Going Underground - animal architects of the natural world

By Charlotte Owen

WildCall Officer

The number of different homes inhabited by animals is huge, so in this blog, we're going to look at underground homes. Because, as The Jam would put it, if you want to create a warm, dry and cosy home safe from prying eyes and predators, going underground is a popular option.

Badgers live in family groups in a system of underground tunnels and chambers called a sett. Some badger setts have been in constant use for centuries, with hundreds of metres of tunnels and multiple entrance and exit holes. Badgers love digging and their strong claws are perfectly suited for powering through mud, clay and chalk and shifting vast quantities of soil, creating characteristic spoil heaps. A hole made by a badger is usually the shape of a sideways D, much wider than it is tall, and this mimics the shape of a broad, bulky, low-slung badger. Badgers live in their sett all year round and each family group has a main sett in the middle of their territory, with several smaller ones – known as outliers – throughout, that may be occupied seasonally or occasionally.

Foxes dig a much simpler structure known as a den or earth, which is a single chamber at the end of an entrance tunnel. They use these mainly during the breeding season, when vixens need a safe place to rear their cubs. Unlike badgers, foxes don’t indulge in any creature comforts and cubs are usually born onto bare earth without any bedding material. Urban vixens will often den under garden sheds and can squeeze through surprisingly small gaps to reach a convenient sheltered spot. If there isn’t a ready-made home, vixens will dig out a den in well-drained soil, sometimes in a flowerbed, under tree roots or beneath a dense hedgerow.

Rabbit warrens can be very complex structures with long, winding tunnels and branching corridors between multiple chambers, creating an underground maze – helping to keep the occupants safe from pursuing predators like stoats and polecats. Rabbit holes are usually a smooth oval shape, and the overall structure of a warren looks very similar to the branching pattern of tree roots as the rabbits tunnel their way through the underground paths of least resistance.

Moles spend their entire lives underground. The only clue to a mole’s presence are the molehills, or spoil heaps, created as a result of their tunnelling activity. They are highly territorial and each mole lives in its own network of tunnels called a fortress. At the centre is nest chamber, where the resident mole sleeps and females raise their young. The rest of the mole’s tunnels serve as a giant worm trap. Unsuspecting earthworms, or any other underground invertebrates, will eventually fall into the mole’s tunnel network and as soon as they do, they are quickly detected by the resident mole thanks to its super-sensitive whiskers. The mole’s saliva contains a toxin that paralyses earthworms, and they will often store their still-living prey in special underground larders to keep it fresh. 

Water voles tunnel into step, grassy banks to create their riverside burrows, which often include at least one underwater entrance. This makes sure they can retreat to safety while swimming or escape unseen into the water if threatened on land. Water vole holes are oval and 5-8 cm wide, and those in the top of the bank may be surrounded by a neat lawn of closely-cropped grass. In reedbeds and other areas where they cannot burrow, water voles will weave a football-sized nest in the stems, securing it above the water line.

Otters make use of couches, holts and hovers to rest, sleep and breed. A couch is a roughly circular hollow, usually in grass, formed by an otter resting up in the same place time after time. The grass around the edges, where it’s not worn down, eventually grows up and forms an enclosing mound, a bit like a giant nest. A hover is a sheltered resting spot or bolt hole, maybe in the roots of an old riverside oak, a crevice in a rock pile or some dense scrub. A holt is used by breeding females, and this is where the cubs will be born and raised. Otters will sometimes tunnel into river banks or make use of existing holes and abandoned burrows, or natural overhangs and other features that provide plenty of cover.

Beavers live in family groups inside a lodge, which is either a system of burrows tunnelled into the bank – a bit like a giant water vole burrow – or built from branches, twigs and soil. The entrance is usually hidden underwater. Sometimes a lodge is built in open water rather than bankside, and resembles a giant pile of logs a bit like an unlit bonfire.