Chemically nosey

Koalas are specialist folivores. This means they eat a diet consisting of leaves, and in this case they have a narrow range of leaves that they are able to eat. Some people say koalas are ‘fussy eaters’, but the reason for their particular choices of which tree, and then which leaves in the tree they will eat, is fascinating.
We often think of the koala as being a laid back, relaxed, easy going creature, living a life of ease. This perception comes from putting a human interpretation on their postures and the amount of time they spend resting and sleeping.

A wild male koala chooses what leaves are chemically ok to digest.
It may look like easy pickings, but it’s a lot tougher than you might think.

The eucalyptus leaf, if studied closely is tough, abrasive and toxic. Have you ever plucked one of these leaves and felt it with your fingers?
The koala must put a lot of effort into grinding the leaves with their molars. The amount of wear on the molars is the most accurate way we have of determining the age of a koala. The wear on the teeth continues as the koala ages, and if the animal remains healthy and lives to an old age, their teeth may wear down so far they become unable to eat.

Every leaf has toxins that must be analysed before consuming. Wild male koala checking leaves while eating.
Every leaf has toxins that must be analysed before consuming.

It is fascinating to see that an animal that has a limited primary food source has actually adapted to living in a diverse range of habitats, and across such a range of climates. The koala is found living in tropical and subtropical areas of QLD, and yet also in areas around Canberra that snow in the winter, and even some of the drier areas of Victoria. We observe certain basic differences in the appearance of the northern and southern cousins, yet they are the same species.
Across this range of habitats there are around 14 eucalypts that are considered their primary food sources. Yet in nearly every area koalas live, there are usually only around 3 of the primary species that the koalas use in that area.
They are observed to feed from around 30 non-eucalypt trees, but again the species used vary from region to region.
Each eucalypt leaf contains a wide range of ingredients, including the things we would expect such as protein, complex carbohydrates and water.
They also contain high levels of tannin, tough fibres, oil and cyanidic substances (which convert into cyanide). Yikes, that sounds like a mouthful of poison doesn’t it? It certainly is, to most mammals, and you can be sure it even tastes that way as well.

Wild koala joey learning to smell leaves while mum is eating.
The first thing a joey learns is what the right leaves smell like.

It is interesting to note that when we pluck a eucalyptus leaf from a tree, we can tell very little about it’s make up without taking it to a sophisticated laboratory. It feels tough and a little dry, because they have a rough surface, but it actually has a high water content – often at least 50%. This means around half the food the koala eats, is actually water.
Most animals could not survive the toxicity of the eucalypt leaf diet, and even the koala has to be careful they to not exceed certain levels of eucalyptol (the main component in eucalyptus oil).
So how do they make sure they can cope with their diet?

Koalas have the ability to chemically analyse the contents of the leaves by smell. That huge nose is like a portable laboratory. If you get the chance to watch a koala feeding in the wild, you will notice how often they smell leaves before eating. This is a constant balancing act they must perform.

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Pap for a joey

Pap – one dictionary definition of the word in general is “bland soft or semi-liquid food such as that suitable for babies or invalids.”

Koala mums don’t have the luxury of a supermarket and kitchen to prepare those first baby meals for their joey of course.

The koala digestive system is unlike that of most other animals.  We know that koalas eat leaves.  Do you also know that a joey drinks mum’s milk and nothing else for around 6 months while developing in the pouch?

Wild koala and joey that has head out of the pouch ready to eat pap.
Joey with head in perfect position for pap feeding.

Most animals that digest leaves and grass have 4 stomachs, and these animals are called ruminants.  Cows, sheep, goats, giraffes and kangaroos are some of the most common ones you might think of.

However, some animals that eat leaves have only one stomach, and they use a really big caecum to do this (that’s the appendix in us, which is tiny). They are called ‘hindgut fermenters’.  Horses, rabbits and koalas are common animals you know that are in this category.

The caecum and the large intestine are filled with microbes that turn it into a big fermentation vat.  This fermenting happens in the extra stomachs of ruminants and they are also called ‘foregut fermenters’.

It’s the microbes that are of most interest here.  Other articles will cover more about digestion.  Joey has been living a very sheltered life inside mum’s pouch, and has been drinking only her milk.  The microbes joey needs to digest leaves are not found in the leaves, so joey needs to get them from somewhere.  A goat kid will begin nibbling dirt within hours of birth and so begins to get microbes into their digestive system.  Not the case for the koala, as joey is still very fragile and high up in the treetops.

So mum has to provide the microbes joey needs to ready the digestive system for those tough eucalyptus leaves.

Wild koala mother and joey in this photo. Joey has head out of the pouch and is eating pap for the first time.
Joey is eating pap.

Joey knows when the desire to eat begins, and puts its head out of the pouch and begins to nuzzle around mums cloaca. (koalas have a single opening like birds and so it has the same name as we use for birds)  This nuzzling and licking stimulates mum to begin to secrete a pasty liquid directly from her caecum for joey to eat.  Koala faeces is shaped into firm pellets, but pap is a pasty semi-liquid.  It is really quite similar in texture to a puree made for a human baby just starting to eat.  However the reason is to kick start the hindgut in the fermentation process rather than to provide nutrition.  You could say, pap is a probiotic infusion.

We usually say that pap is a special faeces, but it is actually contents of mum’s caecum.  Probably not a big distinction for most people, but there is a difference.  Mum’s only produce the pap for a week or two at most while joey stimulates her.  Joey knows when it is no longer needed as joey starts to feed on leaves and is able to easily digest them.

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