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.