Out on a limb: tree-kangaroos

Evolution can take some surprising twists and turns, and none more so than with kangaroos that live in trees, says evolutionary biologist Mark Eldridge.

Drop bears and tree-kangaroos? They both sound like the creation of an Antipodean sense of humour, designed to lead astray unsuspecting tourists.

After all, why would an animal like a kangaroo, so superbly adapted to bounding around on the vast open Australian plains, come to live over 30 metres above ground in the tangled forest canopy where opportunities for hopping are quite limited? Indeed, the return of one branch of the kangaroo and wallaby family (the group we know as macropods) to life in the trees is an enduring mystery of marsupial evolution. But we are getting ahead of ourselves.

Arboreal existence

Unlike drop bears, tree-kangaroos really do exist – but only in the rainforests of Australia and New Guinea. The first time I saw one was as a child in the early 1970s at a wildlife park run by renowned naturalist David Fleay at Burleigh Heads on Queensland’s Gold Coast. These beautiful and marvellously improbably macropods have intrigued me ever since. It was over 20 years before I finally encountered one in the wild, this time on the Atherton Tableland west of Cairns, and only then after several nights of spotlighting.

Tree-kangaroos are excellent climbers, spending most of their time in the forest canopy where they feed on the leaves of trees and vines. In New Guinea, tree-kangaroos are quite diverse, with eight species recognised, mostly inhabiting the cool and very wet highland rainforests.

In Australia, there are now just two species: Lumholtz’s Tree-kangaroo, Dendrolagus lumholtzi, and Bennett’s Tree-kangaroo, Dendrolagus bennettianus. Both are found only in the Wet Tropics of far north-east Queensland, one species on each side of the Daintree River. The fossil record, however, indicates that tree-kangaroos were previously widespread throughout Australia in both rainforest and dry forest habitats with many more species than today.

Rock-wallabies

My interest in tree-kangaroos stems from many years of studying rock-wallabies, a quite different group of macropods, first at Macquarie University and more recently at the Museum. Rock-wallabies, as their name suggests, are adapted to exploit complex rocky environments, which they do with consummate skill and agility, climbing and descending quite steep rock faces with ease and precision. These abilities also enable them to climb trees to find both shelter and food.On field trips with colleagues, we would often joke about rock-wallabies being part tree-kangaroo, so you can imagine our delight and surprise when recent studies confirmed our frivolous speculations. Despite major differences in their morphology, tree-kangaroos and rock-wallabies are indeed closely related, sharing a common ancestor as recently (in evolutionary time) as 8 million years ago.So are tree-kangaroos really just rock-wallabies that moved into the trees? And what drove this evolutionary step? To escape predation? To exploit foliage as an abundant untapped food resource? To escape parasites and infections carried by faeces? Whatever the cause, ancestral tree-kangaroos underwent major and quite rapid morphological changes as they made the transition to an arboreal (tree-dwelling) lifestyle.

Law-breakers

Weighing up to 17 kilograms, tree-kangaroos are the largest arboreal animals in both Australia and New Guinea. Their feet are short and broad, and their forelimbs have become massively developed to facilitate gripping and climbing. Though often slow moving, even clumsy, in trees, they have evolved other specialisations for their arboreal existence, such as flexible ankle joints and, in some species, the ability to walk bipedally (like humans).This ability to move or rotate the ankle joint is an intriguing feature of their evolution. Other macropods evolved rigid ankle joints from a more flexible ancestor as an adaptation for moving by hopping, so tree-kangaroos must then have reacquired flexible ankle joints. Clearly tree-kangaroos have not heard Dollo’s law of evolution biology, which states that evolution is not reversible . . . or was it that Louis Dollo (a nineteenth-century Belgian paleontologist) had never heard of tree-kangaroos?

Path to extinction?

Despite their obvious scientific interest, tree-kangaroos are not easy subjects for study and they remain one of the most poorly known groups of mammals. Fieldwork is hampered by their remote, often-inaccessible, natural habitats and their secretive lifestyles, and so we know little about their ecology, behaviour or reproduction. We are not even sure how many species exist; four new species or subspecies of tree-kangaroo have been described by Australian Museum researchers since 1990 and it is highly likely that more await discovery.The uncertain identification of tree-kangaroos is hindering efforts to develop and implement effective conservation measures, especially in New Guinea. It is greatly concerning that most of the known tree-kangaroo species are seriously threatened with extinction, both as a result of the destruction of their forest habitat (for logging, agriculture and mining) and because they are hunted for food by the growing numbers of people who share their forests. Already, as a consequence of human activity, many tree-kangaroos have been eliminated from large areas where they once thrived.If we are to ensure their survival, we need to urgently identify how many species of tree-kangaroo there are, where they occur and how they are interrelated. What we think of as one widely spread species represented by several populations could turn out to be two or more restricted species. The loss of one population, perhaps due to a misinformed land-use decision, while bad enough, could become the loss of a previously unrecognised species. It is fitting indeed that tree-kangaroos, perhaps because of their large size and charismatic nature, are increasingly being used as flagship species to promote rainforest biodiversity conservation.

Evidence

This lack of basic knowledge also highlights a critical role for natural history museums and their collections. The Australian Museum has the most comprehensive collection of tree-kangaroo specimens in the world and, with funding from the Hermon Slade Foundation, we have recently commenced a research project using DNA sequence data to establish a clear taxonomy for tree-kangaroos that should underpin future conservation efforts. Scientists currently recognise just 10 species of these creatures, but our preliminary DNA sequencing data has already identified 13 highly divergent lineages, each of which appears to represent a distinct species. Clearly tree-kangaroos are much more diverse than previously thought. We will also be using our DNA sequence data to unravel some of the mysteries surrounding the evolutionary origins of tree kangaroos. For example, did the group originate in Australia and subsequently spread to New Guinea, or was it the other way around?So while tree-kangaroos are now slowly revealing their secrets, it remains astonishing that even in the twenty-first century new species of large mammals can still be discovered right on Australia’s doorstep. It makes you wonder what else is still out there . . . maybe even drop bears?

Dr Mark Eldridge, Research ScientistFirst published in Explore 33(3) pp8-11.