Eucalyptus Secrets Key to Koala Survival, Health

Koalas famously eat eucalyptus leaves, and they are one of the very few mammals that can. Eucalyptus leaves contain a veritable cocktail of toxins that koalas have evolved the special ability to manage their diet. It is a blend and concentration of these toxins, called ‘plant secondary metabolites’ (PSMs) that largely determine which leaves koalas choose to avoid, which ones they choose to eat, and in what quantities they can safely consume them.

Between individual trees of the same species.

So, you have two trees of the same species standing side by side and one may be completely inedible to koalas (due to its high PSM concentration) and the other may be totally irresistible due to its, low PSM concentration.

‘Koalas are experts at detecting the chemicals in a gum leaf’

So, a koala sitting in a eucalypt forest has a literal buffet of leaves, but the trees all have varying amounts and mixtures of PSMs.

It is the koalas job to determine which leaves to eat and how much they need, in order to meet their nutritional needs without intoxicating themselves. Of course, koalas are often provided with Tallowwood (eucalyptus microbes) as this tree is on the list of preferred koala food trees. Tallowwood always has a very high protein content. Making it appealing nutritionally, but the concentration of a group of PSM toxins called unsubstituted B-ring flavones (UBFs) varies wildly.

UBFs deter koala feeding above certain concentration

Below this concentration the koalas will eat a lot of Tallowwood to access all that protein. Above this concentration they will eat less and less as the concentration of the UBFs become too much, the koala will not be able to eat it at all. That is one reason why, on some days when we offer koalas tallowwood they eat lots of it and on others they may not eat it at all.

Unlike koalas humans can't tell what the concentration of PSMs will be, unless we do laboratory testing. That's why when koalas are in hospital it is important to always offer a choice of leaf, so that they have the option to eat something else at the buffet if the particular offering is too toxic. Koala conversation Australia (KCA) currently has two eucalypt plantations from which browse (leaf) will be harvested for koalas (residing at the koala hospital) in the future it will be important to properly manage the plantations from nutrient and PSM.

Our team are always researching to help develop best practice management of the plantations as well as to understand how the koalas in the hospital respond to leaves with different compositions.

This knowledge means decision making about what leaf to offer the koalas is evidence based. Plus leaf waste can be minimised while we aid the koalas in making timely recovery from their illnesses!