Why do koalas eat eucalyptus leaves? Koala have evolved to eat eucalyptus leaves because it is the most abundant source of food where they. They question might be, have the evolved to eat the food where they live, or have they moved to those area because it is abundant in the food they can eat?
Eucalyptus leaves are very fibrous, low in nutrition, and quite poisonous, so there are not a lot of animals that can eat them. Some animals eat the leaves along with a lot of other foods, but the koala, the Greater Glider, and the Ringtail Possum are the only animals that can live exclusively on eucalyptus leaves.
Eucalyptus leaves contain a potent combination of toxins that would be very dangerous for most animals, including us, to eat. They contain a compound called eucalyptol, which can be toxic in large quantities. We can actually eat it if the amount is small enough and it is even used as a flavoring in some things. It has a spicy taste and is used to flavor confectionary and drinks. However, it is at levels of 0.002% and lower. If you or I tried to eat eucalyptus leaves, we would get sick, have a stomachache, and probably have diarrhea. The leaves taste really bad and are very hard to chew, so you would have trouble managing to eat enough leaves to be fatal. Although, if you set your mind to it, a couple of hundred grams should be enough. Drinking the oil would be more dangerous. 3.5 ml of pure eucalyptus oil can be fatal in humans.
Eucalyptus trees have evolved to be toxic for the same reason chilis are spicy and some mushrooms are toxic. It is a defense against insects and animals. In fact, this toxin is so effective that it is quite often used in insect repellents. It can be sed to clear head lice as well. There are 700 different varieties of eucalyptus and they all have different amounts of toxin in their leaves. It may appear that koala eat all eucalyptus leaves, but they are actually rather selective.
Koalas are marsupials that live in trees. They look like bears, but they are related to wombats. They grow to be about 75 cm long and weigh 10 kg. They have strong arms and legs, and strong claws so that they can climb trees. They only come down to the ground to move to another tree, or if they fall. Koalas have evolved to be able to eat eucalyptus leaves because it gives them an enormous food source with no real competition as there are very few other animals that can eat the leaves. However, there are some downsides to just eating eucalyptus leaves.
So, how do koalas eat eucalyptus leaves? Koalas have evolved special enzymes in their livers that can break down the toxins in the leaves. They also have different types of gut bacteria that can break down the toxins into less harmful chemicals. This is similar to the mongoose, which has evolved to be able to break down snake venom, and the vulture, which has evolved to be able to survive the bacteria living on rotting carcasses. This doesn’t mean that koalas are immune to the toxin, just as mongooses are not totally immune to snake venom. Koalas can cope with a certain level of toxin, and they select the leaves they eat based on that. They can tell by the smell and by the taste of a leaf if it will have too much toxin and they will leave those leaves and those trees alone. There are only really three types of eucalyptus that koalas eat.
There is more at issue here than just the toxins in the leaves. The eucalyptus leaves have a lot of fiber and they are not very energy dense. Koalas have very strong teeth that can grind up the fiber, and they have a long caecum, the entrance to the large intestine, full of bacteria that can break down the fiber into something the koala can use. The koala’s caecum is 200 cm long, and even then they can only absorb 25% of the fiber they eat. That puts them in the same boat as pandas and sloths. Pandas get around eating food that doesn’t have much nutrition in it by eating an enormous amount of it. They can eat 40 kg of bamboo a day. Sloths go the other way. They don’t eat more, they just move less. They try to use as little energy as possible. Koalas do the same thing and they move very slowly and sleep for 20 hours a day. They only eat about 400 g of eucalyptus leaves a day. Although, that would be enough to kill you or me. And this is what I learned today.
Try these next:
Sources
https://www.savethekoala.com/about-koalas/koalas-diet-digestion
https://koalaconservationaustralia.org.au/blogs/news/eucalyptus-secrets-key-to-koala-survival-health
https://www.webmd.com/diet/eucalyptus-is-it-good-for-you
https://www.wwf.org.uk/learn/fascinating-facts/koala
https://en.wikipedia.org/wiki/Koala
Photo by Thiago Oliveira: https://www.pexels.com/photo/koala-bear-on-tree-17038054/