I have been really sleepy lately. But even with how tired I’ve been feeling, one animal can still out-sleep me: the koala.
Koalas (Phascolarctos cinereus) hold the record for the sleepiest animal, sleeping between 18 and 22 hours a day. These small animals are arboreal and spend almost their entire lives in the eucalyptus trees of eastern Australia. Koalas will only leave their trees when deciding to move to a different one!
There’s only one species of koala, but some scientists think there are multiple subspecies. Koalas living in the south tend to be larger and have thicker, browner fur than their northern cousins. These could be adaptations in response to the colder winters in southern Australia.
Koalas are marsupials like kangaroos and not bears. When Europeans first arrived in Australia, they thought koalas looked like large teddy bears with their fluffy fur and big ears. And in the settlers’ defense, it’s possible that no European had ever seen a marsupial before and had no idea what it was.
But of course, Australia’s Indigenous people knew what koalas were. Koalas are thought to have evolved on Australia before humans ever made it to the continent. Once humans arrived, koalas became an important part of Aboriginal culture. In fact, ‘koala’ is thought to mean ‘no drink’ in an Aboriginal language, referencing the fact that koalas get most of their water from their food.
Like other marsupials, koalas raise their young in pouches. But koalas have an outward-facing pouch, unlike kangaroos, which have an upward-opening pouch. When the young joey leans out of the pouch, the pouch is pulled down, and it appears that the pouch is ‘backward.’ This pouch orientation seems to be a holdover from when prehistoric koalas were a) as large as a bull; and b) lived in underground burrows. Wombats are close relatives of koalas and have backward-facing pouches to stop dirt from getting in their pouch when they dig.
Koalas are one of the few mammals that can eat eucalyptus leaves. In addition to their high water content, eucalyptus is full of fiber and has a high percentage of toxic tannins that are poisonous to most other animals. Plus, there’s very little nutritional value in eucalyptus leaves to begin with. So how are koalas able to eat it?
To start, koalas are surprisingly picky. Out of the over 600 species of eucalyptus trees, koalas only eat around 30 and prefer just two or three species in a particular area. Koalas will even avoid certain trees if they’re growing on less fertile soil, as they seem to have more toxins than those growing on fertile ground. This is where their large, leathery nose comes in – koalas can smell different toxicity levels in eucalyptus leaves and preferentially pick out ones with the least amount of toxins.
Koalas also have a very long caecum for digestion. We humans also have a caecum, but ours are comparatively very short. In humans, the caecum is about 2.4 inches (6 cm) long and aids in absorbing fluids and salts as food moves from the small to the large intestine. In contrast, a koala caecum is 78.75 inches (200 cm) long and full of special microbes that help break down the fiber found in eucalyptus leaves.
But koalas aren’t born with these special, eucalyptus-digesting microbes! When young koalas reach around 22 weeks old, they start peeking out of their mother’s pouch and begin eating a substance called ‘pap.’ Pap is essential to transition koalas from milk to eucalyptus. It’s a specialized form of poop and the source of a koala’s specialized gut microbes. This is another reason koalas might have outward-facing pouches: it’s much easier for the joeys to reach their mother’s pap!
Sleepy little guys
As I said before, koalas spend a lot of time sleeping. They typically spend 90% of their day sleeping, with the other 10% allocated to eating eucalyptus. This ends up being between 18 and 22 hours a day spent snoozing.
A koala’s eucalyptus diet may be to blame for this high level of sleepiness. Remember, eucalyptus doesn’t have much in the way of nutrition. To squeeze out every calorie, koalas have a very slow metabolic rate that keeps food in their digestive tract for long periods. This slow metabolic rate minimizes their energy requirements. In addition, because they don’t get much nutrition from their food, koalas sleep to conserve the few calories they do extract from eucalyptus.
A koala’s bed is the eucalyptus trees it eats from. Because they spend so much time in trees, koalas have gained multiple adaptations to stay comfortable. For example, they only have a vestigial tail that doesn’t get in the way of sitting in the crooks of trees. Koalas also have increased cartilage at the base of their spine and thick, densely packed fur on their rump that serves as a cushion.
Koalas also keep their place in trees because of their unique feet.
Each paw has five digits, which is typical for a mammal. But unlike most mammals, the fingers on a koala’s front paws are organized so that two digits are grouped opposite the other three. Think of it like the koala has two thumbs on each hand. Having their fingers oriented this way helps koalas grip tree trunks extra securely.
And then there are the feet. There is a large, opposable ‘thumb’ that has no claw and is used for gripping branches. The second and third digits on the hind paws are fused together into a double-clawed digit that koalas use to groom their fur.
So now you know what koalas aren’t considered to be bears: they don’t have the correct…koala-fications! (Like ‘qualifications.’ Get it? …I’ll see myself out).