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.
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.
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.
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.
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.
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.