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Why Are Marsupials Found Only In Australia?

Take a look at global animal distribution and you’ll notice how each country in the world has a specific type or species of animal, that isn’t found anywhere else.

One such group of animals is the Marsupials – animals that possess a pouch which they use to raise their young in. Some of the best examples of Marsupials are kangaroos, koalas, possums and wombats.

When we think about marsupials, we always associate them with the Land Down Under. Why is it that marsupials are found only in Australia?

Okay, let’s take a moment to set the record straight. Marsupials aren’t found only in Australia. They are also found in South America, Central America and certain parts of North America & Southern Canada. The best example of an American marsupial is the Opossum

Possum 2
An opossum

Scientists believe that the first marsupials were actually born in South America and they crossed Antarctica to finally land on and inhabit Australia. This was 180 million years ago, when Australia, Antarctica and South America were a single super-continent called Gondwana. A common marsupial ancestor born in South America branched into two distinct species, with one residing in the Americas and the other migrating to Australia. Today, over 200 species of marsupials are found in Australia, 100 in South America and 13 in Central America – all descendants of the single American ancestor. 

Gondwana
A representative image of the ancient super-continent of Gondwana (image source)

So, the question we should be asking now is – “Why are a majority of the animals that are found in Australia, marsupial?” Or, a better question would be, “Why are so many marsupials still alive in Australia, when most of their American counterparts are extinct?” What makes Australia such a fertile ground for the birth (& survival) of so many marsupial species?. 

The answer can be two-pronged. One line of thinking states that the geography of the country-continent is the reason for a high percentage of marsupials in Australia.

Australia has been a landmass that has remained largely separate from other continents for millions of years. This meant, it was subject to weather and soil conditions that was completely different from what was found on other continents. In turn, this affected the type of plants that grew on the continent, which changed the diet of the Australian marsupials significantly from their American counterparts. The researchers who support this theory believe that the diet offered by Australia was more conducive to the development of the marsupial species as a whole, compared to the diet elsewhere.

The second theory is that, since Australia was largely and for a very long time secluded and protected from the invasion of foreign species, the marsupials of yore didn’t have much competition to face for shelter, food and water. Additionally, the predominantly marsupial population ensured the birth of more marsupials and over time, the continent was soon overrun by marsupials. 

On the other hand, the Americas blossomed with many distinct species of animals, leading to intense competition for resources and as a result, the extinction of many marsupial species. We need to remember here that marsupial babies are born underdeveloped due to the lack of a placenta. They need additional time compared to their placental or egg-born cousins to grow into strong & mature creatures. With so many threats lurking around and such few resources to be shared by thousands of animals, it was just a matter of time before the genetically-weaker marsupial species in the Americas went extinct. 

Placenta vs marsupial
A short list of placental mammals and marsupials commonly found in the wild (image source)

The future of Australia’s marsupials

So, what’s next for our pouched friends?

Species around the world are experiencing the brunt of habitat loss and governments are implementing conservation projects to keep them safe. In Australia, the kangaroo is given protected status – with criminals found injuring or killing them, getting a one-way ticket to prison. But not all marsupials have been afforded this luxury, making conservation a challenging endeavour.

Additionally, some species like the antechinus, are going extinct for another (never-anticipated) reason – their suicidal mating tendencies – and have stumped scientists. Experts are now scrambling to save these almost-extinct species, but it may already be too late. 

In terms of whether we’ll see any new marsupial species being discovered anytime soon; only time will tell. For now, the focus is on preserving the population that is present in the Land Down Under.

 

-NISHA PRAKASH 

P.S: Featured image: Pixabay