The Tale Behind the Presence of Hopping Mice in Australia and the Absence of Kangaroos in Asia

The Fascinating Tale of Australia’s Unique Fauna

The frill-necked lizard is one of many land animals that reached Australia from Southeast Asia. Credit: Damien Esquerré, Author provided

Australia is home to a remarkable array of animals that are unlike anything found in Asia. While it’s well-known that Australia boasts peculiar and wonderful creatures like the platypus and the koala, what might surprise you is that many of these iconic critters actually originated from Asia and arrived fairly recently – at least in geological terms. Among Australia’s most recent additions are various lizard species, such as goannas and thorny devils, as well as hopping mice, flying foxes, and the kookaburra. However, the movement of animals between the continents has been mostly one-sided, with significantly fewer representatives of Australian fauna found in Asia compared to Asian fauna in Australia. The reason for this asymmetrical distribution has been a subject of curiosity.

Exploring the Origins

In a study recently published in the journal Science, my colleagues and I delved into the distribution and habitats of 20,433 species of land-dwelling vertebrates over the past 30 million years. We also examined climate and plate tectonics during this period to unravel the reasons behind this peculiar distribution.

Continental Drifts on a Changing Planet

To understand this phenomenon, we must travel back in time over 200 million years. At that point, dinosaurs roamed the Earth and Australia was part of a supercontinent called Gondwana. Gondwana included present-day Antarctica, South America, Africa, Australia, and India. This supercontinent had split from another called Laurasia, which consisted of North America, Europe, and Asia. When Gondwana and Laurasia separated, Australia lost its land connection to Asia. Over time, each piece of Gondwana embarked on its own independent journey. Some of these journeys eventually led them back towards Laurasia.

India collided with Eurasia, giving rise to the magnificent Himalayas. South America crashed into North America, forming the land bridge of Panama. Africa collided with Eurasia, creating the Mediterranean Sea. Finally, Australia began its collision course with Asia.

Around 45 to 35 million years ago, Australia completely detached from Antarctica, when it was situated much farther south than it is today. As Australia started drifting northwards, the expanding gap between Australia and Antarctica led to the formation of the Antarctic circumpolar current, which drastically cooled the planet. Australia found itself isolated and going through a period of cooling and aridification. This unique environment gave rise to a distinct set of animals and plants.

Stepping Stones across Continents

Meanwhile, the Australian and Eurasian tectonic plates collided, resulting in the formation of numerous islands known as the Indonesian archipelago. These islands, including Lombok, Sulawesi, Timor, and the Lesser Sunda Isles, do not belong to either the Australian continental shelf (Sahul), comprising Australia and New Guinea, or the Asian continental shelf (Sunda), consisting of Thailand, Malaysia, Singapore, Sumatra, Java, Borneo, and Bali. This transitional zone is called Wallacea, named after the 19th-century British naturalist Alfred Russell Wallace. He was the first to notice differences in animal species on either side of what is now known as Wallace’s Line. These islands acted as stepping stones between two continents, enabling the exchange of species that had been separated for an extraordinarily long time.

However, as our research reveals, only certain types of animals were able to successfully traverse this divide and establish themselves on the other side.

The Role of Climate and Adaptations

The ability of species to cross the ocean was a crucial factor in determining which animals could move between Asia and Australia. Among all the groups of animals that made this journey, birds were the clear majority. However, this aspect alone was not enough for successful colonization. Animals had to be capable of thriving in their new environment, which often presented distinct challenges. We discovered that animals able to tolerate a wide range of wet and dry conditions were more likely to successfully make the transition. This finding aligns with the contrasting climates of Sunda, which is wet, and Sahul, which is dry. Animals capable of adapting to various points along this wet-dry spectrum were better equipped to move between the regions.

Yet, a fundamental question remains unanswered: why did more animals move from Sunda to Sahul than in the other direction?

The Influence of Time and Changing Environments

To complete the puzzle, we must consider how these essential factors—species’ dispersal ability and their capacity to adapt to new environments—have evolved over time. Most of the animals that migrated from Asia to Australia were birds, including the ancestors of the kookaburra. We know that Sunda has been characterized by dense tropical rainforests since before Australia separated from Antarctica. As the stepping-stone islands emerged, they had a humid equatorial climate similar to that favored by the rainforest vegetation and subsequent animals from Sunda.

Meanwhile, in Australia, rainforests were receding across most areas, making way for grasslands and woodlands. Consequently, as animals moved through the stepping-stone islands to New Guinea and the northern regions of Australia in Sahul, they encountered a band of relatively consistent humid tropical climate. However, most of the animal species in Sahul had evolved on the drier Australian mainland. Thus, traversing from mainland Australia through New Guinea and the stepping stones to Sunda necessitated adaptations to a significantly different environment. Furthermore, Australian animals that managed to reach the stepping-stone islands likely faced competition from pre-existing groups from Sunda that were already well-adapted to the tropical climate they preferred.

Ultimately, examining climate, geography, and the unique courses of evolution provides insight into the fascinating distribution of species we witness today. Questions like why kangaroos are absent in Asia but hopping mice thrive in Australia have complex answers that span millions of years. By exploring the deep history of our planet, we gain a better understanding of the natural world around us.

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