Australasian Science: Australia's authority on science since 1938

High Society

Credit: lassedesignen

Credit: lassedesignen

By Bill Ellis

GPS collars have revealed that koalas are more social than previously realised.

Koalas are mostly arboreal and nocturnal, so their social system has been difficult to study. The koala’s mottled grey fur also makes it vanish among the canopy by day, so while we have been able to discover a lot about what they eat and where they live, how they interact has remained somewhat of a mystery – until recently.

We have been able to use smart phones and GPS-enabled radio collars to study how koalas communicate and move. And we’ve found that the females are much more socially interactive than the males.

Now we just need to figure out why. If we want to conserve their resources effectively we have to work out how they communicate, how they choose mates and why small populations have become extinct. If we don’t know what the critical parts of their social structure are – or even if such things exist – then we are doomed to misunderstand their habitat requirements.

Of all the things koalas do –and they don’t seem to do a lot – their distinctive bellow that rumbles through the forests each spring must be about the most interesting. The bellow is loud, long and like no other sound you hear, but its role in koala biology has only recently been properly studied. We now know that every bellow is an individual signature telling other koalas not only where another one is, but who and how big that koala is.

Koalas are most active at night when it cools down in Queensland, so they are at their busiest when the need for water to stay cool is at its lowest. Koalas don’t sweat but they do pant, and since they get almost all of their water from the leaves they eat, water is a critical consideration for them.

Finding mates and avoiding adversaries in the middle of the night requires more than just good eyesight. Koalas have eyes similar to cats, with a vertical pupil that closes tightly by day but expands to allow good night vision.

Even so, misjudging the size of a competitor when high up in a tree can be costly for male koalas. Falls are common, and can result in debilitating injuries and even death, so koalas need more than the sight of a furry ball in an adjacent tree to decide whether to approach or avoid it.

Our group decided to investigate koala bellows. We established a suite of “listening stations” that consisted of a mobile phone attached to a large battery and solar panels. The phone turns itself on, records for a few minutes each hour and uploads the sounds to a server so we can listen to them almost in real time.

From my desk in Brisbane I can hear the koalas on St Bees Island, 20 km off the coast of Mackay, bellowing away as summer approaches. With the help of researchers from Queensland University of Technology and Sussex University in England, we began a set of experiments to decipher the bellows and interpret the associated koala behaviour we observed.

A key to this was to monitor every koala on our island study site using GPS and proximity logging radio collars. As well as letting us track the koalas by day, these collars recorded how far they moved every few hours, and also which other koalas they came into contact with.

Back in Brisbane, my colleague Ben Charlton recorded the bellows of captive koalas and analysed the spectrograph of each one. We also took the recorded bellows into the field and played them to koalas as well as re-recording them over distances to see how far the information travelled.

We first discovered that koalas tend to bellow very late at night. Although we had heard bellows around dawn and dusk, midnight to 4 am is when most of the bellowing activity occurs. On days when male koalas bellow more, female koalas move further, suggesting that the males were advertising and the females were going on excursions and selecting mates.

Each koala has a unique bellow, and other koalas are able to identify one another by their bellow. The bellow is an accurate indicator of the size of a koala. The koala has unusual vocal folds that it uses to create the bellow sound, and this sound is passed through its vocal–nasal tract. The longer the tube the sound passes through, the lower the frequency of the sound becomes. Female koalas seem most interested in the lower frequencies, which belong to the largest males.

This only tells part of the story, because a genetic analysis of our population revealed that the size of the male koala only accounts for about one-third of the variability in a male’s siring success, with some small males producing more offspring than their larger contemporaries. In addition, females tend to breed with different males each year, so size is not everything. Furthermore, we never found a wild male koala that sired more than three young in any season, regardless of his size. Why is this?

For females it may be as important to know who is out there as it is to know how big he is. For males, paying attention to size is probably more important!

So we figured that the bellow was serving two different purposes: letting males know where they could and couldn’t go, and letting females choose mates from a distance.

Our thoughts were confirmed when we included the proximity data into the mix, which revealed that males avoid one another most of the time. Even in the breeding season between September and March they didn’t have much to do with one another.

Koalas are presumed to be solitary animals, and when many koalas are found in one tree it is considered an unusual event. However, our data suggest that females are frequently in contact with each other, from short stays in the same tree right up to multi-tree associations over several days.

We know from daytime observations that females are more tolerant of their own relatives in their range. Their ranges might overlap but they don’t seem to be in the same tree very often, or even share the same tree much. It’s like the black and white squares on a chequerboard, with one koala avoiding trees that are used by the other.

Because our new technology monitors the koalas 24 hours per day, we gathered a slightly different picture. While males are very good at avoiding one another, the females seem quite tolerant of other females in their area.

But herein lies our next quandary. We assume that two males will be aggressive toward one another and therefore avoid contact, but what if the females are actually competing for trees and mates, and that is why we see so many contacts between females? We don’t have an answer for this yet, but we are working toward it.

What is clear is that the information contained in the koala bellow is critical to help females to find mates and for males to remain safe in the treetops. Thus noisy human suburbs can disturb communication among groups of koalas. Indeed road noise is of low frequency, and the sound of some vehicles and machinery may well confuse koalas in their search for mates. We used to think that it was the young dispersing males that were at most risk of road injury in urban populations, but if the females are searching for mates then they are also at risk.

Across Queensland there are many isolated, small and even extinct koala populations. Our goal has to be to find, connect and protect these habitats and facilitate their repopulation. If we don’t know what factors lead to successful mating seasons for the koalas, we don’t have a great chance of long-term success.

Bill Ellis is a Research Fellow in the School of Agriculture and Food Science at The University of Queensland.