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.

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.

The koala nose

Scents are important to koalas and their sense of smell is highly developed.  We are all aware that they have large noses dominating their faces, and that is not coincidental.

The sense of smell is used for two main purposes – communication and chemical analysis of a potential meal. In both cases the ability to conduct a degree of chemical analysis is involved. This means the koala olfactory ability is above most mammals.

Koalas are solitary animals, yet they do live in a colony. This means they recognise the other colony members, and therefore distinguish koalas that are strangers. They are highly unlikely to do this by sight because their solitary arboreal nature means that they may not really ‘see’ each other often. They do however recognise the smell of other koalas. They also recognise the bellow of other males.

Most of the time however, koalas are quiet. Yet a koala does not move through the territory of another koala unnoticed, even if it makes no vocal sounds.

How does one koala know it is moving through another koala’s territory you might ask? How does a regular colony member know another koala has been in their territory?

We observe that both sound and scent are used to communicate more during the breeding season – a time when koalas are actually communicating the most. However, koalas have a home range within their colony area and they have home trees within their home range.

How does one newcomer know a tree ‘belongs’ to another koala? It must be by some method that humans are not aware of unless they were to be come a scientist (even a citizen scientist) and begin to study what is going on.

A terrific thing about using scents to communicate is that it is not directly threatening, and it conserves energy. Koalas will use both scent gland and urine to mark territory, but the messages they leave behind are probably more complex than simple ownership.

Whenever we observe a koala moving on the ground from tree to tree, we notice the koala will take some leaps and bounds, then stop and smell the ground and shrubbery at intervals and so on.

Pictured are two male wild koalas showing the difference in size between the male scent gland used to mark koala territory and advertise their availability to females within the colony.
You can clearly see the difference in size of scent glands between these two male koalas.

Every koala has a subtle difference in the chemical composition of their smell, making their own scent as unique as their fingerprint. Colony members remember this individual scent because they not only smell it once, but they smell it in layers if you like. A tree will be marked with scent whenever the koala enters and exits the tree. This builds up these layers of scent. These layers add a lot of meaning to the scent as they advertise clearly that a particular koala owns a tree, or uses it frequently. A koala can also tell how recently the scent was left and so have some idea of whether the owner of the scent is likely to be about.

A male koala will rub his scent gland over trees belonging to other koalas in order to leave an indication that he passed through in a similar way to other mammals that use urine to mark territory and indicate their availability for breeding.

When out in the bush even you can sometimes ‘smell’ a koala about, particularly during the breeding season.

While only males have scent glands, and only mature males have well developed glands, all koalas have a highly developed sense of smell.

Koalas and machinery noises

As mentioned previously, hearing is more than the physical process of vibrations being perceived inside the structure of the ear. Hearing is also about interpretation of those vibrations, and the how brain filters the vibrations our ears physically perceive. (How the brain filters and makes sense of what we, and in this case the koalas, hears.)

The history of the koala as an arboreal (tree dwelling) animal with little predator threat has influenced their hearing filters and reactions to what they hear. From the heights of the treetops, koalas filter out noises such as cars and other motorised vehicles going past. Roads encroach so much koala habitat that many koalas find themselves sleeping in places where there is almost constant vehicle noise.

Koala sleeping deeply in fork of a tree within sight and earshot of a busy road.
Koala sleeping deeply in fork of a tree within sight and earshot of a busy road.

Have you ever driven along a busy road and seen a koala fast asleep in a tree right beside that road?

By the same filter, a koala does not really differentiate between the sound of a car, chain saw, mower or bulldozer.

The koala has no way to figure that the bulldozer is going to knock down the tree it is in, when thousands of cars and other machinery have gone right past without incident.

In the same line of reasoning, the koala does not perceive they are in the direct path of the car approaching, when it is down on the road. The car sounds the same as the cars that went right past the tree all day long. The koala does not have the kind of executive thinking to figure that this spot on the ground is the place the cars were.

I have often seen a koala look up and watch me approach on the quad bike however. In nearly all instances the koala was already awake, and actually sees my face, and sees me looking intently into the tree. Any koala that has not become very accustomed to my presence will react to me, rather than the quad bike, and specifically to me looking directly at it.

Bullet has seen me approaching and looking at him so is now watching me. Bullet is a wild male koala.
Bullet has seen me approaching and looking at him so is now watching me.

I have tested this by looking away and riding past the tree, then looping back around and being sure not to directly look at the koala or the tree. The koala will nearly always relax, at least to some degree.

I always obtain a much bigger response if I approach the tree on foot.

An individual koala will have variations to their hearing filter depending on the particular habitat they live in and what is normal there. This means a koala that lives in a forest that is isolated from roads completely may respond differently to machinery noise if it is something they do not normally hear at all.

What do koalas hear?

I have been fascinated by how koalas respond, or do not respond, to sound, when I have been out koala spotting.

It would appear that koalas have a decent ability to hear. They have large, mobile ears and when awake I have found they can hear footsteps, twigs snapping or voices quite acutely.

However hearing is more than the mechanics of the ear perceiving vibrations. Hearing is also the brain interpreting those vibrations.

I have mentioned in other articles here that koalas are neither prey nor predator animals. If we consider this when observing koala behaviour in relation to hearing, it does shed some light.

Prey animals need to be alert as much of the time as possible, and they will particularly need to be alert through their hearing when they are resting and sleeping.

Predators need their hearing for hunting. They may need to hear smaller animals moving through dense bush for example.

Koalas also become familiar with sounds in their own home range and learn to filter out sounds that their experience tells them is not a threat. This is important in understanding that hearing is as much the brain interpreting sounds as the ear detecting the vibration.  I have researched this topic and found little, so have conducted my own research with the koalas here in my daily spotting.

Compare the posture and expression on Legion in the video above to Pinky in the photo below and you can see the difference in response to sound, through being familiar with the sounds.

Photo taken on the first day Pinky was seen on the property.
Pink’s first day here and she is very alert to sounds as nothing is familiar.

An observation I have made with many different koalas over a range of years is that I can elicit a strong reaction from a koala that is new to the property with certain sounds such as pretending to sneeze, cough or even growl. However, with repetition, over a period of time, the reaction will decrease and for many koalas the reaction will completely cease. This means a koala that arrives at the property may startle and stare at me, then move up the tree if I approach and make a sneezing noise. If I repeat this each time I approach on different days, it may take as little as 3 or 4 days for the koala to completely ignore the same noise. Some koalas take longer to reach a point of ignoring me, but I have noticed that every koala will have a significantly reduced response within 3 or 4 days of me approaching and making a particular noise.

There are however, some types of noises they will always respond to, such as stomping footsteps approaching the base of the tree, particularly if long grasses are rustled and twigs snapped. Even though koalas are not truly prey animals, there is an instinct to understand that anything that can make heavy footstep noises, and is approaching the tree, is a possible threat.

Mist peering at the camera after been awoken by stomping footsteps.
I had to make stomping noises to get Mist’s attention as she is very familiar with me.

This does not mean that the footsteps are the loudest noise, at the time.   I have often witnessed a koala ignoring the sound of the quad bike approach the tree, but jumping from the bike to the ground and stomping has brought the head up immediately. The bike motor had obviously been making a far louder noise, yet had been ignored.

The next article will discuss koalas and machinery noises, as this becomes important in understanding another layer of why they are at such risk with cars.

How koala eyesight impacts on them today

Koalas are trying to adapt to the modern world but their eyesight is made for the tree tops. Maxine and Enigma are pictured here showing koala eyes.
Koalas are trying to adapt to the modern world but their eyesight is made for the tree tops.

We have discussed the way koala eyes work, or rather how they don’t work as well as the eyes of either predators or prey animals.  See koala eyesight post for more background information.

It’s more than their physical eyes however, it’s how their brains are hard-wired as well.

The koala is in an unusual position, being neither prey or predator on any real level. They are just going about their daily business of trying to get enough calories from all those eucalyptus leaves.  In the process of doing this, they really are not taking much notice of anything else (except other koalas).

Let us now remember we no longer have large areas of old growth forest providing acre after acre of trees that can be accessed through the branches in the canopy, or at least by coming partway down and jumping across.  The koala only had to see around itself, and use their highly developed sense of smell to know which trees were good and move through them.

The koala now finds itself too often, in a single tree, in the middle of an open paddock.  He eats his fill, has a sleep then has to climb down to the ground and set off in search of the next tree which may be tens or even hundreds of metres away.

Mist showing typical female koala head and eye shape
When a koala is awake they are usually hungry and needing calories.

But let us think about the fact that the eyes of the koala are not made to make sense of this – they do not see well at long distances. Their brains are not wired up to think – this trip could be dangerous and tell their eyes to be really sharp either.

Yes a koala that lives well into adulthood is going to learn to traverse particular areas and deal with particular dangers, but the problem is that less and less koalas are living through their early encounters with the dangers on the ground.  On top of this, the koalas are using up a lot more calories in order to take in each meal they find because they have to travel down the trunk, across the ground and then up the next trunk again.  This is significant as eucalyptus leaves are low in calories and they have to eat a large quantity of them already.

When they are travelling they are usually hungry, and so their primary focus is going to be on getting to a good tree for their next meal.  A koala with a full belly nearly always goes to sleep to aid digestion quite quickly.

So a koala walking through a paddock is needing calories and is unlikely to even see a dog coming for it unless the dog makes some good noise.  A car approaching from a distance does not register with the koalas eyes at all.  We will talk further about how koalas seems to respond to mechanical noises in another post.

Even what they can see is usually distanced from them as they should be up in a tree.  Jordan is pictured here sitting in a tree looking at me without a care as I am on the ground and he is in a tree.
Even what they can see is usually distanced from them as they should be up in a tree.

Finally, when a koala does see something approach them, it is normally on the ground and they are up in a tree.  Their tactic if concerned is to go higher up the tree.  If they are on the ground, by the time they realise there is something of concern it is usually too late.

This highlights the importance of planting clumps of trees and rows of trees connecting good feed clumps together so that koalas can be on the ground for less time and eating more quickly.


Do koalas have good eyesight?

There is a reason koala eyesight is not so good over distances. Even when a koala looks directly at you, it will have heard you first, then had to find you by eye.
There is a reason koala eyesight is not so good over distances.

If you pose this question through Google the answer is pretty quickly returned as “no they do not”.

Now why might that be?

To understand why we need to consider the environment they had before humans came ripping through their homeland and changing everything.

Koalas are arboreal marsupials which means they live in trees, and are mammals that give birth to tiny under-developed young that crawl into a pouch to complete development for around 6 months.

Wild male koala looking down from a pink bloodwood tree - he can see me but koalas do not have good eyesight
Wild male koala looking down from a pink bloodwood tree

However, we need to think of incredible old growth forests thick with tall trees when we think of the arboreal koala.  When you live high in the canopy of a forest, there is not a lot you can see long range.  The ground is distant and hidden by the lower branches and smaller young trees.  Anything beyond the tree you are in, is likely obscured by branches and leaves from your tree or the next.

Stella is just over 12 months old and though she has bright eyes, she relies more on her hearing than her sight.
Stella is just over 12 months old and though she has bright eyes, she relies more on her hearing than her sight.

Koalas are far from blind, but their long range sight is pretty poor because it was not needed.  Koalas are skilled at jumping from trunks and branches to move about, and they look carefully to judge the distance before taking a leap.

Koalas have an acute sense of smell, and they do have keen hearing.  Sometimes it seems their hearing is not so keen because they sleep so deeply.

If you can imagine huge forests with hundreds of koalas moving through the treetops, you can maybe imagine that they didn’t bother looking to the ground or out into the sky too often.  Their focus was within the short range area of the tree they were currently occupying.

Koalas are said to have no natural predators, but this isn’t completely true.  However the only predators that were of any real concern to koalas in the past were usually only a concern for joey’s and small juveniles, and only if they were not in the treetops.  The main predators that may take a small number of young koalas are goannas, dingoes, pythons and some powerful owls and eagles.  The incidence of these predators seems to have always been fairly low.  Coastal pythons are the only one of these predators that may attempt to take a lone young koala in the canopy.

Stevie is looking at me, but it was sound that caught her attention fast, while it took her time to find me with her eyes.
Stevie is looking at me, but it was sound that caught her attention fast, while it took her time to find me with her eyes.

This all means that koalas do not have a historical need to be on a constant lookout for predator threats, and they do not hunt as predators themselves, and so their eyesight, for long distances is somewhat poor.

Another important feature of koala sight is that their eyes are ‘forward facing’ like our eyes. This puts the koala outside the general rule which says – ‘eyes to the side, run and hide; eyes to the front, love to hunt’.  Koalas don’t hunt, and yet they did not have any real need to run and hide either, and so they have eyes that face the front which give better depth perception. It’s also one of the features that makes them look so endearing to humans.

Enigma shows us a perfect example of the forward facing eyes of a koala. Koala eyesight however is not very good over long distances.
Enigma shows us a perfect example of the forward facing eyes of a koala.

The impact for koalas in adapting to the environment we have created by invading their home is huge when you understand their eyesight limitations.