Leaves ain’t leaves

The constant balancing act a koala is involved in while feeding, goes way beyond what can be seen. The koala analyses the leaves to ensure they contain enough water and not too much toxic eucalyptol.

Eucalyptus leaves are very high in fibre, which means the koala’s stomach fills up quite quickly. If they were also high in calories, then a nice full stomach would mean fuel to give them energy.

Male wild koala asleep in a forest red gum tree after feeding
Often the position seems impossible for sleeping.

However, because eucalyptus leaves are low in calories, they feel full quite quickly, and they also feel tired.

I have often observed a koala actively awake and eating, then reaching a point where it will literally fall asleep where it takes in the last leaf. It always appears that they eat until they can fit in no more and the need to sleep overtakes them.

To complicate the choice of leaves to eat, the chemical composition of leaves on any given tree is not a constant, but a variable. Many factors affect the composition of the leaves such as weather events, seasons, insect loads, disease, soil events and amount of browsing.

This koala is eating leaf that is spotted brown, but obviously passed the test. Eucalytpus leaves are toxic to nearly all other mammals.
This koala is eating leaf that is spotted brown, but obviously passed the test.

Adding to the complications is the fact that you cannot tell a ‘good’ leaf by looking at it. What this means to the koala, is that even though it has trees it knows in its own home range, they need to be analysed at every meal.

Kita the koala is eating leaf that is spotted brown, but obviously passed the test. Eucalytpus leaves are toxic to nearly all other mammals.
Grinding the leaf with the molars until the entire leaf is eaten.

Koalas are not looking for the youngest, softest, greenest, brightest, freshest leaf or tree. At times, certainly a koala may eat the youngest tips ravenously, but that does not mean only young tips are preferred.

Many times I have seen koalas eating leaves that appear old, dry and even spotted with brown areas. At other time I see koalas plucking only the leaf, yet another time they eat a large amount of the stems.

Joey is learning from mum what good leaf smells like as they eat together. Wild koalas in forest red gum tree.
Joey is learning from mum what good leaf smells like as they eat together.

I have also observed, with my daily record keeping of koala movements and use of trees on the property, that some trees are only used seasonally. This is not a consistent occurrence in itself. Some trees of the same species and a similar age (mature trees at least 50 years old), are used year round, some are used only at one time of the year, and some another time of year. There is nothing for the human eye to see that would indicate why this is so.

This supports the research, showing that koalas do stop and smell the leaf and analyse what to eat when. This is more intricate than choosing a tree or leaf for the taste, or to obtain a varied diet.

In order to cope with the low calorie content of their food source, the koala has a lower metabolic rate of about half that of most mammals.  This is significantly low.

The result that we observe is that koalas spend 18-22 hours a day either sleeping or at least resting, so that their digestive system can work on the leaf they have eaten.

Wild koala male sleeping in a pink bloodwood.
Sleeping after a large meal to aid digestion.

Even though koalas are often thought of as nocturnal, this is not strictly true. Koalas will wake at any time of the day or night and move about and eat as needed. They eat when their stomach empties, and sleep or rest to digest. They are most likely to have their most active period during the night, and most likely this will be in the early hours of the morning before dawn.

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.