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)