How do mammals adapt to life underground?

How do mammals that live underground survive in dark, airless burrows? What are the main challenges they face, and how do they adapt and modify their environments?

How do mammals adapt to life underground? spread

How do mammals that live underground survive in dark, airless burrows? What are the main challenges they face, and how do they adapt and modify their environments?

A surprising variety of mammals head underground. The burrows of aardvarks are used by steenbok to evade pursuing predators, and by warthogs as nocturnal refuges and breeding sites.

But most mammals that rely on tunnels and dens also dig their own. Some, such as badgers, emerge at night to forage but still spend about 80 per cent of their existence underground.

While we know a great deal about their behaviour above the surface, we know remarkably little about life in their subterranean labyrinths.

The drive to live underground first occurred 45–35 million years ago, as the world became much cooler and more arid.

Surface survival became more challenging, whereas more stable environments could be found underground.

However, this brought many new challenges: perpetual darkness, high humidity, low oxygen but high carbon dioxide levels, and increased disease transmission.

To overcome these problems, many of the specialist burrowing (fossorial) mammals show very similar characteristics that evolved independently.

The most extreme adaptations are seen in several hundred species of rodents, insectivores and marsupials.

Common senses

While most have poor vision, their circadian rhythms are still determined by the outside photoperiod, presumably to ensure that they do not emerge when predation risk is highest, and to time their reproductive cycles.

Instead, many have highly developed tactile senses, with sensory hairs all over the body, especially the tail. Even otherwise hairless species such as naked mole rats have tactile hairs.

Perhaps most remarkable are the 22 appendages ringing the nostrils of the star-nosed mole, each covered with sensory domes called Eimer’s organs, used to identify potential prey.

This animal’s sense of touch is so sensitive that it is able to identify and consume a small prey item in 120 milliseconds – the fastest-known mammal forager.

Many subterranean mammals are adapted to hear low-frequency sounds, which travel best underground, and are insensitive to the high frequencies used by above-ground species.

Some even have highly developed middle ears capable of detecting low-frequency seismic sounds transmitted through the soil. Since they still need to be able to orientate underground, some, at least, rely on magnetic orientation.

The energetic costs of digging are high, especially for species that have to burrow to obtain their food. Most fossorial mammals are small, so need to excavate smaller tunnels that are easier to dig.

Being compact also has the advantages of limiting general energetic costs, which are further reduced by having lower metabolic rates, low body temperature and poor thermoregulation – adaptations that also help to minimise water expenditure.

The air down there

Since burrows may not be well ventilated, especially in wet or heavy soils, fossorial mammals must function in compromised air conditions.

Mole-rat burrows have been recorded with oxygen concentrations of a mere 7.2 per cent – fatal to humans – and carbon dioxide levels up to 6.1 per cent, which would induce unconsciousness in us.

Fossorial mammals are extremely important in many environments, where they have widespread ecological impacts.

They can relocate large quantities of soil, often changing its structure in the process: North American pocket gophers excavate on average 18m3 of earth per hectare per year, while a single mole fortress can incorporate 750kg of soil.

So it is hardly surprising that many subterranean mammals are also considered agricultural pests.



The Naked Truth

Naked mole rats show some of the most extreme adaptations to subterranean life.

They cannot thermoregulate and can only survive between 15 and 38°C.

However, the temperature in their burrows remains constant around 29–30°C. Being naked helps them to transfer heat from the soil; when cold they huddle together or bask in shallow tunnels, moving to the deeper (cooler) tunnels when hot.

During periods of food shortage they can reduce their metabolic rate by up to 25 per cent.

Living life in pulses may prevent oxidative damage to their tissues and explain why they are so extraordinarily long-lived for a small mammal: up to 28 years has been recorded.


The Benefits of Burrows

Many burrowing mammals have a disproportionately large impact in relation to their population size.

Siberian marmots in Mongolia have declined dramatically due to overhunting; their burrows are important refuges for Pallas’s cats and corsac foxes.

Similarly, pikas are poisoned on the Tibetan Plateau because they damage grazing, yet their burrows provide homes for a variety of small birds and lizards, their digging increases plant species richness, and they are an important food source for many predators.

The loss of these mammals would have dramatic impacts on the local ecosystems.


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