A meal nothing else wants
How Does a Koala Eat Poisonous Leaves?
Eucalyptus leaves are toxic to almost every animal. A koala took the job nobody was competing for.
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Step 1 of 5
The tree is defending itself with chemistry.
Eucalyptus leaves are loaded with oils and bitter compounds. They are not there for us to smell nice — they are there to make eating the tree a bad idea.
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Step 2 of 5
A koala's liver takes the poison apart.
Its liver makes unusually large amounts of the enzymes that break those oils down, so the koala can pass them out instead of being harmed.
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Step 3 of 5
Then a very long gut squeezes out what little food there is.
A koala has a pouch in its gut about two metres long, full of bacteria that ferment the leaves slowly and release the last of the goodness.
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Step 4 of 5
A joey is not born able to do any of it.
The gut bacteria have to be got from somewhere, so the young one eats a soft dropping from its mother. That is how the bacteria are handed on.
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Step 5 of 5
The price is a very quiet life.
Leaves this poor in food will not pay for much activity, so a koala keeps still for most of the day and does not travel far from its trees.
Koalas are fussy even within eucalyptus — of several hundred kinds, most koalas will touch only a handful.
The short version
A koala can live on poisonous eucalyptus because its liver breaks the toxins down and a huge gut ferments what little food is left.
Try it yourself
Tear a eucalyptus leaf and smell it. The strong oil you can smell is the poison — and it is why almost nothing else eats that tree.