The cute, grey marsupial can be found on t-shirts, hanging off people’s bags and pencils, and decorating any decent souvenir shop. But seeing a real koala in the wild has become increasingly tricky in some parts of the country. The iconic marsupial is now listed as endangered in Queensland, New South Wales and the Australian Capital Territory.

But koalas have been in a similar situation before.

As my new study published in the journal Molecular Biology and Evolution shows, koalas experienced a population crash about 100,000 years ago. This finding rewrites our understanding of the genetic history of koalas in Australia – and overturns previous theories about what caused their decline in ancient times.

Turning to the genome

Fossil records of koalas are extremely rare. This makes it difficult to estimate how many koalas were present in the past.

Instead, genomes provide important clues about their evolutionary history. The genome acts as a historical record. It preserves genetic information from ancestral populations that can be used to determine their population size.

Previous genomic studies of koalas have estimated koalas experienced a major population decline roughly 40,000 years ago. This was shortly after the arrival of humans in Australia, suggesting this may have been a contributing factor.

Yet the impact of human arrival on Australian fauna is hotly debated. Some researchers use it to explain the widespread extinction of megafauna during this period.

My new study challenges this theory.

Pushing the timeline back 60,000 years

My colleagues and I set out to construct the first estimate of the koala mutation rate. This is simply the number of mutations that appear in each generation.

Estimating the historical population sizes that have shaped mutation patterns in the genome relies heavily on knowing how often new mutations arise. The problem is that each species has its own unique mutation rate.

To estimate the mutation rate in koalas, we sequenced the genomes of 12 koalas from three families, comprising seven parents and five offspring. This allowed us to count the number of new mutations over each generation.

The whole koala genome has about 3.4 billion sites where changes could occur. We found only 25 mutations per offspring. That’s the equivalent of searching for 25 wrong letters scattered across more than 1,000 copies of The Lord of the Rings trilogy.

We then applied this mutation rate to 457 koala genomes sampled across their entire range. This allowed us to investigate how koala populations have changed over time – including when their numbers crashed.

We found koala population declines occurred around 100,000 years ago – well before humans arrived in Australia. This effectively rules out humans as a cause of the population crash.

Although the mutation rate is a fundamental evolutionary concept, we surprisingly have very few estimates for Australian species. Our estimate is the first from Diprotodontia, the marsupial order which also includes wombats, kangaroos and possums.

Previous studies estimating historical population sizes in koalas have had to rely on mutation rate estimates from distantly related placental mammals such as humans and mice. Applying the koala mutation rate has rewritten the genetic timeline for koalas.

So, what caused the crash?

The koala population crash 100,000 years ago matches a period of intense environmental change across Australia.

The Pleistocene (2.5 million to 11,700 years ago) saw repeated glacial periods, characterised by cold and dry conditions, as well as repeated interglacial periods, characterised by warmer and wetter conditions.

As Australia became drier, the expansion of the Nullarbor Plain established a vast semi-arid shrubland across southern Australia, shrinking suitable koala habitat and separating eastern and western koala populations.

Unfortunately, the population west of the Nullarbor Plain (which was recently described as a distinct species from the modern koala) went extinct around 28,000 years ago.

Although eastern populations were restricted to a small patch of forest on the east coast, they persisted through harsh glacial conditions. Over the last 17,000 years, as conditions became warmer and wetter, they expanded and formed the five genetic groups that are now distributed along the east coast of Australia.

Given our results, we’re now curious to see if other Australian species, including the closest relatives of extinct megafauna, also experienced population declines before humans arrived.

Koalas are back to hard times

Koalas are once again experiencing population declines across Australia.

One similarity between modern and ancient declines is they are both largely driven by reductions in the amount of suitable habitat. The ancient decline was driven by global glacial cycles – an unavoidable result of Earth’s orbit.

However, recent declines have generated a similar bottleneck over a much shorter time window, due to the historical and continued removal of suitable koala habitat. This is made worse by other threats such as hunting, disease, vehicle strikes, feral dog attacks and bushfires.

Fortunately, most koala populations have only recently started losing genetic diversity, and rapid population recovery can prevent further loss and inbreeding.

Hopefully the eastern koala will persist once again.

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Despite their name, these dark and blubbery marine mammals aren’t technically whales – they’re large oceanic dolphins which are believed to have a navigator or lead for each pod. Hence the “pilot” part of their name.

There are two types of pilot whales – short and long-finned. They’re generally found in deep offshore waters but can appear in coastal areas. And like other dolphins, they use high frequency sounds to talk to each other in their pods. These clicks and squeaks travel shorter distances compared with the melodic songs of humpback whales.

And as a new paper led by Milou Hegeman from Aarhus University in Denmark and published in the Journal of Experimental Biology shows, the pilot whales that live in the Strait of Gibraltar are having to shout at the upper limit of their range in order to hear each other over human noises.

What’s making all that noise?

The ocean is full of sounds.

Some of these are natural, such as the sounds from fish, seals and waves. Other sounds are produced by human activities, either deliberately (for example seismic and sonar exploration) or unintentionally (for example, the sound of moving ships or other vessels).

The ocean continues to get noisier because of human-made sound – even in isolated Arctic regions. And because of its strategic location, the Strait of Gibraltar is especially noisy with the drone of cargo ships.

Spying on pilot whales

To investigate the communication and behaviour of the population of pilot whales in the Strait of Gibraltar, scientists used 6-metre poles to attach small tags to the creatures (kind of like an Airtag used to track your suitcase) with sterile suction cups positioned between the dorsal fin and blowhole.

Between 2012 to 2015, the steam attached tags to 23 different long-finned pilot whales who live in the region year-round.

These tags remained on pilot whales for up to 24 hours collecting sounds and tracking individual behaviour. The tags then floated to the surface where scientists could locate them using an antenna and collect the data from their diving activities.

More than 84 hours of recordings were made, with 1,432 pilot whale calls extracted. The tags also recorded ship noise in the area.

The researchers found there was a scarcity of pilot whale calls during periods of shipping noise. And the volume of the calls they did make were louder by about half the increase in background noise.

This means the animals are adapting to communicate in times when it is noisy – kind of like having a conversation in a crowded place and you having to raise your voice to be heard.

Other noises, other impacts

This study focuses on just one location in the ocean. But there’s increasing evidence that human-made noise is also impacting other species in other places.

For example, a 2012 study found that ship noise increases stress in right whales. Another study from 2024 found sea turtles travelling in the Galapagos were more vigilant because of increased ship noise.

But it’s not just ship noise that is impacting the animals that live in the ocean. Sonar disrupts whale diving behaviour and feeding behaviour, sometimes even potentially resulting in strandings.

Thankfully, work is being done to reduce noise pollution in the ocean – from building quieter ships to rerouting ship activity, helping ship operators drive more quietly and dialling down the noise from all human activities.

This new study is just one of many scientific contributions to learning more about our impact on our blue backyard. We can only protect what we know. And as we celebrate the 100th birthday of Sir David Attenborough, it’s worth remembering one of his many pieces of wisdom: “If we save the sea, we save our world”.

Part of this involves being more aware of sound in our sea. Because sometimes, it’s not always the visible impacts such as plastic pollution that need our attention. It might also be the impacts we can only hear.

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But making sounds can be a double-edged sword: it can help them communicate, sometimes over long distances, but it can also reveal them to predators.

In new research published in the Journal of Mammalian Evolution, my colleague and I studied how far the sounds of 103 different mammal species travel, and discovered some surprising patterns.

What’s more, these patterns hint at an overlooked impact humans may be having on our fellow creatures: not only changing their sonic landscapes through our own noise, but also changing the world their sounds are travelling through, with unknown effects.

What’s happening in the water?

In aquatic mammals, the relationship between the size of an animal and the farthest distance its call travels is simple. Bigger animals can be heard farther away.

On a perfect day in perfect conditions, the call of a blue whale (the largest animal in history) can travel up to 1,600 kilometres. Its (slightly smaller) cousin the fin whale can be heard over a similar distance.

These are the longest-travelling animal sounds ever reported.

What’s happening on land?

On land, the story is very different. Environmental factors are crucial to how far the sound of a terrestrial mammal travels.

Things that matter include the size of an animal’s home range (the area in which it lives and defends resources), whether a call is territorial (to defend against other animals), whether the environment is open versus densely vegetated, and if the animal is very social or solitary.

On a good day in the savannah, lions and elephants have sounds that travel 8km and 10km, respectively.

How does this work?

Our research is centred around the idea that your sound reveals you to predators, and that revelation leads to a higher risk of injury and death (potentially before you pass on your genes, and hence reducing what evolutionary biologists call “fitness”). This would be because the predator can more quickly locate its calling prey.

There is a delicate balance between using sounds to communicate and using sounds in the wrong place and at the wrong time.

If sound is revealed at the wrong distance, it may mess up the reason an animal uses the sound in the first place.

Animals that cannot adapt to changes in the sound environment may reveal themselves and be eaten, or may be unable to find their friends.

Where does this fit?

In the midst of human-induced environmental and species change, understanding how animals use sounds to communicate and find each other has become valuable to conservation. Many ecosystems are being cleared on land to make way for development and agriculture.

Our finding that land mammals in closed habitats have evolved to have relatively farther sound distances is important because of what happens when the environment changes.

If a possum has evolved in a eucalyptus forest, for example, and the forest is cleared, its sounds will travel farther (because there are fewer trees to muffle it). As a result, the possum may reveal itself to a predator when it doesn’t mean to.

This in turn means the animal’s call leaves it more exposed than it “should” in evolutionary terms. The animal may not have the same tools to escape predators that animals evolved for open environments do, and so may be more easily eaten.

What are humans doing?

Many species have reduced in body size due to things like harvesting activities and climate change.

It’s a well documented fact that many whale species have been getting smaller as a result of human whaling activities and environmental impacts.

Since 1981, for example, the length of northern right whales has become about 7% smaller. Among gray whales, animals born in 2020 are estimated to be 1.65 metres shorter than animals born in the 1980s.

Given our finding that larger body sizes mean farther-travelling sounds in aquatic mammals, smaller whales may not be able to be heard as far away.

This means that when smaller whales call to their friends or family members, their calls may not reach these individuals over the enormous distances the species travel.

What can humans change?

Our findings add a new dimension to our understanding of how humans are affecting animals, and may help inform future conservation decisions.

Do they mean anything in our everyday lives?

For one thing, they remind us to take a moment to listen to the world around us.

We might find out where an animal is. We might observe a new species.

We might even find a quiet space in the landscapes around us to sit and connect again with the world and ourselves.

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