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Echidnas Have a Nose for Ecological Engineering

Echidnas move in the wild at an average speed of 1.3 km/h

Echidnas move in the wild at an average speed of 1.3 km/h, with a maximum speed of only 2.3 km/h.

By Christine Cooper

Activity loggers have revealed that echidnas turn over 200 cubic metres of soil each year, making them one of Australia’s most important remaining ecosystem engineers.

To meet their energy and water needs, short-beaked echidnas (Tachyglossus aculeatus) dig to find around 40,000 ants and termites daily. In the process they turn over, or bioturbate, large amounts of soil.

New, custom-built miniature activity loggers have enabled us to show that a single echidna can move about 200 cubic metres of soil per year; this is the equivalent of an Olympic-sized swimming pool for every 12 echidnas. Echidnas are therefore important bioturbators; they break up and loosen the soil, improve water penetration, reduce run-off and erosion, and incorporate organic matter into the soil profile.

Since many of Australia’s other digging mammals – such as bettongs, potoroos, bilbies and other bandicoots – have suffered dramatic declines in their distribution and abundance since European settlement, the widespread echidna is vital to maintaining ecosystem health throughout Australia. However, warmer temperatures associated with human-induced climate change might impact on the extent of their important ecosystem function.

Echidnas are remarkable mammals. They are monotremes, which means they lay eggs like reptiles but have hair and feed their young on milk like other mammals.

Marsupial and placental mammals diverged from monotremes 170 million years ago, so monotremes represent an ancient lineage of great general interest to scientists. There are five living monotreme species: the Australian platypus (Ornithorhynchus anatinus), three long-beaked echidnas (Zaglossus)in New Guinea, and the short-beaked echidna found in Australia and New Guinea.

The short-beaked echidna is the most widespread Australian mammal, occupying all major terrestrial Australian habitats from snow-covered alpine areas to hot, dry deserts. Due to the protection offered by its sharp spines, introduced predators such as cats and foxes have not had the impact on echidnas they have had on other native Australian mammals.

Echidnas remain relatively common throughout Australia, and this, together with the large amounts of soil they move while foraging, mean they are important ecosystem engineers throughout the continent, maintaining bioturbation and enhancing ecosystem function where the impact of other digging mammals is now limited or entirely absent over much of the Australian landmass.

As well as having a curious mammalian reproductive strategy – females lay an egg that they carry in their pouch for about 11 days before it hatches – short-beaked echidnas have other highly unusual characteristics. They have a much lower resting body temperature than other mammals (only 30°C compared with 35°C for a kangaroo or 37.5°C for a human); their body temperature is also quite variable, and their metabolic rate is only 22% as high as an equivalent-sized mammal.

Echidnas also have an unusual anatomy, partly as a consequence of their evolutionary history and partly as they are adapted to a digging lifestyle. Their shoulders and hips have retained some primitive reptilian characteristics, and they have a sprawling gait and short, stout limbs that generate power when digging. Their hind feet are so modified for digging that they are rotated and point backwards!

Observations of these characteristics raise questions about how these structural and physiological features impact on the echidna’s ability to move around in their environment, how their movement is impacted by seasonal weather changes, and how much they dig.

The widespread availability of miniature electronic devices for applications such mobile phones and fitness trackers has changed the way we can study animals in their natural environment. We can now collect data on the private lives of wild animals without having to observe them directly, which might influence their behaviour. This means that it’s now possible to remotely investigate questions about a particular species’ biology, and also about its interrelationships with the broader ecosystem. Miniature custom-made activity trackers glued onto the spines of echidnas provided us with three values concerning echidnas’ movements – pitch, roll and yaw – up to 200 times per second for a week in summer and again in spring. Small GPS units provided information about the echidnas’ location, and radio transmitters enabled them to be re-located to retrieve the logged data.

From these data we determined when and where echidnas were resting, walking and digging. Their rolling gait even allowed for the identification of individual steps, providing a detailed insight into the echidna’s day-to-day activity, movements and biomechanics, and how they survive in, and impact on, their environment.

We found that echidnas in the wild moved at an average speed of 1.3 km/h, with a maximum speed of only 2.3 km/h. These slow movements are due to the structural constraints of the echidna’s short, powerful legs. While these provide the mechanical force to dig into soil and even termite mounds, they limit the rate and particularly the length of their steps, thus reducing their walking speed. Indeed echidnas have the shortest relative stride length of any mammal.

The structural constraints of the limbs forces echidnas to change their speed in an unusual way. To move faster, most mammals take longer rather than quicker steps, but echidnas can’t increase the length of their steps sufficiently due to the structure of their short legs, so instead they must take more, faster steps. This means that echidnas can’t move particularly quickly, but their armour of sharp spines protects them from predators. Other mammals such as armadillos and pangolins that have also traded off speed for an improved ability to dig are also armoured; it is not necessary to run quickly if predators can’t eat you.

During the hot Western Australian summer, when temperatures reached 45°C during the day, echidnas were nocturnal. They sheltered in burrows, caves, logs, and under leaf litter through the heat of the day, and then at night “sprinted” at their top speed to foraging sites, where they would dig for ants and termites before returning to their shelters after 3 hours of activity.

In spring, when it was 25°C cooler, echidnas were sometimes active during the day, and they wandered between retreats and foraging sites at a more leisurely pace. Hot weather constrains their foraging to more favourable periods at night, while in milder conditions there is more time suitable for activity.

Remaining inactive during temperature extremes makes it easier for echidnas to maintain their low body temperature and saves water, and also aligns their foraging to times when their food is most available. Termites are closer to the soil surface when temperatures are mild, and retreat deeper underground when it is very hot or cold.

Despite the limitations of their limbs and their low metabolic rates, echidnas cover large distances each day of 2.7 km in summer and 3.6 km in spring, moving to and foraging at wide-spread feeding sites. Echidnas move further than expected for a similar-sized mammal, reflecting the sparse and patchy availability and low energy density of their insect prey.

Termites and especially ants are difficult to digest as their tough external skeletons are made of chitin, a typically indigestible protein. They also use chemical and physical defences to deter predators, so the length of an echidna’s feeding bouts are limited by the soldiers that defend the colony.

By licking up ants and termites with a long tongue coated with sticky saliva, echidnas also ingest dirt when feeding, which further reduces the total energy content of a meal. Echidnas must therefore forage widely to find sufficient food to meet their daily requirements.

Echidnas dig up an area of 0.03–0.04 km2/day, moving about 0.56 m3 of soil in this time. This digging, combined with the continent-wide distribution of echidnas and their relative abundance, means that they are one of Australia’s most important remaining ecosystem engineers. Their digging plays an important role in improving ecosystem health by enhancing the structure and fertility of the soil. This role used to be shared by a multitude of digging mammals, but various threatening processes, such as introduced predators, land clearing, increasing aridity, changed fire regimes and introduced herbivores have reduced them to such low levels – or driven them locally or even globally extinct – that their bioturbation is no longer sufficient to maintain this ecosystem function.

Despite their current high digging rates, reduced activity during periods of high temperature may mean that echidnas’ digging, and therefore their role in providing this ecosystem service, may be reduced with human-induced global warming. As temperatures increase, the energy needed to keep warm may decrease, so echidnas may need to find less food. Food availability may also decrease, and time with sufficiently low temperatures for activity may be impacted, together leading to less digging.

It remains to be seen how the role of echidnas in maintaining Australian ecosystems continues in an ever-changing environment.

Christine Cooper is a Senior Lecturer in the Department of Environment and Agriculture at Curtin University.