Australian Wildlife

Podcast Transcript

Australia is unique among the countries in the world.

It is a continent, a country, and an island all wrapped up into one.

Australia is also home to some of the most bizarre species in the world, most notably its large number of marsupials, which comprise 70% of all marsupial species worldwide.

Why do Australia’s animals differ so significantly from those of the rest of the world, and what led to the dominance of marsupials?

Learn more about the animals of Australia on this episode of Everything Everywhere Daily.

As I noted in the introduction, Australia is different. Geologically, it is very old, with very little in the way of geological activity over the last several million years.

It is also very remote. Africa, Europe, and Asia form a single, vast landmass that was once connected to North and South America as recently as 20,000 years ago.

Australia, however, is off on its own.

As such, it shouldn’t come as too much of a surprise that the wildlife in Australia is also markedly different than what is found on the rest of the planet.

When I say different, I’m really mainly talking about Australia’s mammals.

To start this discussion, we should define what a mammal is.

If you recall my episode on biological taxonomy, there are eight different taxonomic levels: domain, kingdom, phylum, class, order, family, genus, and species.

Mammals are a taxonomically defined group of animals that fall under the class Mammalia.

Under the class Mammalia are two subclasses. The subclasses are defined based on how they give birth. There are monotremes, which are mammals that lay eggs, and therians, which give live birth.

The therians are further subdivided into two infraclasses, which are essentially sub-subclasses. There are two main types of mammals: pouched mammals, also known as marsupials, and placental mammals, commonly referred to as eutherians.

If we take monotremes, marsupials, and eutherians as the three most basic types of mammals, Australia is the only continent that contains all three. In all three types, their presence in Australia has an interesting story.

Let’s start with Marsupials.

Marsupials share many common traits with the larger mammalian class, including having hair and mammary glands, which are specialized glands that produce milk.

Marsupials are a group of mammals that give birth to relatively undeveloped young, which then continue to develop outside the womb, typically in a pouch on the mother’s belly.

Technically, marsupials do have placentas, but theirs are very short-lived and much less complex than those of eutherians. The marsupial placenta provides nutrients to the developing embryo for only a brief time before birth.

After this short gestation, the tiny, underdeveloped young are born and crawl into the mother’s pouch to continue developing while nursing.

In this phase, the child is roughly the size of a jelly bean. In this embryonic condition, the child’s hind legs and skull are not fully developed. But their forelimbs and mouth are more developed.

This gives the offspring the tools to climb to their mother’s pouch, or cling to their mother if they do not have a pouch. They then fuse to the mother’s nipple for the rest of their development.

There are certain evolutionary advantages to having a pouch.

In a dangerous situation, marsupials can abandon their young. This might not seem like a good evolutionary strategy, but it could mean the mother lives to have more offspring, which can better help the species survive.

Likewise, the embryo in the pouch also has some benefits as well.

The mother’s immune system is less likely to attack the embryo and allows the mother to maintain eating the same amount of food.

Risks of childbirth are also usually avoided as the offspring is born at such an early stage.

Some of the most common and well-known types of marsupials reside in Australia, including kangaroos, koalas, wombats, among many more.

Due to their embryonic stage, biologists have long believed that marsupials represent an intermediate stage of development between egg-laying mammals and placental mammals.

One of the reasons it is thought that marsupials survived this stage is because of their isolation in Australia.

Interestingly, despite developing separately, marsupials and placental mammals developed in similar behavioral and structural ways.

This is an example of convergent evolution, similar to how fish and aquatic mammals share a similar appearance.

Such examples include sugar gliders and flying squirrels, marsupial moles and placental moles, or the extinct Tasmanian tigers and wolves.

Marsupials can also fill the same ecological spaces as cats, bears, and rabbits, among others. A good example of this is the kangaroo and the deer, which fill a similar ecological role.

This process is known as “adaptive radiation,” which occurs when a species diversifies rapidly to fill different ecological roles and utilize available resources.

I recall driving around the country and coming to the realization that kangaroos were essentially Australia’s equivalent to the white-tailed deer in North America.

However, there is a reason why there are fewer marsupials compared to placental mammals in the rest of the world. Marsupials are generally considered to be less intelligent than placental mammals. This is due to their brains being smaller.

Their skulls are smaller and more compacted, and their brains don’t have a corpus callosum, which is what connects the two cerebral hemispheres of the brain.

Marsupials also have less social organization when compared to their placental counterparts. Even those, like kangaroos, that move in groups do not live in a true hierarchical social structure.

The big question then is why is there such a disproportionately large number of marsupials in Australia?

Marsupials likely originated in South America during the Cretaceous period, over 100 million years ago. Fossil evidence suggests that the earliest marsupials evolved there after diverging from a common ancestor shared with placental mammals.

From South America, some marsupials migrated to Antarctica, which at the time was connected and much warmer. From there, they then migrated to Australia, likely around 50 to 60 million years ago.

Once in Australia, marsupials diversified extensively due to the absence of competing placental mammals, leading to the wide variety of marsupial species found there today.

Marsupials were not the only species to attempt this crossing, but they were the main survivors of the journey. According to biologists, this could be because the journey to Australia was easier for marsupials because of their reproductive system.

Other biologists are unsure if marsupials developed before or after the placentals, but if placental animals did cross over to Australasia, they died out.

Regardless, plate tectonics eventually resulted in the continents spreading further apart from each other, leading to Australia’s isolation 65 million years ago.

While marsupials dominate Australia’s mammal population, they are not the only type. The subclass is monotremes, the egg-laying mammals.

They are the only mammals that lay eggs instead of giving birth to live young. Monotremes also retain some reptilian traits, such as a cloaca, or a single opening for excretion and reproduction.

There are only five living species of monotremes, all found in Australia and Papua New Guinea: the platypus and four species of echidna, also called spiny anteaters. Despite their unusual reproduction, monotremes are true mammals in that they produce milk and have fur.

Monotremes are the most primitive group of living mammals, and they represent an ancient lineage that split off from the common ancestor of all mammals more than 200 million years ago, during the Triassic or early Jurassic period.

Fossil evidence and genetic studies suggest that monotremes diverged before the evolution of the two other main mammalian groups: marsupials and placental mammals.

This early split explains their many unique traits, such as egg-laying, a cloaca, the lack of a corpus callosum, and certain skeletal features more reminiscent of reptiles.

The oldest known monotreme fossils date back to approximately 110 million years ago, during the Early Cretaceous period, and were found in Australia. These fossils confirm that monotremes were already distinct and specialized in the age of dinosaurs.

Monotremes likely evolved in Gondwana, the southern supercontinent that included Australia, Antarctica, and South America. As these landmasses drifted apart, monotremes became isolated in Australia and New Guinea, where they survived and evolved into the modern platypus and echidnas.

I should address the placental mammals, or eutherians, that exist in Australia. While marsupials and monotremes get most of the attention, eutherians do exist.

The most common are bats and small rodents such as rats. I’ll explain why they are in Australia in a bit, but there is one large animal which is also an eutherian: dingos.

Dingos likely arrived in Australia around 3,500 to 4,000 years ago, brought by Austronesian seafarers or other maritime peoples from Southeast Asia.

This was long after humans first arrived in Australia around 50,000 years ago and well after marsupials had become the dominant mammals on the continent.

Dingos spread across mainland Australia and became top predators, often filling ecological roles that were once occupied by now-extinct native species, such as the Tasmanian tiger.

Dingos are essentially an invasive species, albeit an ancient one.

A question some of you might be asking is if the isolation of Australia results in the dominance of marsupials on the continent, why are there any eutherians in Australia at all?

The answer lies in the natural boundary known as the Wallace Line, a topic I covered in a previous episode.

During previous ice ages, sea levels dropped, allowing Australia to be connected by land to Papua New Guinea and for most of Indonesia to be connected to Southeast Asia.

However, Australia never connected to Asia during these sea level drops. This is because there is a deep ocean trench between the islands of Bali and Lombok, and Borneo and Sulawesi in Indonesia.

No matter how low sea levels dropped, the trench prevented these two land masses from being connected. That means large mammals that traveled by land couldn’t migrate between Australia and Asia.

However, flying mammals, like bats, and small animals like rats, if they got lucky enough to get suck on a piece of debris, could cross the gap.

It also explains why marsupials didn’t cross into Asia and stayed in Australia.

The vast majority of marsupial species in the world reside in Australia, with 70% of these species inhabiting the Australian continent and the nearby islands on the same side of the Wallace Line.

The remaining 30% live in the Americas, with the majority of those marsupials residing in South America, and one species, the Virginia Opossum, living in North America.

As stated earlier, the lack of marsupials is likely due to competition with placental mammals present in South America, which caused the larger marsupial species to go extinct.

Modern-day marsupials are facing new challenges and threats.

Since the British colonized Australia over 200 years ago, it has rapidly lost much of its unique biodiversity.

One of the most severe impacts on Australia’s biodiversity has been the introduction of invasive species.

Australia has had the highest rate of mammal extinction of any country.

House cats and foxes have decimated the populations of some smaller marsupial species, as these species have never evolved to protect themselves from such predators.

Competition prevents resources, such as food or water, from reaching some native species. This can be seen in the addition of grazing species, such as cattle and sheep, which can overgraze vegetation.

The introduction of new weeds can outcompete the native plant population, altering the food sources for marsupials.

Finally, new species can introduce disease to the native population as they have never developed immunity.

However, not all marsupials are at risk, and some are thriving. Kangaroos and wombats have been adapting surprisingly well to modern environments.

The isolation of Australia led to an explosion of biodiversity on the continent, resulting in the vast majority of marsupials in the world today being found there.

Geology, evolutionary biology, and a strategically placed oceanic trench were all factors that contributed to the mix of animals that live in Australia today.

The Executive Producer of Everything Everywhere Daily is Charles Daniel. The Associate Producers are Austin Oetken and Cameron Kieffer.

Research and writing for this episode were provided by Olivia Ashe.

Today’s review comes from listener smarter thanthou on Apple Podcasts in the United States. They Write:

Very good and rarely biased

Just caught up, I’m now part of the Montreal chapter of the Completionist club!

This podcast is very informative and enjoyable. Gary is generally very

good at covering topics without much bias and from an objective

standpoint. I recommend listening to this podcast as you will most

likely learn a lot from it.

Thanks, smarter thanthou! Congratulations on your membership in the Montreal Chapter of the Completionist Club! You will find that the Montreal Clubhouse has a fine selection of bagels, deli meats, and poutine.

Please make sure you pick up your key and member’s jacket on your next visit.

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