The Special World of Koalas
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The Special World of Koalas

By Valentina Ménager

Koalas are fascinating animals. These cute and somewhat aggressive creatures are part of a specialized class of mammals called marsupials, animals that carry their young in a pouch. However, koalas are so different from every other marsupial; they survive off toxic leaves, sleep all day, and are extremely susceptible to chlamydia.

Koalas are known for living in eucalyptus trees, off which they eat bark, leaves, and branches. Interestingly, these trees contain toxic molecules that are poisonous to nearly every other living species. Koalas, however, are somehow able to flush these toxins out of their system and eat their way through pounds of eucalyptus leaves every day without getting sick. Scientists hypothesize that the part of the koala genome that codes detoxifying proteins could be about twice as big as other animals, even humans. Perhaps this special characteristic of the koala genome arose from random mutations, and eventually, all koalas had an abundance of detoxifying proteins as a result of natural evolution.

Scientists have also noticed another odd habit: koalas sniff eucalyptus leaves before eating them and occasionally throw some away. This observation was coupled with a discovery that koalas have extra genes that allow them to sniff subtle differences in the minty smell of eucalyptus leaves. Building on these findings, some theorize that these special creatures can sniff out the nutritional value or toxicity of each leaf and assess whether or not to eat it. In the end, though, all eucalyptus leaves contain so few calories that koalas have to spend a whopping 22 hours a day resting or sleeping.

The gut microbes in koalas are so complex that they are not only experts at digesting poisonous plants but are also very sensitive to common antibiotics used to treat diseases like chlamydia, a sexually transmitted bacterial infection. This sensitivity is problematic because many koalas suffer from a retrovirus similar to HIV that weakens the immune system and increases their susceptibility to diseases like chlamydia. Unfortunately, they cannot be treated with antibiotics, as ingesting them interferes with the ability of their special gut microbes to break down eucalyptus and could cause them to starve. This has made it nearly impossible for scientists to treat koalas for infections. A vaccine to prevent koala chlamydia is currently being developed in hopes of one day letting these creatures roam the eucalyptus trees chlamydia free.


Blackman, B. (2022, March 24). Koala facts | Environment | Department of Environment and Science, Queensland. Queensland Department of Environment.

Morrison, J., & Phillips, B. (n.d.). History of Koalas. Australian Koala Foundation.

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Tamisiea, J. (2022, September 30). Meet the Smithsonian Scientist Unlocking Crucial Conservation Clues in the Genetic Code of Koalas. Smithsonian Magazine.

Lazunova, T. (n.d.). Cute cartoon koala. lazy koalas with eucalyptus. iStock.