by Tim Flannery
A change in the world’s climate may have contributed to the meteoric rise of the macropodines, for around 6 million years ago the Earth entered a dry and hot phase that is marked by the Messinian crisis. This is when the Mediterranean dried out, leaving a massive bed of salt behind, which is still preserved under the sandy bottom of that azure sea. It is also when our first upright ancestors appear in Africa, marking the expansion of drier, more open environments there, and also when many extinctions occur among the mammals of North America.
Changes in Australia’s environment at the time of the Messinian crisis may have benefited macropodines in two ways. As the continent dried out, distances between watering points and areas of good feed would have increased, and the hopping mechanism perfected by macropodines would have provided an enormous competitive advantage. And if the drying trend led to the expansion of grasses (as some suspect it did) and other tough-to-digest plants, innovations in the stomach may have helped as well, for by this time the macropodine kangaroos had developed a method of digestion every bit as radical as hopping itself.
Kangaroos, acting alone, are incapable of digesting grass. Instead they harbour bacteria and other organisms in their expansive, sack-like stomachs that chemically attack and break down fibrous plant matter. It is a neat symbiosis, the kangaroo acting as a sort of mobile fermentation vat, while its stomach fauna feed the host with waste products and dead bodies. It provides a striking parallel with the ruminants, whose multi-chambered stomachs are well known. Kangaroos even ruminate (chew the cud) after a sort, though in their case the process is known as ‘merycism’ and can result in the forceful ejection of a bolus of food from the mouth. While a little more like expectoration, perhaps, it is close enough to dub the kangaroos as a type of marsupial ruminant.
There are, however, significant differences between the digestive systems of cows and kangaroos. The stomach-guests of cows are mostly microscopic—single-celled organisms and bacteria—while those of kangaroos can be grotesquely large. Inside the stomach of a kangaroo that has been feeding on prime quality green pick you will see a mass of wiry worms, as thick as a hairpin and twice as long. Sometimes these creatures are so densely packed that you can’t even see the grass the kangaroo has swallowed. Known as strongyles, these worms were long thought to be parasites, and a fine excuse they were too for those agitating against the consumption of kangaroo meat. More detailed studies, however, have revealed that they are the kangaroo’s little helpers, ingesting the grass that the kangaroo cannot break down, and feeding the kangaroo with their waste products. Their abundance is thus a sign of a healthy kangaroo.
So are kangaroos best described as herbivores or carnivores? In 1994 I located a tree-kangaroo entirely new to science. Dingiso is the size of a labrador, strikingly marked in black and white (somewhat resembling a panda) and it lives in the alpine scrubs atop the highest mountains in West Papua. When I asked Lani hunters what the creature ate they unhesitatingly said, ‘Worms.’ I scoffed at this, but upon investigating the stomach of one of the creatures I discovered a knotted mass of more than 250,000 large strongyles, including two species unknown to science, and between which hardly a vestige of the leaves the kangaroo had swallowed could be seen. The Lani were absolutely right, for the kangaroo is host to the worms, which it feeds chewed leaves, while the creature itself feeds upon the worms and their by-products. Thus, in a very real sense, some kangaroos eat worms even though they swallow leaves or grass. It is unusual, incidentally, for tree-kangaroos to have strongyles. This is because they get into the stomach as eggs, from kangaroo faeces carried to the mouth when feeding. Tree-kangaroos do not usually get infected because their faeces drop many metres to the forest floor, breaking the cycle. Dingiso, however, is the only tree-kangaroo to live on the ground, allowing it to benefit from this unusual relationship.
Strongyles are only useful in certain circumstances. If a creature with such stomach-guests feeds on more digestible food the guests will simply beat the host animal to its meal. It is only where they can break down a food that the kangaroo cannot tackle by itself that the relationship benefits the kangaroo. Grass is just such a food, and with the help of their stomach-guests the macropodines have been able to utilise a once unavailable food source. Grass, in effect, became the kangaroo’s new frontier.
Was hopping or digestion more important to the success of macropodines? The tree-kangaroos provide an answer. As you might imagine, hopping through the canopy can be dangerous, for one slip means death, and tree-kangaroos can be clumsy creatures. Indeed, so unsuited is hopping to life in the treetops that some species of tree-kangaroo have re-learned to walk when aloft. Despite this, though, the tree-kangaroos have been wildly successful. The ten living species dominate the large herbivore niche in Australasia’s rainforests, presumably because their digestive efficiency more than makes up for their clumsiness. In their case the macropodine stomach found a somewhat different use, for their stomach-guests help destroy the toxins that the leaves of rainforest plants are often laced with, as well as helping break down the leaf’s cuticle.
It has long been believed that the success of the macropodines was the result of drying around the time of the Messinian crisis. The example of the tree-kangaroos, however, along with the abundance of macropodines in the ancient rainforests of Hamilton, leave me unconvinced. Is it possible, I wonder, that the macropodines arose in rainforests and initially evolved into many types there before spreading into drier environments? As long as the ‘ghastly blank’ in our fossil record from around 7 to 4 million years ago persists, we are unlikely to be able to answer this fundamental question. We do know, however, that in time the macropodines would reach their finest flowering in the grasslands and woodlands of Australia, and the story of that episode in their history is best read at a fossil locality 1000 kilometres northeast of Hamilton, in the Hunter Valley of New South Wales.
15
Grass for the Kangaroos
Bow Creek is a small waterway near the town of Merriwa in the Hunter Valley, around 200 kilometres northwest of Sydney. It wends its way through a bucolic landscape of managed pasture and stately river red-gums typical of the western slopes of Australia’s Great Dividing Range. A chance discovery by a road worker in the 1960s revealed that the region once had a far richer fauna. He had found the jawbone of an extinct giant rat-kangaroo allied to Propleopus, an exciting find which had palaeontologists from the University of Sydney scurrying out to investigate what turned out to be a very rich fossil field.
By the time I arrived on the scene in 1982 the locality had been well worked over and a small collection had been assembled. Reflecting on the fossils and the proximity of the site to Sydney, Michael Archer decided this was a good place to give students a taste of life in the field. The fossils were preserved in an old river ‘terrace’ about twenty metres above Bow Creek. The terrace was once the creek bed and its winding course can still be traced for kilometres across the countryside. The bones proved impossible to date, but comparisons with the Hamilton site indicate that the Bow fossils were around 4 million years old. This interested me greatly, for it was a critical period for the evolution of kangaroos.
Road-cuttings can be dangerous places, and the one at Bow Creek was narrow and steep, so with twenty or more enthusiastic students clambering about its sides, the passing of a cattle truck often had me—a tutor responsible for the students—thinking more of liability and death by misadventure than ancient bones. Other difficulties emerged from the Department of Main Roads, which objects to people undermining their road cuts. This, combined with the fact that the owners of a house perched beside the cut had visions of their abode tumbling into the traffic, made our excavations at Bow less extensive than we would have liked.
Despite such impediments, our understanding of the fauna grew each dusk as we gathered around the barbecue at the Merriwa caravan park to spread the day’s booty on a picnic table. Then, over a glass or two of the wine the Hunter region was rapidly becoming famous f
or, we would put our deductive powers to work. What species did this jawbone belong to? Why was this backbone still articulated, while that bone was rounded to a nubbin? The owner of the backbone, we concluded, must have died very close to where its remains were interred, while the nubbin could have come from a creature that lived and died miles upstream. Thus was a picture of Bow Creek as it was 4 million years ago gradually assembled.
Most of the species were entirely new to science, and it became part of my doctoral research to sort out which bones belonged with which set of teeth, and to name the species. Around nine out of every ten bones unearthed at Bow belonged to a member of the kangaroo family, the old river gravels being crammed with the remains of animals that, were they living today, would be called wallabies—a catch-all name for smaller kangaroos.
The fossil ‘wallabies’ were breathtakingly diverse, the most common belonging to a genus known as Troposodon. They are very rare in most fossil deposits and their heyday was around 5 to 3 million years ago. Though five times larger, they are related to the cat-sized banded hare wallaby (Lagostrophus fasciatus), which is the last survivor of a diverse and numerous subfamily of kangaroos. Known as the Sthenurinae, it included the troposodons and the short-faced kangaroos, which we will hear more of later.
The banded hare wallaby is a beautiful soft grey, its rump is marked with a series of deep chocolate-and-silver bands. It is now, sadly, a relic of a relic, for not only is it the last of its lineage, but it clings to existence on just two islands in Western Australia’s Shark Bay. You can trace its decline on the mainland through specimens lodged in various European museums. The first population to vanish lived in South Australia. Until 2003 it was thought to have become extinct before the arrival of the Europeans, but then I recognised a specimen collected in 1863 in the Gawler area, which had lain unidentified in a glass-fronted cabinet in the Humboldt University museum in Berlin for 140 years. It represented a new subspecies, which my student Kris Helgen and I named.
The South Australian banded hare wallaby was mostly reddish rather than grey, and sported a very handsome and luxuriant black crest on its short tail. We might know more about it had not a second specimen been thrown out in the nineteenth century because it was ‘too rotten’. But I suppose that we should be grateful for small blessings, for beside the sole surviving example hung a number of scorched wallaby skins, one of which bore a handwritten label indicating that it had caught fire on 1 May 1945—seven days before war’s end—as Russian troops, engaged in hand-to-hand fighting in the museum, tossed a grenade at the retreating Germans. Had that grenade veered a metre to the side we may never have known what South Australia’s unique banded hare wallaby looked like.
The western subspecies of the banded hare wallaby fared a little better. Collections in London’s Natural History Museum and at Oxford University indicate that it persisted in the forests of the Darling Escarpment inland from Perth until at least 1906, where it may have been protected by a peculiar native plant known as poison peas. Gastrolobium is so toxic that even a mouthful can be fatal to a sheep. The active ingredient is fluoroacetate, better known as 1080, and through long exposure to this toxin the marsupials of the southwest have become remarkably immune. By a quirk of its chemical structure, members of the dog family are exquisitely sensitive to fluoroacetate, and the tiniest dose is fatal for a dingo or a fox. So while the poison-pea bush remained dense the little wallaby persisted. Nevertheless, changes in the early twentieth century—to do with land-clearing and fox predation—led to its rapid extinction on the mainland, leaving only a few hundred on Bernier and Dorre islands.
The other ‘wallaby’ fossils discovered at Bow belonged to creatures that approximated the living swamp wallaby (Wallabia bicolor) of the east coast and the agile wallaby (Notamacropus agilis) of Australia’s north. These animals are browsers (meaning that they eat a variety of foliage, but not much grass) or browser-grazers (which eat more grass, but are not dependent on it). No remnant of the vegetation that fed Bow’s kangaroos has survived, but a crocodile tooth preserved nearby suggests that conditions were then warmer than today, perhaps allowing more-tropical plants to thrive.
From the abundant remains of browser-grazers we can surmise that the vegetation of the site was relatively open, perhaps a woodland with some grass and thickets. Two waterworn teeth of a tree-kangaroo testify that in the upper reaches of Bow Creek patches of rainforest persisted, while a few teeth of a euro—the only larger kangaroo found at the site—indicate the presence of rocky slopes and grass. Not a single scrap of the Propleopus-like giant rat-kangaroo was ever recovered during our dig. The dozer driver clearly hit the jackpot the day he jumped off his machine to pick up the jaw of this most elusive of fossils.
In effect, Bow Creek marks the modernisation of the kangaroos, for by then almost all of the major genera known today, as well as the immediate ancestors of many of the huge ice-age types, had come into existence. The fossils from Bow Creek demonstrate how the dominant trend in kangaroo evolution has been to colonise ever more open habitats. But as always there is the exception that proves the rule, for one kangaroo has forsaken the plains to return to life in the dense scrub. The creature was known to the Eora people of the Sydney region as bugaree (pronounced something like ‘buggery’), while the European settlers knew it by the misleading and bland name of swamp wallaby (Wallabia bicolor). Although molecular studies reveal that it is closely related to the large red and grey kangaroos and the striped wallabies, its teeth are fundamentally different, indicating a reversion to a life of eating soft herbage. It is a great survivor, for it is found over a wide area of eastern Australia—often far from swamps—wherever vegetation is dense.
The fur of bugaree is a delicious licorice colour, tipped with silver, while its underside is often a deep, rich ochre; yet the beauty of the animal is difficult to appreciate in nature, for it is rarely seen outside dense thickets. But if you are bushwalking in any of Sydney’s larger national parks and you go quietly, you might catch a glimpse of one. If it is a warm day the animal might be sunning itself beneath the north face of a sandstone boulder, its eyes half closed in pleasure, its rich, red belly exposed—much in the pose of a sunbather and as relaxed as any Bondi beauty. It will not be easy to spot, for its ochre belly will mirror the iron stains on many rock surfaces. Perhaps a twitch of one of its shortish ears will alert you; or maybe all you will see is the white tip of its long tail disappearing into the bushes as the vigilant bugaree, having seen you approaching long before you saw it, flees for safety.
Bugaree has a paltry number of chromosomes: eleven in the male and ten in the female—the smallest number of any marsupial. It nevertheless uses its genetic inheritance to great effect, for it is able to eat toxic plants—even introduced ones such as the hemlock that saw off Socrates—with no evident ill-effect. But it is the tweak that it has given to the kangaroo method of reproduction that is its most astonishing achievement.
Unlike other kangaroos bugaree’s females do not have to await the birth of their young in order to mate and conceive again. Instead she mates up to eight days before the birth, thus gaining an extra week to grow her embryo before it is thrust out into the world. No other marsupial can do this, for none, except some tree-kangaroos, can maintain a pregnancy for more than the length of their oestrous cycle. Just how the animal manages this in terms of hormones, immune responses and such like, is not known. But bugaree must have many secrets that we have not even begun to guess at, for it is the last of a once numerous tribe of kangaroos to survive around Australia’s eastcoast cities. If you live in Melbourne, Sydney or Brisbane, it’s a fair bet that bugaree is closer than you think. In Sydney it still appears as close to the city centre as the Manly reservoir, just twelve kilometres from the Opera House, while around Melbourne the Dandenongs and Cranbourne areas remain favoured haunts. In his memoirs written around 1883, Sydneysider Obed West recalled that they were Very numerous’ in the vicinity of what is now the Australian Museum on College Str
eet, and ‘many scores of them have I shot’ in a wetland just south of the museum site. Once upon a time potoroos (another Eora word), bettongs, rufous rat-kangaroos, rock-wallabies, red-necked wallabies and eastern grey kangaroos all thrived around Sydney. And there is no reason why these creatures should not flourish once more, for Sydney is blessed with parks and reserves, and kangaroos are adaptable creatures. All they require is a little assistance from the city’s humans.
Every year, usually around June, the first winter winds blow in from the west over Sydney, chilling the estuaries and wetting the vegetation. It’s a hard time for bugaree, and it is then that you might notice something furry floating in the waters of Botany Bay or the Hawkesbury estuary, carried by the tide towards the open sea. If such a sight passes you by as you sit fishing on a jetty or pontoon, you might wish to spend a moment of two examining these last mortal remains of bugaree. You will find that her teeth are worn to the gums and her claws blunted by years of clambering over rough sandstone boulders. She is old—very old—and has not fed well for months now, for her swollen joints make travel painful and her worn teeth cannot chew the plants that give her sustenance. But still she breeds, the long nipple in her pouch indicating that right up until the last she suckled a joey. She is a survivor—perhaps she too has ‘the will to fail’—and she commands our salute as she goes to her final rest.
16
Not Formed for Such Work
On Sunday 12 October 1872, the explorer William Hann was in the rainforest wilds of northeast Queensland and, judging from his diary entries, the humidity and rugged topography were leading to a shortness of temper. The last straw, it seems, was a story told by his Aboriginal guides of a kangaroo that lived in trees. This ‘fable’ was, to Hann’s chagrin, taken seriously by the expedition’s doctor, Thomas Tate, who claimed to have located a ‘nearly complete skeleton’, with which he wanted to burden the expedition horses.