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Kangaroo
Portrait of an Extraordinary Marsupial
By Stephen Jackson, Karl Vernes Allen & Unwin
Copyright © 2010 Stephen Jackson and Karl Vernes
All rights reserved.
ISBN: 978-1-74269-108-4
CHAPTER 1
FANGAROO TO KANGAROO
The Evolution of Macropods
The Quadrupeds we saw but few and were able to catch few of those that we did see. The largest was call[e]d by the natives Kangooroo. It is different from any European and indeed any animal I have heard or read of except the Gerbua of Egypt, which is not larger than a rat when this is as large as a middling Lamb; the largest we shot weighed 84lb. It may however be easily known from all other animals by the singular property of running or rather hopping upon only its hinder legs carrying its fore bent close to the breast; in this manner however it hops so fast that in the rocky bad ground where it is commonly found it easily beat my greyhound, who tho he was fairly started at several killd only one and that quite a young one.
In the vernacular, a 'kangaroo' is any one of the 77 species of hopping macropod marsupials of Australia and New Guinea. We know that several species have been confirmed as extinct since European settlement but the fossil record shows that Australia once proliferated in macropods — some the size of rabbits, others giants weighing several hundred kilograms. Many of these ancient species, and all of the largest ones, became extinct shortly after the Australasian region was settled by its indigenous peoples, more than 45,000 years ago, while many others survived until the arrival of the first European settlers.
There were 57 species of macropod known to have occurred in Australia and its islands at the time of European settlement, with a further 23 species found on New Guinea and its surrounding islands, three of which are shared with Australia (see Appendix 1). The 77 modern species of macropods encompass a wide variety of shapes and sizes and range from the diminutive Musky Rat-kangaroo of the Australian Wet Tropics that weighs just 500 grams to the massive Red Kangaroo of the arid centre that can weigh as much as 85 kilograms. The group includes species that burrow, others that climb trees, some that live among the steepest rocky gorges and others that inhabit the seemingly endless desert plains. They live in rainforest, woodland, heathland, grassland and desert — there is not a vegetation type in Australia or New Guinea where you will not find at least one species of macropod.
THE MACROPOD FAMILY TREE
Living macropods are so diverse that they are divided up into three families within the Superfamily Macropodoidea. The most primitive of the three is the strangely named Family Hypsiprymnodontidae. The family has only one living member, the Musky Rat-kangaroo, the smallest of all the macropods and generally considered to be a missing link between the possums and kangaroos. The Musky Rat-kangaroo is unique to the rainforests of North Queensland and retains some of its possum heritage by having five toes on its hind foot. Every other macropod species has four digits, because evolution has done away with the first digit, thus allowing the animals to hop more efficiently. The Musky Rat-kangaroo is the only macropod that does not hop — it appears to have separated from the other macropods before hopping evolved. It is also unique in that it has a simple stomach, and females can give birth to twins and even triplets while all other macropods typically produce only one young at a time.
Distant relatives of the Musky Rat-kangaroo are the potoroos, bettongs and rat-kangaroos of the Family Potoroidae. These small macropods, generally referred to 'potoroids', all weigh less than 3 kilograms, and are represented by 11 species, of which three have become extinct since European settlement. Potoroids once occurred across the length and breadth of Australia — indeed, the Burrowing Bettong once had the greatest geographic range of all the macropods — but the family has no relatives within New Guinea. The potoroos live in the dense rainforest undergrowth of Australia's east coast, and parts of the south-west, whereas the bettongs prefer drier conditions such as open forests and woodlands or scrublands covered in Spinifex and other grass tussocks of central Australia. As we shall see in Chapter 6, the potoroids are also unusual in that their diet is typically dominated by the underground fungi known as truffles.
The third and by far the largest family, in both number and body size, is the Family Macropodidae, generally referred to as the macropodids. There are 65 species in this family, including the kangaroos, wallaroos, wallabies, rock-wallabies, nailtail wallabies, hare-wallabies, tree kangaroos, the quokka, pademelons and several species of New Guinea forest wallabies. Macropodids can be found throughout Australia and New Guinea — from the highest mountains to the most arid deserts. The most unusual macropodids are probably the tree kangaroos, which are amongst the family's most recently evolved lineages, in evolutionary terms, but which have taken to the trees to return to an arboreal lifestyle like their early marsupial ancestors. To this end they have developed several unique features, including forelimbs that are of similar size to the hind limbs, short broad feet, and the ability to move the hind feet alternately, which other macropods only do if they are swimming. Intriguingly, one species of tree kangaroo, the Dingiso — a species from the Indonesian province of Papua (previously known as Irian Jaya) described in 1995 by Professor Tim Flannery — has come full circle and readapted to a terrestrial lifestyle. The Dingiso has a more slender skeleton and longer hind feet than other tree kangaroos and therefore appears to be a tree kangaroo in evolutionary reverse — making the transition back down from the trees to the ground.
WHAT'S IN A NAME?
Common names for many of the macropods such as 'kangaroo', 'wallaroo', 'wallaby', 'pademelon', 'potoroo' and 'bettong' seem to be derived from the multitude of indigenous Australian languages. The local inhabitants of a region would have had specific meanings for these words, in that they would have given each name to an individual species of macropod. The first European explorers and settlers later adopted these names to describe a range of species and even several genera.
The word 'kangaroo' is sometimes used as a generic term for all of the Superfamily Macropodoidea, but it is often reserved for the larger members such as the Eastern Grey Kangaroo, the Western Grey Kangaroo and the Red Kangaroo. There is some disagreement about both the origin of the word and its original meaning. When Lieutenant James Cook was delayed at Endeavour River in 1770 while his ship underwent repairs, he wrote in his journal on 14 July:
Mr Gore, who went out this day with his gun, had the good fortune to kill one of the animals which had been so much the subject of our speculation. ... This animal is called by the natives Kanguroo.
Captain Cook's 'Kanguroo' is now assumed, after considerable debate, to have been the Eastern Grey Kangaroo (for more about Cook's encounter with kangaroos, see Chapter 3). Regardless of the (sometimes heated) discussions about which species was being referred to, it seems certain that the local inhabitants used the word to describe a large macropod. Folklore suggests that, on seeing a large creature hopping around, Cook asked a native what it was called and was told 'kanguroo' meaning 'I don't know', but this is almost certainly untrue. In the early 1970s, John Haviland's investigation into the language of the area, Guuyu Yimidhirr, concluded that there was no evidence that the language had changed radically between 1770 and the present day. Modern Guuyu Yimidhirr retains the word gangurru for 'kangaroo'. The myth perhaps stems from the records of Captain Phillip King, who in 1820 believed he had heard the word men-u-ah used as the name for the kangaroo. What King probably heard was minha, a Guuyu Yimidhirr word that literally means 'meat' or 'edible animal'.
We are familiar with the term 'wallaby', but what is a wallaby? Wollabi is an Eora word, from the language spoken by the people of the Sydney region, and was their name for the Brush-tailed Rockwallaby. The term was quickly adopted by the earliest settlers around Sydney, along with other Eora words such as walaru (wallaroo),budaru (potoroo) and betong (bettong). Eora also gives us the names for the wombat and the dingo. Today, 'wallaby' is often used loosely to describe the smaller macropodids of the Family Macropodidae, but there is no hard-and-fast rule as to when a kangaroo becomes a wallaby. A quick look over the various body weights suggests that, typically, wallabies weigh less than 20 kilograms and kangaroos weigh more than 20 kilograms, but apart from that there is no real difference between them. One could argue that the only true wallaby is the Swamp Wallaby, as it is the sole member of the genus Wallabia, though even this species has recently been proposed to be placed back in with the other wallabies and kangaroos in the genus Macropus. The Eora word for Swamp Wallaby is banggaray, which brings us no closer to a definition of wallaby! The name Pademelon, which today is given to small forest-dwelling wallabies of the genus Thylogale, is also an Eora word, being a transliteration of the word badimaliyan.
CLIMATE CHANGE AND THE RISE OF THE MACROPODS
How and when did such a wide variety of macropods evolve? In order to understand this question, we need to first understand how Australia's climate has changed over the last 35 million years. Continental plates have collided, merged and separated again several times during the Earth's history. The super-continent Pangaea ('the entire Earth') began to break up about 250 million years ago, forming two smaller super-continents — Laurasia, which included much of present North America, Europe and Asia; and Gondwana, the major southern land mass. About 160 million years ago, Gondwana began to fragment into the present southern continents (and many smaller fragments), which have been drifting apart ever since. Australia and Antarctica were the last joined fragments of this super-continent. Their rift began about 45 million years ago, a separation which was complete by about 35 million years ago. Australia then began drifting northwards, finally colliding (perhaps 5 million years ago) with an arc of islands lying to the south of South-east Asia. These islands became part of what we now call New Guinea, at the leading northern edge of the Australian plate. Australia is continuing its northward drift towards Asia, causing the highlands of New Guinea to rise and slowly closing the distance between it and the Indonesian archipelago.
The climate trigger for the evolution of the macropods appears to have occurred some 33–23 million years ago when a global cooling episode led to Australia's warm and moist climate becoming progressively cooler and drier. This process was initiated by the opening up of the circumpolar current around Antarctica, which led to a dramatic change in the surrounding ocean currents and associated climate, and the development of ice over Antarctica. Before Gondwana fragmented, its northern tips reached up towards the Equator, but those at the south were still connected to Antarctica. That meant that ocean currents could move only between the Equator and higher latitudes, producing a mixing that kept all oceans at a similar, warm temperature.
Uniformly warm ocean surface temperatures create uniformly warm, wet climates, so prior to the separation of Australia and Antarctica the planet's land masses were covered in forest adapted to the prevailing conditions. The opening up of a route for a circumpolar oceanic current effectively separated a pool of sub-Antarctic waters that rapidly cooled, from the warmer oceans that then circulated water from the tropics to temperate latitudes. This was the first stage in the 'polarisation' of the world's climate: the poles became deeply cold, while the tropics became much warmer. The polarisation of the southern hemisphere's climate, into a polar south, temperate mid-latitudes and a tropical equatorial zone intensified through the Miocene and into the Pliocene (24 million years ago to 2 million years ago) as Australia drifted further into the tropics. The overall effect on Australia's climate was a gradual drying of the interior, leaving only the eastern and northern margins as wet as the whole continent had been at the start of the Miocene.
It was this 'drying out' of Australia which led to the expansion and diversification of the sclerophyllous plants, with their leathery leaves which were better adapted to reducing water loss. The genus Eucalyptus became particularly common and widespread from about 120,000 years ago (but in many places only in the last 40,000–50,000 years). The increasing frequency of Eucalyptus, grass and forb (non-woody herbaceous plants other than grasses) pollens in lake-bed sediments is matched by a corresponding decrease in rainforest fossil remains and an increase in charcoal particle frequency, indicating a greater frequency or intensity of fires. It was this opening up of the forests and expansion of the grasslands and grassy woodlands which led to such a marked change in the terrestrial marsupial fauna, with the macropod component becoming more conspicuous, providing as it did an abundant supply of food for ground-dwelling browsers and grazers.
Much of Australia's land surface has been exposed to the elements for tens or hundreds of millions of years, so it has been weathered to the point where most of its soluble nutrients have long gone, leaving soils that are generally of low fertility. Much of Australia's vegetation has evolved to tolerate these low-fertility soils and is therefore of low nutritional value. Larger animals are more efficient at digesting low-nutrition foods such as grass as the energy requirements relative to body mass decrease as body mass increases, while gut capacity remains a constant proportion of body mass. The increasing availability of low-nutrient grasses has resulted in an increasing diversity of large marsupials, including giant kangaroos. In contrast, smaller macropods such as the potoroids need food with a higher nutritional value and lower fibre content. Over the last several million years, similar changes in vegetation structure were happening in other parts of the world, leading to the concurrent evolution of the various antelopes, horses and other large, open-country grazing mammals on other continents.
What did the macropod's extinct relatives look like? As it turns out the long-lost relatives of today's macropods were typically larger, sometimes had very different diets, and some may not have hopped. The macropods evolved from tree-dwelling possum-like ancestors that came down from the trees to the rainforest floor, and probably resembled the Musky Rat-kangaroo. Genetic studies suggest the ancestors of modern-day macropods separated from these ancient possum-like relatives in the Eocene some 40 million years ago, but this has yet to be confirmed by the fossil record as the oldest fossils found to date are from some 26–23 million years ago.
How were the ancestors of modern macropods related to each other and, in turn, to the modern species? The relationship between the extinct relatives of the Musky Rat-kangaroo and the potoroids, for example, has been much discussed. Until quite recently, the Musky Rat-kangaroo was placed in the same family as the potoroids, but it is now recognised as a distinct family with a sister lineage to all other macropods. The Musky Rat-kangaroo's primitive status is also supported by DNA research that suggests they diverged from the other macropods shortly after the macropods diverged from their possum-like relatives. The Musky Rat-kangaroo's recognition as a separate lineage gives rise to a number of related issues. Did bipedal hopping and the macropods' other unique characteristics evolve independently in the potoroids and macropodids? Did the Musky Rat-kangaroo's extinct relatives lose these characteristics after evolving into the common ancestor of all macropods or, as seems most likely, did bipedal hopping and a complex digestive system capable of fermenting difficult to digest plant material evolve after the Musky Rat-kangaroo's ancestors separated from the other macropods?
(Continues...)
Excerpted from Kangaroo by Stephen Jackson, Karl Vernes. Copyright © 2010 Stephen Jackson and Karl Vernes. Excerpted by permission of Allen & Unwin.
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