Koalas up a gum tree, but having the last laugh..?

Mention koalas to most people who’ve heard of them, and the knee-jerk response is to imagine cute, cuddly creatures. But, other than their presumed cute- and cuddlesome-ness, what else do we know about this iconic animal (Desley Whisson & Kita Ashman, Conservation Science and Practice. 2020; 2:e188; https://doi.org/10.1111/csp2.188), commonly called a koala bear? What are the facts that members of the public can state when invited to? Well, maybe that they aren’t actually bears, but are marsupials (mammals with pouches, related to kangaroos and (o)possums…), that live in eucalyptus trees (also commonly called gum trees) – feeding almost exclusively on their leaves – in Australia.

OK, so far *… and, finally, probably everybody has heard that koalas either don’t, or don’t usually need to, drink because they get all their water from the eucalyptus leaves they eat. For years we’ve just accepted that latter assertion as ‘fact’, and would probably have been happy to continue doing so. But, footage of parched koalas taking water from fire-fighters ** and others during the fires that ravaged large areas of the koala’s habitat in Australia in late 2019/early 2020 suggest that they don’t just rely on leaves. Or, maybe they do – when there are leaves left on the trees to rely on that is! In those unprecedentedly, and intensely, inflammatory times this YouTube-evidenced drinking behaviour may be viewed as unusual. After all, bottles and trays of water aren’t part of the normal life of a koala, and should probably be ignored in terms of the natural biology of these animals. So what is the truth of the matter regarding koalas and drinking water?

Scientists are inquisitive types and don’t just accept ‘what is generally held to be so’ – in this case that koalas don’t normally drink water; they require evidence. And that is just what Valentina S. A. Mella et al. have provided (Ethology. 2020;00:1–6; https://doi.org/10.1111/eth.13032). In summary, their research reports koala drinking behaviour in the wild for the first time. Collating 44 observations *** made over a 13-years’ period by citizen scientists of the Koala Clancy Foundation in the You Yangs Regional Park (YYRP), Victoria (Australia), and other observations from Liverpool Plains (New South Wales, Australia), they show that koalas drink by licking the water that runs down tree trunks following rainfall. As the authors admit, this discovery was rather serendipitous because observations in YYRP are not normally conducted during heavy rainfall or thunderstorms out of concerns for the safety of the observers. “However, every time behavioural observations were conducted in the rain, koalas were recorded drinking” (Mella et al., 2020).*****

Additional support for the naturalness of this tree-drinking behaviour is provided by the fact that the YYRP has several ephemeral and permanent dams which could have provided access to free water to koalas when the observations took place. [Although one could suggest that presence of human observers might have caused the koalas to avoid terrestrial-located sites of water in favour of the safer, tree-trunk-derived option.] However, for the Liverpool Plains observations, no free-standing water was available at the sites when observations took place. Considering all the evidence gathered in this study [read the paper for the details – it’s only a few pages long], the authors conclude that “this behaviour is likely to represent how koalas naturally access free water” (Mella et al., 2020)..

The derivation of the word ‘koala’ ****** is insightful. Apparently it comes from an Aboriginal Australian word that means ‘no water’ [here, here, and here], ‘no drink’ [here, here, and here], or ‘doesn’t drink’. Whichever phrase is the true origin, all seem to focus upon the notion that the animal doesn’t appear to drink (or require a source of free water). In view of Mell et al’s evidence-based confirmation that koalas do naturally drink, we may need to find a new name for the animal formerly known as koala.

In science, it’s often the case that one piece of research may raise more questions that inspire further work. A follow-on study that might be of interest is to see if koala-interception of bark-flowing water deprives moss and other bark-dwelling epiphytes – on the same trees – of water that may be essential for their own survival, and whether there’s a negative correlation of epiphyte cover and residence in, or bark-licking use of, the tree by koalas…

* Probably the main ‘known’ that’s been omitted from this list is the fact that koalas have chlamydia. In humans, chlamydia is a well-known sexually-transmitted bacterial infection. It is also passed on by sexual activity between koalas, and can cause blindness, severe bladder inflammation, infertility and death. Pre-sexual, young koalas can also contract the infection by consuming their infected mother’s ‘pap’ – faecal material “excreted by the mother, which is more concentrated in nutrients and microbes than normal faeces” (Katherine Dahlhausen et al., 2018. PeerJ 6:e4452; https://doi.org/10.7717/peerj.4452) – a practice which otherwise helps to ensure that the youngster’s gut is colonised by the unique microbial flora required for digestion of eucalyptus leaves (Adam Polkinghorne et al., Veterinary Microbiology 165: 214–223, 2013; https://doi.org/10.1016/j.vetmic.2013.02.026).

** Should you chance upon a parched koala and are tempted to give it life-saving water, beware! There is a wrong way and a right way to do this; the latter could be fatal to an animal that’s already classified as vulnerable in the wild.

*** Interestingly, although 41 of the trees involved in the YYRP data set were various species of Eucalyptus, one observation was made in Corymbia maculata (spotted gum) ****. So, koalas aren’t exclusively associated with Eucalyptus trees – another ‘fact’ debunked? Not quite; “C. maculata is one of around 80 eucalypts which were transferred in 1995 from the genus Eucalyptus to the newly created genus Corymbia. The species was formerly known as Eucalyptus maculata”. It’s just that when it comes to taxonomic niceties of Eucalyptus – and which are matters of life and death to these endangered marsupials – koalas are probably more ‘lumpers’ than ‘splitters‘…

**** OK, I know what some of you are thinking: What about the other 2 – of the 44 – observations? These were noted as ‘NA’ regarding their identity in Mella et al’s Table 1.

***** The more sceptical amongst you might now just be wondering if koalas are having a bit of fun and deliberately misleading the scientists, only doing this when they know they’re being observed. A reasonable question then is: Do we know that koalas drink rainwater from tree trunks when there’s nobody there to observe them..? A case for use of remotely-operated video cameras? Getting strong evidence for one view or another is not always easy or straightforward; Science is hard work(!)

****** Whilst on the subject of etymology [“facts of the origin and development of a word” – https://www.etymonline.com/word/etymology], the scientific name of the koala is Phascolarctos cinereus [https://environment.des.qld.gov.au/wildlife/animals/living-with/koalas/facts#toc-4]. The generic name is a combination of two Greek words, ‘phaskolos’ meaning pouched (in keeping with its status as a marsupial), and ‘arktos‘ meaning bear [https://en.wikipedia.org/wiki/Koala] (which unfortunately perpetuates the myth that koalas are bears…). The specific epithet ‘cinereus‘ is from a Latin word that means ‘ash-coloured’ [http://ielc.libguides.com/sdzg/factsheets/koala/taxonomy], which seems about right – the fur of a koala is naturally ashy-grey in colour (however, most of the koalas one’s likely to have seen on TV or on-line in early 2020 were – rather sadly! – ash-covered…).