Over half the named species of tree-kangaroos in existence today occur in the high montane forests of New Guinea.  These all fall in the category of the highly-derived species and some are reported to be quite specialised in diet and behaviour (Martin, 2005). New Guinea's highlands are very young, with most tectonic uplift occurring over the last 5 million years, with evidence that uplift is still happening (Riker-Coleman et al, 2006). 

Ancestral tree-kangaroos would have happily moved into these lush, well-forested mountain tops and from there, just like Petrogale species isolated on rocky outcrops, speciated.  It may be that this speciation occurred purely as a result of long isolation by time: it's quite a long way from one mountain-top to another, but equally likely is vicariance brought about by periods of glaciation in the Pleistocene. These glacial periods are well-known to have affected the distribution of vertebrates in Australia's Wet Tropics (Winter, 1997).

Palynology tells us that over the past 190,000 years, the rainforests of the area have undergone repeated expansions and contractions and that as a result, mammal distributions have become determined by the persistence of refugia (Winter, 1997).  In the Wet Tropics region, for example, mammal distribution can be divided into two discrete refugia: the Thornton Unit to the North and the Atherton unit to the south, divided by an 80km wide gap of dry forest known as the Black Mountain Corridor (Bell et al, 1987).  It is thought that mammals were confined to either one or the other of these two refugia and a subsequent warm and wet vicariant phase determined the distribution of cool-adapted upland isolate species (Winter, 1997).  While isolation into these refugia has not been cited as a determining factor for tree-kangaroo distribution in the Wet Tropics, it is interesting that the ranges of Australia’s two species do fall generally into one of these two units.  It is therefore not entirely unlikely that the same basic pressures that cause speciation in many organisms, i.e. geographic and reproductive isolation brought about by vicariance, have been the cause of the diversity of tree-kangaroo species in existence today.

Figure 1.  Wet Tropics region showing the Black Mountain Corridor (black bar). Source: Taberlet, 1998

 

 





Figure 2.  Comparative distribution maps of the two Australian species of tree-kangaroo. Source: www.tenkile.com Accessed 22/5/15






Bell, F. C. (1987, May). Distribution, area and tenure of rainforest in northeastern Australia. Royal Society of Queensland.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne. 

Riker‐Coleman, K. E., Gallup, C. D., Wallace, L. M., Webster, J. M., Cheng, H., & Edwards, R. L. (2006). Evidence of Holocene uplift in east New Britain, Papua New Guinea. Geophysical research letters, 33(18).

Taberlet, P. (1998). Biodiversity at the intraspecific level: the comparative phylogeographic approach. Journal of Biotechnology, 64(1), 91-100.

Winter, J. W. (1997). Responses of non-volant mammals to late Quaternary climatic changes in the wet tropics region of north-eastern Australia. Wildlife Research, 24(5), 493-511.

The story of the previous posts is that tree-kangaroos came from rock-wallabies that evolved to take advantage of an abundant and underutilized Malesian flora that invaded Australia in the mid-Miocene.  The slight hiccup in this tale is that the Malesian flora did not extend a long way either south or west into the Australian continent (Sniderman and Jordan, 2011).  This might seem to be of little consequence until we take into account the existence of Bohra.  As you will recall, fossils of these large, assumed ancestral tree-kangaroos were found as far west and south as the Nullabor Plain, a long way from the proposed site of tree-kangaroo evolution.
This is puzzling. If these were in fact ancestral, why were they so far away and why are there no refugial populations of tree-kangaroos anywhere else in Australia? The Malesian flora certainly didn't extend that far.  Maybe the answer lies once again with rock-wallabies...
Rock-wallabies are widespread and their proposed ancestral range encompassed all the sites where Bohra fossils have been found (Fig. 1).

Figure 1. Current (coloured areas) and ancestral (within dashed line) ranges of the genus Petrogale.  Source: Potter et al, 2012

A recent molecular phylogeny constructed for Petrogale concluded that this genus originated in a mesic environment within Australia (characteristic of the East Coast today, also the area the basal Proserpine Rock-Wallaby calls home) and dispersed into more arid environments from the Late Miocene on. From here, further diversification happened during the Plio-Pleistocene when it was likely that glaciation caused repeated range contractions and resulted in isolated refugia.  The current arid-adapted taxa are considered to have arisen quite recently, allowing the persistence of Petrogale in the arid central and western regions of Australia (Potter et al, 2012).

If we adopt the assumption for now that rock-wallabies originated somewhere around the current range of P.persephone and radiated out, then perhaps it is not drawing too long a bow to suggest that ancestral tree-kangaroos followed a similar route, adapting to different food resources as they went.  The difference in current distribution of the two groups can be probably explained by the ability of Petrogale to adapt to forest contraction and aridfication, which of course would have severely disadvantaged tree-kangaroos.  Remember that the Nullarbor plain was treed when Bohra was roaming the land there (Prideux and Warburton, 2008) so perhaps this contraction of habitat has been a feature of tree-kangaroo evolution from the early days.




Sniderman, J. M., & Jordan, G. J. (2011). Extent and timing of floristic exchange between Australian and Asian rain forests. Journal of Biogeography,38(8), 1445-1455.


Potter, S., Cooper, S. J., Metcalfe, C. J., Taggart, D. A., & Eldridge, M. D. (2012). Phylogenetic relationships of rock-wallabies, Petrogale (Marsupialia: Macropodidae) and their biogeographic history within Australia. Molecular Phylogenetics and Evolution, 62(2), 640-652.

Prideaux, G. J., & Warburton, N. M. (2008). A new Pleistocene tree-kangaroo (Diprotodontia: Macropodidae) from the Nullarbor Plain of south-central Australia. Journal of Vertebrate Paleontology, 28(2), 463-478.

So it would seem that the arrival of Malesian forests presented a high-quality, underutilized niche into which tree-kangaroos evolved from their forest-loving rock-wallaby ancestors.  This idea is further supported by a similar evolutionary trajectory taken by another denizen of these Malesian forests: the striped possum (Dactylopsila trivirgata).  These diminutive creatures, unlike their folivorous cousins, feed nearly exclusively on insects, by preference on wood-boring beetle grubs (Rawlins and Handasyde, 2002).  They possess adaptations to accomplish this, such as chisel-like lower incisors that chip into dead trees and an elongated fourth finger which it uses to winkle the fat morsels out of their woody redoubts. This niche these possums fill is nearly exactly the same niche that is filled by woodpeckers north of Wallace's Line (Handasyde and Martin, 1996; Martin, 2005).

The south-western part of Papua is part of the Queensland section of the Australian plate.  Sea-levels have risen and fallen and coastlines have changed greatly over the last few million years but reconstructions of the likely coastline for when sea-levels were about 120 metres lower than now show a joined Australia/New guinea landmass covered with mixed forest containing elements of monsoon forest, dry forest and tropical woodland (Martin, 2005).
It is possible that ancestral tree-kangaroo species once lived throughout this contiguous forest and this probability is supported by the present day distribution of D. inustus (the third ancestral species, you'll recall).  D. inustus  occupies the north west and northern coasts of the New Guinea land mass (Flannery et al, 1996).

This may be explained by the concept of vicariance.  Vicariance occurs when the geographical range of a species, our posited ancestral tree-kangaroo, is split up into parts by a physical barrier, in this case sea-level change, and is a necessary precursor to allopatric speciation (Freeman and Herron, 2007).  D. inustus is suspected to be closest to the basal species (Bowyer et al, 2003).  The disparate distribution of the three ancestral species may be explained by the unusual band of contiguous dry woodland that stretches from Eastern Cape York and continues along the southern slopes of New Guinea's alpine spine (Ray and Adams, 2001).   Thanks :)





Bowyer, J. C., Newell, G. R., Metcalfe, C. J., & Eldridge, M. B. (2003). Tree-kangaroos Dendrolagus in Australia: are D. lumholtzi and D. bennettianus sister taxa?. Australian Zoologist, 32(2), 207-213.

Flannery, T., Szalay, A., Martin, R. W., & Johnson, P. N. (1996). Tree kangaroos: a curious natural history. Reed Books Australia.

Freeman, S. and Herron, J.C. (2007). Evolutionary Analysis. Pearson Educational.

Handasyde, K. A., & Martin, R. W. (1996). Field Observations on the Common Striped Possum (Dactylopsila Trivirgata) in North Queensland. Wildlife Research, 23(6), 755-766.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne. 

Rawlins, D. R., & Handasyde, K. A. (2002). The feeding ecology of the striped possum Dactylopsila trivirgata (Marsupialia: Petauridae) in far north Queensland, Australia. Journal of Zoology, 257(02), 195-206.

Ray, N., & Adams, J. (2001). A GIS-based vegetation map of the world at the last glacial maximum (25,000-15,000 BP). Internet Archaeology, 11.

Australia was covered in Gondwanan rainforest for millions of years after its split from that great southern supercontinent, but as it drifted north, climatic changes, specifically drying and increased seasonality, a new drought adapted flora emerged and spread until it covered most of the continent.  Relicts of the Gondwanan rainforests clung only to the eastern coastline, where wetter conditions enabled it to persist (White, 1986).
A clade of marsupials evolved to take advantage of the new grasslands, and some of these found a niche in the rocky outcrops, becoming rock-wallabies.
Around 15 million years ago, Australia's northward drift brought it into collision with Asia and Malesian flora invaded the tropical lowlands, aided by birds and bats, including much of the habitat of rock-wallabies.  In this new flora they found an abundant and nutritional food source, and likely pressed their already-good climbing abilities into the service of accessing what the new trees had to offer (Martin, 2005).

In terms of tree-kangaroo evolution, the ancestry of rock-wallabies is quite noteworthy.  The karyotype, that is, the arrangement of chromosomes, of the Proserpine Rock Wallaby (Petrogale persephone) has been found to be basal to all other species (Eldridge and Close, 1994).
This is interesting as P. persephone lives within closed forest and seems equally happy climbing trees as bounding among the rocks, illustrating the remarkable similarity between the life-history traits of rock-wallabies and tree-kangaroos.

Proserpine rock-wallaby.  Source: http://matthewsyres.com/ Retrieved 8/5/15

The presence of Wallace's line (a deep ocean channel between the islands of Lombok and Bali in Indonesia), while not presenting a major barrier for plant dispersal, did stop the movement of fauna largely in their tracks (Wallace, 1869).  The absence of large folivorous and/or frugivorous creatures such as monkeys,  meant that the Malesian flora in Australia initially went largely unmolested.  Nature abhors a vacuum, as Aristotle once noted, and here were niches waiting to be filled.  Australian fauna quite happily took the baton, as has been observed in the New Guinea lowlands (Martin, 2005).  The role of large folivore is occupied by leaf-monkeys north of Wallace's line (Brandon-Jones, 1998) and it seems probable that rock-wallabies simply filled this niche.

Dusky leaf-monkey.  Source: Wikipedia.org Retrieved 8/5/15

References

Brandon-Jones, D. (1998). Pre-glacial Bornean primate impoverishment and Wallace’s line. Biogeography and geological evolution of SE Asia, 393-404.

Eldridge, M. D. B., & Close, R. L. (1994). Chromosomes and evolution in rock-wallabies, Petrogale. Australian Mammalogy, 19, 123-136.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.

Wallace, A.R. (1962). The Malay Archipelago. Dover Publications Inc, New York (unabridged republication of 1869 Macmillan and Company, London edition)

White, M. E. (1986). Greening of Gondwana: The 400 million year story of Australia's plants. Reed Australia.

Charles Darwin proposed a gradualistic approach to evolution, with organisms changing by infinitesmal degrees over time (Darwin, 1859).  This view has been added to over time by suggestions that evolution occurs abruptly, perhaps when a mutant organism happens by chance to have some adaptive advantage, rather than the deleterious results of most mutations.  More recently, the concept of "punctuated equilibria" was put forward, examining the premise that lineages are by nature, over geological time, generally conservative in the amount of change exhibited, and that these periods of stability are interspersed with events or periods of rapid speciation (Eldridge and Gould, 1972). 

The theory of speciation thought to be the most widely applicable to all organisms is that of allopatry.  Allopatry occurs when a peripheral population is geographically and reproductively isolated from the main population.  Often these peripheries are at the edge of ecological tolerance for a species, so variation that extends a fitness advantage is more rapidly taken up by a population, with the usually smaller population size aiding in this genetic dispersal.(Freeman and Herron, 2007).

In Petrogale, allopatric speciation appears to have been the mode of speciation resposible for diversity (Sharman et al, 1990) but in Dendrolagus, the mode of speciation was unclear, with ancestral forms (the two Australian species) seemingly on the outer edge of the stronghold of the more derived forms (most New Guinea species) (Martin, 2005).  This conundrum was even met with a proposal of a new pattern of speciation, called centrifugal (Groves, 1990). 

In light of the recent discovery of the dry-forest Bohra fossils, the dry-forest incursions of present-day D. bennettianus and taking into account the relationship between dry country Petrogale and Dendrolagus, it may well be that tree kangaroos originated in Australia under dry conditions (Martin, 2005).  If this is the case, then the pattern of allopatric speciation fits, with the more derived species occuring in very different environments to the ancestral group, with different groups becoming periodically isolated by contraction of their montane forest habitats (Winter, 1997).

Despite the general paucity of knowledge about these animals, it seems that we are piecing together a picture of the evolution of an amazing creature. 

An example of the dry habitat inhabited by a Petrogale species (P. xanthopus) in Western Queensland.  Source: www.ehp.qld.gov.au Retrieved 24/4/2015

Darwin, C. (1859). The origin of species by means of natural selection: or, the preservation of favored races in the struggle for life. Reprinted 2008. Ed. Quammen, D.  Sterling. New York 

Eldridge, N. and Gould, S. J. (1972). Punctuated equilibria: an alternative to phyletic gradualism, in Essential Readings in Evolutionary Biology. John Hopkins University Press. Baltimore

 

Freeman, S. and Herron, J.C. (2007). Evolutionary Analysis. Pearson Educational.

 

Groves, C. P. (1990). The centrifugal pattern of speciation in Meganesian rainforest mammals. Memoirs of the Queensland Museum, 28, 325-328.

 

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.   

Sharman, G. B., Close, R. L., & Maynes, G. M. (1990). Chromosome evolution, phylogeny and speciation of rock wallabies (Petrogale, Macropodidae). Australian Journal of Zoology, 37(3), 351-363.

 

Winter, J. W. (1997). Responses of non-volant mammals to late Quaternary climatic changes in the wet tropics region of north-eastern Australia. Wildlife Research, 24(5), 493-511.

 

Petrogale is a diverse genus containing the rock-wallabies, 21 species in all.  These engaging creatures inhabit rocky environments like cliffs, gorges, boulder piles and rocky outcrops throughout Australia, using these rocky fastnesses as their primary defence against predation.  They range from 0.9 to 9kgs in weight and are agile and quick amongst steep terrain (Eldridge and Close, 1992).
Not surprisingly, their pedal morphology differs somewhat from their flat-land relatives.  Their feet are packed with fatty tissue, they have reduced claws and they have highly developed transverse ridges at the ends of their toes, much like a human fingerprint (Flannery et al, 1996).   These features combine to produce an appendage that is quite capable of enhanced grip and climbing ability.

A comparison of hind-feet. Left to right: Brushtail possom, Musky rat-kangaroo, Black Dorcopsis, Black-footed rock-wallaby, Bennett's tree-kangaroo, Grizzled tree-kangaroo, Lumholtz's tree-kangaroo. Source: Flannery et al, 1996

In 1887, the Reverend Charles de Vis, curator of the Queensland museum, suggested a link between rock-wallabies and tree-kangaroos, noting the similarities in seating and balancing and suggested that the, "...passage of one into the other may appear of easy accomplishment by insensible degrees," although he went on to dismiss this line of thought as fanciful (Martin, 2005).

In 1989, however, a molecular study was performed using albumin proteins from across the macropod clade.  The basis of this technique is that the more similar the proteins, the more closely related are the species.  To their suprise, they found a strong relationship between Petrogale and Dendrolagus (Baverstock et al, 1989).
  This relationship was backed up and reinforced by another group of molecular biologists using a different technique with higher resolution and indicated a close association between pademelons (Thylogale spp.),  Petrogale and Dendrolagus, going as far to say that the latter two genera are sister taxa (Kirsh et al, 1995).  Further work along this line allowed researchers to estimate the divergence of these three genera from a common ancestor as occurring no ealier than 8 million years ago and that the latter two split from pademelons about 500,000 years after that.

These results seem to satisfactorily answer the answer the question of  the direct ancestors of tree-kangaroos and my next post will discuss the possible modes of speciation that have led to the diversity of tree-kangaroo forms in existence today.  Thanks :) 


References:

Baverstock, P. R., Richardson, B. J., Birrell, J., & Krieg, M. (1989). Albumin immunologic relationships of the Macropodidae (Marsupialia). Systematic Biology, 38(1), 38-50.

 Campeau-Péloquin, A., Kirsch, J. A., Eldridge, M. D., & Lapointe, F. J. (2001). Phylogeny of the rock-wallabies, Petrogale (Marsupialia: Macropodidae) based on DNA/DNA hybridisation. Australian Journal of Zoology, 49(5), 463-486.

Eldridge, M. D. B., & Close, R. L. (1992). Taxonomy of rock wallabies, Petrogale (Marsupialia, Macropodidae). 1. A revision of the Eastern Petrogale with the description of 3 new species. Australian Journal of Zoology, 40(6), 605-625.

Flannery, T., Szalay, A., Martin, R. W., & Johnson, P. N. (1996). Tree kangaroos: a curious natural history. Reed Books Australia.

Kirsch, J. A., Lapointe, F. J., & Foeste, A. (1995). Resolution of portions of the kangaroo phylogeny (Marsupialia: Macropodidae) using DNA hybridization. Biological Journal of the Linnean Society, 55(4), 309-328.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.

It is a curious thing to consider the secondary adaptation of kangaroos, animals that are seemingly perfectly designed for a terrestrial existence, to a life back in the trees.

It may be here that a brief discussion of macropod evolution would be interesting background knowledge.

Macropods are thought to have diverged from a possum-like common ancestor in the early Eocene, from around 56 Ma (Meredith et al, 2009).

Hypsiprymnodon moschatus, the musky rat-kangaroo.
Creatures similar to these are thought to be the ancestor of modern
macropods. Source: www.kpbs.org Retrieved 8/4/15

These animals are thought to have been rabbit-sized, solitary, nocturnal, omnivorous dwellers of dense forests (Kaufmann. 1974) similar to extant Hypsiprymnodon (rat-kangaroos), which are among the smallest macropods in existence today.

After Australia separated from Godwana some 40 million years ago, its northward drift brought large climatic changes including the drying of the continent and concomitant loss of the central Australian forest habitats (White, 1986).  This period has been shown to coincide with the rapid diversification of macropods and the emergence of their hallmark mode of locomotion, the bipedal hop (Meredith et al, 2009).

The most spectacular radiation of forms, though, began about 12 million years ago (Meredith et al, 2009) and this time period coincides with a major contraction of Australia's rainforests, associated with the further drying and cooling of the environment and the subsequent spread of grasslands (Martin, 2006).  This period also saw the widespread evolution of high-crowned molars, an adaptation for grazing abrasive grasses (Martin, 2005).  All lineages from which modern taxa are derived were in existence by 5 million years ago (Meredith et al, 2009).

"So what about tree-kangaroos?" I hear you cry with impatience!  Well, as we saw in the last post, tree-kangaroos were well in existence by 101 thousand years ago, but from whence did they come?  Can they be related to any extant taxa?  Why did this lineage move into the trees in a land of shrinking forests while their cousins were flat out adapting to the plains?

All very interesting (and perplexing) questions and unfortunately ones that will have to wait for future blog post, I'm afraid.  Thanks :)


References:

Kaufmann, J. H. (1974). The ecology and evolution of social organization in the kangaroo family (Macropodidae). American Zoologist, 14(1), 51-62.

Martin, H. A. (2006). Cenozoic climatic change and the development of the arid vegetation in Australia. Journal of Arid Environments, 66(3), 533-563.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.

Meredith, R. W., Westerman, M., & Springer, M. S. (2009). A phylogeny and timescale for the living genera of kangaroos and kin (Macropodiformes: Marsupialia) based on nuclear DNA sequences. Australian Journal of Zoology,56(6), 395-410.

White, M. E. (1986). Greening of Gondwana: The 400 million year story of Australia's plants. Reed Australia.

In 2002, in the unwooded steppe of Australia's Nullarbor Plain, cavers stumbled upon a palaeontological treasure trove: an astonishing assemblage of exceedingly well-preserved fossils of 70 taxa,  from the middle Pleistocene era (the minimum age for these fossils was optically dated at 101 thousand years old). Largely victims of pitfalls into the numerous collapsed caves that abound in this limestone karst area (named by researchers the Thylacoleo Caves, after the marsupial lion, remains of which were found here), this assemblage included 23 kangaroo species, 8 of them undescribed (Prideaux et al, 2007).

Figure1. Map showing the location of Australian tree kangaroo fossil finds, along with the current distribution of Dendrolagus. Site 1 is the Thylacoleo caves. Source: Prideaux and Warburton, 2008

Among these 8 were the partial skeletons of two species of tree-kangaroo of the extinct genus Bohra, the best-preserved tree-kangaroo fossils to date.  Tree-kangaroos are rare in the fossil record (Dawson, 2004), and until the Thylacoleo Caves find, fragmentary.  Bohra was a genus tentatively erected from from fossil fragments, some collected a hundred years previously,including hind limb elements (B. paulae; Flannery and Szaly, 1982) and a partial juvenile tooth (B. wilkinsonorum; Dawson, 2004).
The evidence supporting these tentative assignations to the tree-kangaroo fold was not particularly strong (Martin, 2005) but the intact Thylacoleo Caves specimens have linked the two previous fossils, and shown that they are congeneric (Prideaux and Warburton, 2008).  This illuminating find gave the first described specimen its specific name of  illuminata.
The second specimen was named B. nullabora (Prideaux and Warburton, 2009).  The genus is large compared to modern tree-kangaroos.  The heaviest modern species weighs about 15 kg (Martin, 2005) while Bohra species are thought to have weighed about 40 kg (Flannery and Szaly, 1982), which is very large for an arboreal mammal. The genus also had longer, larger hind-limbs than Dendrolagus, although these were still morphologically well-suited to an arboreal lifestyle (Prideaux and Warburton, 2009).
Interestingly, the Thylacoleo fauna has a high number of mixed feeders and general grazers as opposed to arboreal folivores, suggesting a dry, open environment (Prideaux et al, 2007) .  This contrasts with the current habitat occupied by Australia's two tree-kangaroo species, namely, rainforests, although the habit of D. bennettianus of traversing fingers of gallery forest stretching into dry savannah (Martin, 2005) may hint at an ancestral tolerance of dryer conditions amongst this most interesting group of marsupials.

References

Dawson, L. (2004). A new Pliocene tree kangaroo species (Marsupialia, Macropodinae) from the Chinchilla Local Fauna, southeastern Queensland.Alcheringa, 28(1), 267-273.

Flannery, T.F. and Szaly, F.S. (1982).Bohra paulae: a new giant fossil tree kangaroo (Marsupialia: Macropodidae) from New South Wales, Australia. Australian Mammalogy 5: 83–94.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.

Prideaux, G. J., Long, J. A., Ayliffe, L. K., Hellstrom, J. C., Pillans, B., Boles, W. E., ... & Warburton, N. M. (2007). An arid-adapted middle Pleistocene vertebrate fauna from south-central Australia. Nature, 445(7126), 422-425.

Prideaux, G. J., & Warburton, N. M. (2008). A new Pleistocene tree-kangaroo (Diprotodontia: Macropodidae) from the Nullarbor Plain of south-central Australia. Journal of Vertebrate Paleontology, 28(2), 463-478.

Prideaux, G. J., & Warburton, N. (2009). Bohra nullarbora sp. nov., a second tree-kangaroo (Marsupialia: Macropodidae) from the Pleistocene of the Nullarbor Plain, Western Australia. Records of the Western Australian Museum,25, 165-179.

The taxonomy of tree-kangaroos is not crystal clear.
Before the advent of genetics, morphological characteristics were the means available for discerning between groups of organisms.
Previous attempts to classify these animals had attempted to use the position of hair whorls to group them and there were some very flawed taxonomies proposed in the first half of last century.
Colin Groves (1982) provided a taxonomy based on many characteristics, amongst them similarities between molars and foot dimensions, which tidied the mess up somewhat but more work was needed.
On the first Dutch mission to describe tree-kangaroos, the same one that assigned them the culinary appellation of "tree hare," Schlegel and Muller (1845) noticed a significant difference between two of their specimens, D. inustus and D. ursinus, namely, that the tibia and fibula were far more greatly separated in the latter (Fig. 1).
The resulting lack of contact between these two long bones is thought to increase the rotational ability of the hind-foot and thereby improve gripping and climbing ability and has been used to group tree-kangaroos into two groups: one more primitive and the other more highly derived (Flannery et al, 1996).  In this grouping, D. inustus was included with the two Australian species in the primitive group.

Figure 1.  A comparison of tibio-fibula contact between a) Dendrolagus inustus and b) Dendrolagus ursinus.  Source: Martin, 2005

A genetic insight

An analysis of mitochrondrial DNA performed on D. lumholtzi and D. bennettianus (along with several New Guinean species to a lesser extent) has suggested that these two Australian species are indeed sister taxa (that is, they have a close evolutionary relationship).  Interestingly, though, the differences between the two species were great enough to suggest to the researchers that speciation had occurred a long time ago, before the mid-late pleistocene (Bowyer et al, 2003).  Their results also suggested that D. inustus, the New Guinean species, is ancestral to the Australian species, although this data is considered to be fairly weak (Martin, 2005) and goes against previous assertions of the Australian species being basal (Flannery et al, 1996).

Palaeontologists have recently made an amazing discovery in the karst caves of Australia's dry, tree-less Nullarbor Plain that potentially nails down the question of tree-kangaroo ancestry once and for all, but I'll examine that in the next post.
Thanks :)







References:

Bowyer, J. C., Newell, G. R., Metcalfe, C. J., & Eldridge, M. B. (2003). Tree-kangaroos Dendrolagus in Australia: are D. lumholtzi and D. bennettianus sister taxa?. Australian Zoologist, 32(2), 207-213.

Groves, C.P. (1982). The systematics of tree kangaroos.  Australian Mammology, 5(3), 157-187

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.

Flannery, T., Szalay, A., Martin, R. W., & Johnson, P. N. (1996). Tree kangaroos: a curious natural history. Reed Books Australia.

How do they do it?

Tree-kangaroos have a suite of adaptations that enable them to lead a life among the high leaves but still retain essential kangaroo-like features.  Alfred Russel Wallace (1869) noted that, "...They move along by short jumps on their hind feet, which do not seem particularly well adapted for climbing trees," and indeed, they can move with some pace once on the ground.  Carl Lumholtz (for whom Lumholtz's tree-kangaroo is named) wrote that local Aboriginal people told him that, "Boongary [local language name for D. lumholtzi] plenty walks about," suggesting ample capability on the ground (Lumholtz, 1884)

So how do these beasts climb trees?

Most noticeable are the curved claws on the forepaw (Fig 1), reminiscent of other arboreal marsupials such as koalas and possums .  These are much longer and more curved than those of terrestrial kangaroos.

Figure 1. D. bennettianus forepaw.  Source: Martin (2005) 

Maintaining a grip on the trees they climb is the most apparent function of these claws but they are also, perhaps surprisingly, of considerable use in manipulating food items (Iwanuik et al, 1998).  In the same study, it was found that tree-kangaroos showed marked freedom of movement in the shoulder girdle, certainly more than other macropods, another way an arboreal existence is made more feasible.

An examination of the hindfoot (Fig 2.) shows a very typical macropod pattern, with an enlarged fourth and fifth toe.  Where they differ is in the broadness of the foot and the enlargement of the claws.

The pads on tree kangaroo paws are also indicative of climbing ability, being large, fleshy and covered in small protrusions, called papillae, which are thought to enhance grip whilst climbing (Martin, 2005).

 Figure 2. D. bennettianus hind paw.  Source: Martin (2005)

Tree-kangaroo tails are  not prehensile, like possums or many New World monkeys.  It appears that they use them simply as a balancing aid while climbing along branches (Martin, 2005). Interestingly, different species have different tail lengths, with those in the group thought to be more derived, that is, further removed from a common ancestor, having shorter tails (Groves, 1982).

The reasons behind and the means of classifying these different groups of tree-kangaroos is a fascinating one, with some amazing recent finds shifting our knowledge, but that, I'm afraid, will have to wait for a future post.  Thanks for reading!

Figure 3. D. lumholtzi. Note the exceptionally long tail and well-developed hind-limbs, reminiscent of terrestrial kangaroos. Source: www.australianmuseum.net.au

References:

Groves, C.P. (1982). The systematics of tree kangaroos.  Australian Mammology, 5(3), 157-187

Iwaniuk, A. N., Nelson, J. E., Ivanco, T. L., Pellis, S. M., & Whishaw, I. Q. (1998). Reaching, grasping and manipulation of food objects by two tree kangaroo species, Dendrolagus lumholtzi and Dendrolagus matschiei..Australian Journal of Zoology, 46(3), 235-248.

Lumholtz, C. (1884). Notes upon some mammals recently discovered in Queensland. In Proceedings of the Zoological Society of London 52(3),406-409).

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.

Wallace, A.R. (1962). The Malay Archipelago. Dover Publications Inc, New York (unabridged republication of 1869 Macmillan and Company, London edition)

The rainforested areas of Australia and New Guinea are home to a little known, rarely seen and largely understudied creature, the tree-kangaroo.  Kangaroos are not the first creatures that spring to mind when contemplating tree-dwelling mammals.  Most people's idea of macropods is of large, bouncy animals soaring across red-soil plains, but tree kangaroos have very successfully made their living in the canopy.  Indeed, they represent a respectable 16 species and sub-species of about 73 species of macropods, nearly 22%.

Tree-kangaroos were first discovered in 1826, when Dutch scientists on a mission to collect natural history specimens landed at Lobo, on the NW coat of New Guinea and acquired 4 live specimens of an as yet unknown mammal, with the intention of shipping them back to Europe.  Unfortunately, particularly for the tree-kangaroos collected, misfortune intervened.  Malaria struck the ship and the scientists on board fell gravely ill.  The ship's officer immediately had the animals killed and turned into a nourishing dish to aid the invalids, based on a Dutch dish called hazenpeper, or peppered hare.  This interesting side note in scientific history is how tree-kangaroos acquired their scientific name of Dendrolagos, or tree-hare (Martin, 2005).

Figure 1. Dendrolagus inustus, the grizzled tree-kangaroo.  This was the first species
of tree-kangaroo discovered by Western science.  It was also probably the
first to be eaten by Westerners.  Source: jumbhoanimal.blogspot.com

Australia possesses two of the 16 species of tree-kangaroo, Bennett's and Lumholtz's, restricted to the rainforests and vine thickets of far north Queensland.  The first discovery of this genus made in Australia was in 1872.  William Hann was commissioned by the Queensland government to explore the mineral potential of Cape York Peninsula.  Hann's Aboriginal guide, Jerry, had mentioned the existence of a tree-climbing kangaroo, a story which Hann scarcely believed.  Scratches on trees were found, however, followed by a complete skeleton.  This, the first hard evidence of tree-kangaroos in Australia, was collected by the party but subsequently lost (Martin, 2005).

Figure 2. Lithograph of Bennett's tree kangaroo by Joseph Smit, published in the
 Proceedings of the Zoological Sociey of London, 1894.  Source: Wikimedia Commons 

Tree-kangaroos are a cryptic animal, as anyone who has spent time looking for them will wearily tell you, and the latest discovery by Western science of a new species, D. mbaiso, known by locals as Dingiso, was made in 1992, in West Papua's Central Highlands, an amazing find for a relatively large mammal (Martin, 2005).   This little explored and largely inaccessible country may yet produce new species, an excitingly tantalising prospect.





                                    Table 1. Tree kangaroo taxonomy.  Adapted from Flannery et al. (1996)

Dendrolagus bennettianus		Bennett’s tree-kangaroo
Dendrolagus lumholtzi		Lumholtz’s tree-kangaroo
Dendrolagus inustus	D. i. inustus	Grizzled tree-kangaroo
	D. i. finschi	Finsch's tree-kangaroo
Dendrolagus ursinus		Vogelkopt tree-kangaroo
Dendrolagus goodfellowi	D. g. goodfellowi	Goodfellow's tree-kangaroo
	D. g. buergersi	Timboyok
	D. g. pulcherrimus	Golden-mantled tree-kangaroo
Dendrolagus matchiei		Matschie's tree-kangaroo
Dendrolagus spadix		Loland tree-kangaroo
Dendrolagus dorianus	D. d. dorianus	Doria's tree-kangaroo
	D. d. mayri	Wondiwoi tree-kangaroo
	D. d. notatus	Ifola
	D. d. stellarum	Seri's tree-kangaroo
Dendrolagus scottae		Tenkile
Dendrolagus mbaiso		Dingiso

References:

Flannery, T. F. et al. Tree Kangaroos. Port Melbourne, Vic.: Reed, 1996. Print.

Martin, R., and Simpson, S. Tree-Kangaroos Of Australia And New Guinea. Collingwood, VIC: CSIRO Pub., 2005. Print.


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