Anyone who has visited a koala sanctuary hoping to see one in action knows the feeling: a grey, furry ball wedged in the fork of a tree, fast asleep. So why do koalas sleep so much? The answer is food. Koalas live almost entirely on gum leaves, one of the toughest, least nourishing and most poisonous diets of any mammal, and sleeping for most of the day is how they survive on it.
How much do koalas sleep?
Koalas may sleep for 18 to 22 hours a day, according to National Geographic, which puts them among the sleepiest animals on Earth. They’re mostly active at night and around dusk, which is why the koala you see on a day visit is almost always snoozing.
A diet that runs on empty
Koalas eat almost nothing but eucalyptus leaves, and an adult can eat more than half a kilogram of them a day. The problem is that gum leaves are a poor fuel, so the tank is always close to empty. They’re packed with tough fibre that’s hard to break down, low in protein and nutrients, and laced with chemicals that are toxic to most other animals. National Geographic explains that the koala’s digestive system has to work hard just to break down the toxins and extract what limited nutrients there are.
What’s left over in energy is very little. So koalas have adapted to live on a tight energy budget: they have a slow metabolism, they move as little as possible, and sleeping is the ultimate way to save energy.
A digestive system built for gum leaves
Koalas manage to live on such an unpromising diet thanks to some impressive internal equipment. They chew leaves thoroughly with strong, grinding teeth to break them into tiny pieces. Then, deep in their gut, they have an unusually large caecum, a pouch-like part of the intestine where bacteria slowly ferment the tough fibre and help break down the leaves’ toxins. Their liver is also specialised for dealing with those toxins.
The process is slow, so food stays in the koala’s gut for a long time, letting it squeeze every last bit of energy from each mouthful. It’s a system that works, but only if the koala doesn’t waste energy on anything else.
Koalas are also fussy eaters. They don’t eat just any gum leaves. Each population prefers a small number of eucalyptus species, and individual koalas pick and choose leaves from particular trees. That pickiness makes sense when every leaf is a compromise between nutrition and poison, but it also makes koalas very dependent on having the right trees nearby.
Are koalas stoned on eucalyptus?
You’ve probably heard that koalas sleep all day because they’re “high” on gum leaves. It’s a fun story, but it isn’t true. Koalas aren’t drugged or intoxicated by eucalyptus. Their bodies are well adapted to deal with its chemicals, and they sleep so much simply because the leaves give them so little energy. A sleepy koala isn’t stoned, just running on a very small battery.
A couple of other popular misconceptions are worth clearing up. Koalas aren’t bears, despite the old name “koala bear”. They’re marsupials, carrying their young, called joeys, in a pouch.
The name “koala” is often said to come from a word in the Darug language of the Sydney region meaning “no water”. That would fit, because koalas rarely drink from streams or ponds, getting most of their water from the leaves they eat and by licking rain off tree bark.
The tree-hugging secret to keeping cool
Sleeping in a tree has one big drawback in an Australian summer: there’s no escape from the heat. Koalas can pant or lick their fur to cool down, but both use up precious water. The tree-hugging secret was uncovered in 2014, when researchers led by Natalie Briscoe at the University of Melbourne used thermal cameras to watch 30 koalas on French Island in Victoria during hot weather.
They found that on hot days, the koalas moved lower down the trees and pressed their bodies against cooler trunks, as the team reported in Biology Letters. Trunks can be several degrees cooler than the air. Briscoe said that access to these trees can save about half the water a koala would need to keep cool on a hot day.
Interestingly, some of the coolest trees were acacias, which koalas don’t eat at all. According to the Royal Society’s summary of the study, the acacia trunks the koalas used more in hot weather were up to 8.9°C cooler than the air at the base. So a good koala habitat needs more than just food trees.
What koalas do when they’re awake
In their few waking hours, koalas mostly eat, usually at night. They move slowly between branches, or occasionally climb down to cross the ground to another tree, which is when they’re most at risk from dogs and cars.
During the breeding season, roughly from spring into summer, males also become much more vocal. If you’ve ever heard a loud, deep, snoring-and-belching bellow coming from the bush at night, it was probably a male koala advertising himself. It’s one of the more unsettling sounds in the Australian bush.
Why a sleepy lifestyle makes koalas vulnerable
Living on such a tight energy budget works well in a stable forest, but it leaves koalas very little room to cope when things change. In February 2022, the federal government upgraded koalas in Queensland, New South Wales and the ACT from vulnerable to endangered under national environment law, with the change taking effect on 12 February, as the Environmental Defenders Office explains. Reports by ecological consultants Biolink, submitted in support of the listing, found Queensland’s koala population had fallen by about half since 2001, according to the International Fund for Animal Welfare. In April 2022, the federal government released the first National Recovery Plan for the koala.
Koalas face land clearing, bushfires, drought, dog attacks, car strikes and disease, and they lost large areas of habitat in the 2019-20 bushfires. Drought adds another problem: as National Geographic notes, koalas are forced to stop napping and come down from their trees in search of water, burning precious energy and exposing themselves to predators. Rising carbon dioxide may also make gum leaves even less nutritious over time.
There are a few simple things that genuinely help:
- Slow down in signposted koala zones, especially at dusk and at night, when koalas are on the move.
- Keep dogs inside or contained at night if you live near koala habitat.
- Plant local eucalypts and other native trees if you have space.
- If you see a koala that’s injured, sick, or sitting on the ground in an unusual place, contact your local wildlife rescue service rather than trying to handle it yourself.
Not lazy, just very efficient
So koalas aren’t lazy, and they’re certainly not stoned. They’re one of the most specialised animals on Earth, living on a diet that almost nothing else can touch, and sleeping up to 22 hours a day is the price of that remarkable lifestyle. Next time you see one asleep in a tree, you’ll know it’s hard at work digesting its dinner.
Bodies everywhere are constantly balancing energy and temperature, as I explain in my post on homeostasis. And for another animal whose size and lifestyle were shaped by what it eats, see why are whales so big?
Sources
- National Geographic: Koala
- Briscoe et al: Tree-hugging koalas demonstrate a novel thermoregulatory mechanism for arboreal mammals, Biology Letters (2014), via PubMed Central
- PBS NewsHour: Koalas are tree-huggers because it’s cool
- Royal Society: Tree-hugging keeps koalas cool (2014)
- Environmental Defenders Office: Koalas formally declared at greater risk of extinction (2022)
- IFAW: Koalas uplisted to endangered (2022)
- Australian Government (DCCEEW): National Recovery Plan for the Koala (2022)