Why there are no kangaroos in Bali (and no tigers in Australia)

If you travel to Bali, you won't see a cockatoo, but if you go to the neighbouring island of Lombok, you will. The situation is similar with marsupials: Australia is home to numerous marsupial species, such as the kangaroo and the koala. The further west you go, the sparser they become. While you will find just two representatives of these typically Australian mammals on the Indonesian island of Sulawesi, you will search in vain for them on neighbouring Borneo. Australia, on the other hand, is not home to mammals that you will typically find in Asia, such as bears, tigers or rhinos.

This abrupt change in the composition of the animal world already caught the eye of the British naturalist and co-discoverer of evolutionary theory Alfred Russell Wallace, who travelled through the Indo-Australian Archipelago from 1854 to 1862 to collect animals and plants. He described an (invisible) biogeographical line running between Bali and Lombok and Borneo and Sulawesi that marked the westernmost distribution of Australian fauna.

Fascinating change of wildlife

Biodiversity researchers have long been fascinated by this abrupt change of creatures along the Wallace Line. How these distribution patterns came about, however, has not yet been clarified in detail.

One explanation is plate tectonics. Forty-five million years ago, the Australian Plate began to drift northwards and slid under the mighty Eurasian Plate. This brought two land masses closer together that had previously been far apart. It became easier for land creatures to colonise one continent from the other. Tectonic movements also gave rise to the creation of countless (volcanic) islands between the two continents, which animals and plants used as stepping stones to migrate westwards or eastwards.

More Asian animals in Australia than vice versa

But why more species found their way from Asia to Australia -- countless poisonous snakes, thorny lizards (Moloch horridus), hopping mice (Notomys sp.) or flying foxes bear witness to this -- than the other way round has been a mystery until now.

In order to better understand this asymmetrical vertebrate distribution along the Wallace Line, researchers led by Loïc Pellissier, Professor of Ecosystems and Landscape Evolution at ETH Zurich, have created a new model. It combines reconstructions of the climate, plate displacements between 30 million years ago and the present day and a comprehensive data set for around 20,000 birds, mammals, reptiles and amphibians that are recorded in the region today.

Climates in areas of origin decisive

In the latest issue of Science, the researchers now show that adaptations to the climates in the areas of origin are partly responsible for the uneven distribution of Asian and Australian faunal representatives on both sides of the Wallace Line.

In addition to plate tectonics, the environmental conditions that prevailed millions of years ago were decisive for the exchange between the two continents. Based on simulations, the researchers found that animals originating from Asia were more likely to "hop" across the Indonesian islands to reach New Guinea and northern Australia.

These islands featured a tropically humid climate, which they were comfortable with and had already adapted to. The Australian wildlife was different, having evolved in a cooler climate that had become increasingly drier over time, and was therefore less successful in gaining a foothold on the tropical islands than the fauna migrating from Asia.

The Asian climate thus favoured creatures that reached Australia via the tropical islands of the faunal region known as Wallacea, especially those that could tolerate a wide range of climates. This made it easier for them to settle on the new continent. "The historical context is crucial for understanding the biodiversity distribution patterns observed today and was the missing piece of the puzzle explaining the enigma of Wallace's line," says first author Alexander Skeels, a postdoctoral researcher in Pellissier's group.

Competitive advantages for tropical species

Traits of species that evolved in tropical habitats include faster growth and higher competitiveness to enable them to withstand the pressure of coexistence with many other species. In harsher climates, such as the colder and drier regions of Australia, organisms usually have to evolve special adaptations to cope with drought and heat stress. These include behavioural adaptations such as nocturnal activity and physiological adaptations to minimise water loss. "Many Australian frogs bury themselves in the ground and remain dormant for long periods for this reason," Skeels points out. "Something that is rare in tropical frogs."

The findings are important for the researchers: "They make it clear that we can only understand today's distribution patterns of biodiversity if we include the geological development and climatic conditions of prehistoric times in our considerations," says Pellissier.

The heritage of long past epochs has shaped the patterns of biodiversity right up to the present. It also helps us to understand why more species are found in the tropics today than in temperate latitudes. "To fully understand the distribution of biodiversity and the processes that maintain it in the present, we need to find out how it came about," says the researcher.

Read more at Science Daily

Skipping evolution: Some kangaroos didn't hop

Extinct kangaroos used alternative methods to their famous hop according to comprehensive analysis from University of Bristol and the University of Uppsala scientists.

Although hopping is regarded as a pinnacle of kangaroo evolution, the researchers highlight that other kinds of large kangaroos, in the not too distant past, likely moved in different ways such as striding on two legs or traversing on all fours.

In the review, published in Alcheringa: An Australasian Journal of Palaeontology, the team shows that there are other ways to be an evolutionary successful large kangaroo and that large-bodied kangaroo weren't only specialised in endurance-hopping.

The review is an extensive discussion of the fossil evidence of the locomotion of kangaroos and their relatives (including wallabies, tree-kangaroos, rat-kangaroos, etc.) over the last 25 million years, and presents new analyses of limb bone and ankle bone metric data that add weight to previous locomotor hypotheses.

Together they indicated that the higher speed-endurance hopping, typical of modern large-bodied kangaroos, was probably rare or absent in all but a few large-bodied lineages, including the direct ancestors of modern large kangaroos like red and grey kangaroos. However, the diversity of kangaroo gaits disappeared with the Late Pleistocene extinctions of larger animals (in Australia as well as on other continents).

While almost all kangaroos today, small and large, use hopping gaits to some extent, the fossil record reveals that the locomotory capabilities of some extinct kangaroos were comparatively diverse.

The earliest recognized late Oligocene-middle Miocene (25to 15 million years ago) basal types of kangaroos most likely employed quadrupedal bounding, climbing and slower speed hopping as their primary modes of locomotion. (All kangaroos today use quadrupedal locomotion at slow speeds, which manifests as pentapedal locomotion -- using the tail as a fifth limb -- in larger species.) Yet, all these early forms were small-bodied, below 12kg, with larger bodied kangaroos over 20kg not appearing until the late Miocene (around 10 million years ago), coinciding with increasing aridity and the spread of openly vegetated habitats.

Hopping is functionally problematic at larger body sizes. Consequently, some members of the later kangaroo radiation achieved a more specialized anatomy for efficient higher-speed hopping at body sizes over 35kg. Modern large kangaroos are spectacular hoppers but none today are over 100kg (most individuals under 70 kg) and many extinct forms were well above this size and physically too big to hop.

Lead author Professor Christine Janis from Bristol's School of Earth Sciences said: "We want people to appreciate that large kangaroos were much more diverse as recently as 50 thousand years ago, which may also mean that the habitat in Australia then was rather different from today.

"In fact, modern large hopping kangaroos are the exception in kangaroo evolution."

While hopping apparently originated early in kangaroo evolution, in small-bodied forms, with the emergence of larger-sized kangaroos in the late Miocene there were several different options: to become more specialized for large-bodied endurance hopping, as in the ancestors of modern kangaroos, or to adopt other forms of locomotion at higher speeds, as in two main extinct lineages. The protemnodons (so-called 'giant wallabies', closely related to modern large kangaroos) likely relied upon a more quadrupedal type of locomotion most of the time, and rarely hopped. The sthenurine short-faced kangaroos, a lineage that split from all modern kangaroos around 15 million years ago, apparently adopted bipedal striding at all speeds.

The new data presented on the length of the tibia (shin bone) and calcaneum (ankle bone) reinforce these earlier hypotheses of locomotor differences from modern kangaroos in these two extinct groups. Co-author Adrian O'Driscoll, a former Master's student in the Palaeobiology program at Bristol and now a PhD student at the University of York made this contribution. He explained: "Especially supported by this new data is the notion of bipedal striding rather than hopping in the sthenurines, as their calcanea lack the anatomy (a long calcaneal heel) that would help resist rotational forces at the ankle experienced during hopping, and suggests a more-erect limb posture rather than the crouched posture essential for hopping."

Professor Janis concluded: "The assumption that increasing continent-wide aridity after the end of the Miocene selectively favoured hopping kangaroos is overly simplistic. Hopping is only one of many gait modes employed by kangaroos both in the past and today, and the fast endurance hopping of modern kangaroos should not be regarded as some "evolutionary pinnacle'.

Read more at Science Daily