a “snot bulb.” Or, to quote the authors:
We identify the beak tip of short-beaked echidnas as a unique type of evaporative window. The beak tip, containing a large dorsal blood sinus, is kept moist to facilitate electroreception. An additional role of this moist surface is evaporative cooling of the underlying blood within the sinus; with a slope equivalent to 1 and minimal intercept, the beak tip functions as a wet bulb globe thermometer. At high Ta [air temperature] echidnas blow mucus bubbles, adding moisture to the beak tip . This unique nasal evaporative window is of particular value for echidnas (which do not pant, lick or sweat) especially under conditions where environmental temperature exceeds Tb [core body temperature] and evaporation is the only avenue available for heat loss.
Here’s a video from Science News about the cooling. Note that here the lightest areas are the hottest and the darkest are the coolest. Check out the snout tip, circled at 5 seconds in. It’s very dark!
Upshot: Echnidnas have evolved to cool themselves off by blowing snot bubbles when it’s hot. The bubbles’ bursting keeps the animal cool, especially because the snout is well equipped with lots of blood vessels that radiate the heat.
Here’s the authors’ description about how other species use evaporative cooling, including the fact that kangaroos lick their forearms to cool off when it’s hot:
. . . there have been few descriptions for endotherms of specialized evaporative windows where endogenous water is behaviourally applied to areas with specialized vasculature. The classic examples of evaporative windows are for storks and turkey vultures, which urinate on their legs that contain extensive subcutaneous vascularization, facilitating EHL. Seals on rocks similarly urinate to wet their ventral surface and vascularized flippers to enhance EHL, while the licking of vascularized forearms by macropods is the best-known mammalian example.
Storks and seals piss on themselves to cool off! I bet you didn’t know that.
Here’s an Attenborough video of red kangaroos (Osphranter rufus) cooling themselves by licking their highly vascularized forearms (you can skip to 2:05 to see it, as well as thermal images showing the cooling). They also stay in the shade and dig down into the cooler soil beneath the surface and lie down on the cool soil.
Now we can add to these examples of evaporative cooling the snot bubbles of echidnas.
h/t Greg Mayer
Cooper C. E. and Withers PC. 2023. Postural, pilo-erective and evaporative thermal windows of the short-beaked echidna (Tachyglossus aculeatus).Biol. Lett.19: 20220495
Flannery, T.F., T.H. Rich, P. Vickers-Rich, T. Ziegler, E.G. Veatch, and K.M. Helge. 2022. A review of monotreme (Monotremata) evolution. Alcheringa 46(1): 3-20.