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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