Why Do Wombats Have Backward Pouches? Digging Explained

How does a backward pouch work?

Like other marsupials, the common wombat gives birth to a tiny, undeveloped baby after a very short pregnancy of about 20–30 days. The newborn is about the size of a jelly bean, weighs less than 1 g (0.04 oz), and has to crawl through the mother’s fur into the pouch on its own.

Inside, it attaches to one of two teats and stays there for its first weeks while it grows. In a wombat the pouch opening faces backward and down, toward the base of the mother’s tail. Around the opening is a ring of muscle called a sphincter, which can close the pouch tightly like a drawstring bag. This is why a joey does not fall out, even though the opening points toward the ground.

A digging wombat throws soil backward with all four feet and shoves loose dirt behind her with her body. A forward-facing pouch would sit directly in the path of that soil. With the opening at the rear and held shut, the joey stays clean and protected while its mother tunnels.

A wombat joey’s journey from pouch to independence

Wombat young develop slowly, a pattern that matches their slow metabolism and long lives.

When the joey first leaves the pouch, it is already furry and can weigh several kilograms. It follows its mother closely as a “young-at-foot,” shelters in her burrow and still pokes its head back into the pouch to drink milk for months.

Why does a koala have a backward pouch too?

The koala spends its life in trees and never digs a burrow, yet its pouch also opens toward the rear. The usual explanation is shared ancestry. Wombats and koalas are each other’s closest living relatives and belong to the same group of marsupials, the Vombatiformes. Most researchers think their common ancestor was a ground-dwelling digger, and the koala kept the rear-opening pouch after its ancestors moved into the trees.

In koalas the trait turned out to be useful in another way. Before a koala joey can eat eucalyptus leaves, its mother produces a soft, runny dropping called pap, which contains the gut microbes the young koala needs to digest the tough leaves. Because the pouch opens near the mother’s rear, the joey can easily reach it. A strong sphincter keeps the joey from slipping out while its mother climbs. Read more about the koala’s family tree in are koalas bears?

How do pouches differ among marsupials?

Not all marsupials have the same kind of pouch, and some barely have one at all.

• Forward-opening pouches: Kangaroos and wallabies have deep pouches that open toward the head. An upright, hopping mother needs the opening at the top so the joey does not fall out. A red kangaroo joey stays in the pouch for around eight months; see what a baby kangaroo is called.
• Rear-opening pouches: Wombats, koalas, bandicoots, bilbies and the burrowing marsupial moles all have pouches that open backward. Several carnivorous marsupials, including the Tasmanian devil, have rear-facing pouches too.
• Little or no pouch: Some small marsupials, such as the numbat and a number of mouse-sized species, have only folds of skin around the teats, and the young simply cling to the mother’s belly.

The direction of the pouch often matches how the animal lives. Diggers and four-legged ground animals tend to have rear-opening pouches, while upright hoppers have forward-opening ones. It is a good example of how anatomy fits a way of life.

Myths and facts about the wombat pouch

Myth: wombat joeys get covered in dirt. The point of the rear opening and the tight sphincter is exactly the opposite. The pouch stays remarkably clean even in a busy burrow.

Myth: the pouch is upside down, so joeys fall out. The joey is attached to a teat for its first months and the muscle around the opening holds it in. Older joeys climb in and out on purpose.

Fact: male wombats do not have pouches. As in other marsupials, only females have a pouch. Males are similar in size and shape, which makes the sexes hard to tell apart at a distance.

Fact: rescued joeys need specialist care. In Australia, orphaned wombat joeys from road-killed mothers are raised by licensed wildlife carers. If you find an injured wombat or a joey in Australia, contact a local wildlife rescue organization rather than handling it yourself.