Echidnas blow snot bubbles to cool down

The evaporation of mucus helps spiny anteaters survive in temperatures once thought lethal

17 JAN 2023 7:01 PM
BY JACK TAMISIEA

Short-beaked echidnas may be one of nature’s strangest looking creatures. Resembling a pug-size hedgehog with the schnoz of an anteater, they are one of only five living species of monotreme, that rare mammal that lays eggs. They’re also encased in a coat of stiff spines and lack the ability to sweat—features that should kill them in the scorching deserts of Australia they call home.

Now, a team of scientists has discovered some of the strategies the echidna employs to stay cool. It blows snot bubbles and flexes its spine, scientists report today in Biology Letters, allowing it to shed heat in temperatures that can approach 40°C.

The find isn’t just a cool discovery, says Tahlia Perry, a zoologist at the University of Adelaide who was not involved in the new study—it could help scientists conserve these mysterious mammals, whose populations are declining in some areas because of habitat loss and vehicle strikes.

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Postural, pilo-erective and evaporative thermal windows of the short-beaked echidna (Tachyglossus aculeatus)

Christine Elizabeth Cooper and Philip Carew Withers
Published:18 January 2023https://doi.org/10.1098/rsbl.2022.0495

Abstract
We identify for wild, free-living short-beaked echidnas (Tachyglossus aculeatus) a novel evaporative window, along with thermal windows, and demonstrate the insulating properties of the spines, using infrared thermography. The moist tip of their beak, with an underlying blood sinus, functions as a wet bulb globe thermometer, maximizing evaporative heat loss via an evaporative window. The ventral surface and insides of the legs are poorly insulated sites that act as postural thermal windows, while the spines provide flexible insulation (depending on piloerection). These avenues of heat exchange likely contribute to the higher-than-expected thermal tolerance of this species. Our study highlights how technological advances that allow for non-contact measurement of thermal variables allow us to better understand the physiological capacity of animals in their natural environment.

1. Introduction
Monotreme and therian mammal lineages diverged between 160 and 250 million years ago [1] and monotremes retain many plesiomorphic anatomical, reproductive and physiological traits not shared with therians. Consequently, their biology provides important insights into mammalian evolution, with the short-beaked echidna (Tachyglossus aculeatus) considered a particularly useful living model for proto- or baso-endothermy [2,3]. One ongoing question is whether their perceived poor thermoregulatory ability is primitive or derived. While the echidna's low and variable core body temperature (Tb) can be considered a primitive trait reflecting their early divergence from therians, other thermoregulatory characteristics such as metabolic rate, thermal conductance and evaporative heat loss (EHL) capacity are typically therian [4] after accounting for their lower Tb. Despite this, laboratory studies suggest that echidnas have a low thermal tolerance, with a Tb of 38°C and air temperature (Ta) of only 35°C considered lethal [5,6].

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