Wombat

The wombat is a stocky, muscular Australian marsupial built for one job above all others: digging. It spends its days underground in burrow systems it excavates itself, and it has a set of adaptations — from its pouch to its backside to its famously geometric droppings — that all connect back to that burrowing life.

Overview

Common wombats live in the cooler, wetter parts of southeastern Australia and across Tasmania, wherever the soil is soft enough to tunnel and there is native grass to eat. They are mostly nocturnal, emerging at dusk to graze and retreating underground by morning, and they are largely solitary, though burrow systems are sometimes shared or passed between animals.

Physical characteristics

A wombat is about a metre long, low to the ground, and can weigh up to 35 kg — nearly all of it muscle. It has powerful shoulders, short splayed legs, and long flattened claws on broad feet for excavating soil. Its teeth, like a rodent’s, grow continuously to keep up with a diet of gritty, abrasive grass.

Two features stand out:

• The pouch faces backwards, so it does not fill with dirt while the mother digs.
• The rump is armoured — a shield of fused bone, cartilage, and thick skin that the wombat uses to plug a burrow entrance and to crush an intruding predator against the tunnel roof.

The cube-shaped droppings

The common wombat is the only animal known to produce cube-shaped faeces — around 80 to 100 small dry cubes a day, deliberately deposited on raised surfaces as territorial scent markers. The shape forms in the final part of the intestine, where the wombat’s unusually dry droppings are compressed against a gut wall that has alternating stiffer and stretchier regions, moulding flat faces and corners. Cubes have the practical advantage of not rolling off the rocks and logs the wombat wants to mark.

Diet and digestion

Wombats are grazers, feeding on native tussock grasses, sedges, roots, and bark. Their metabolism is one of the slowest of any mammal, and food can take up to two weeks to pass through the digestive tract — an adaptation to a low-nutrition diet and a dry environment that lets the wombat extract nearly every calorie and drop of water.

Reproduction

A single joey is born after a pregnancy of about 21 days, crawls into the backward-opening pouch, and stays there for six to nine months. It then rides at the mother’s heels and shelters in her burrow for several more months before becoming independent. Females breed roughly every two years.

Threats and conservation

The common wombat is listed as Least Concern but is declining in parts of its range. The main pressures are:

• Habitat loss to farming and development, and disturbance of burrow systems.
• Sarcoptic mange, a mite-borne skin disease introduced with foxes and dogs, which is often fatal and has devastated some local populations.
• Vehicle collisions, a major cause of adult death.
• Persecution as an agricultural pest, since wombat burrows undermine fences and paddocks.

The northern hairy-nosed wombat, a separate species, is one of the rarest mammals on Earth — down to a single population of about 300 animals — and is the focus of intensive fenced-reserve conservation.