If you have ever visited Australia, you will no doubt be familiar with the phrase “Bloody possums!” Arguably the most successful of all marsupials, these agile vegetarians are renowned for both their resourcefulness and their big appetites. Eucalyptus remains an all-time favourite of both ringtail and brushtail possums; in fact, a particularly tasty Eucalyptus tree can be stripped bare of its leaves within days of discovery. Although tree-dwelling ringtails specialise in eating Eucalyptus leaves and brushtail possums eat a variety of other vegetation in addition to Eucalyptus, both species are picky about the Eucaluptus leaves they will eat. So why are certain Eucalyptus leaves more irresistible than others? Recent research by Karen Marsh and her colleagues at the Australian National University provides some insight into possum preferences. Marsh and her team examined the degree to which chemicals called plant secondary metabolites influence feeding preference in ringtail and brushtail possums. Their results reveal that,whilst both species adore Eucalyptus leaves, they prefer different leaves for different reasons.
Plant secondary metabolites can directly influence feeding behaviour in herbivores. Some of these compounds encourage feeding whilst others, such as tannins and formylated phloroglucinol compounds (FPCs), deter it. Eucalyptus leaves are especially famed for the variety of secondary metabolites they contain. The Australian researchers hoped to discover how tannins and FPCs affected feeding behaviour in ringtail and brushtail possums.
Marsh and her colleagues collected Eucalyptus leaves containing naturally different levels of FPCs. By collecting leaves from several trees,the researchers ensured that the FPC content of their leaves spanned the full range known for the species. Next, the team manipulated the tannin content of the same leaves by coating some with a substance able to neutralise tannin's deterrent effects. The scientists then monitored how popular the altered leaves were with the possums. Finally, they collected possum faeces in order to measure the digestibility of the different leaves.
The brushtail possums avoided eating leaves that contained tannins, while the specialist ringtail possums avoided eating leaves that contained FPCs. Although neither plant secondary metabolite affected how well ringtails digested the leaves, the specialist brushtails were able to gain more nutrients when the tannins were neutralised. Therefore, brushtails foraged to find Eucalyptus without tannins, while ringtails foraged to find Eucalyptus without FPCs.
So why do the two species avoid different plant secondary metabolites? One explanation is that the ringtail, which avoids FPCs, lacks the mechanisms to tolerate FPCs but has evolved anatomical and physiological adaptations that allow it to cope with ingested tannins. And why can't ringtail possums tolerate FPCs? Logic suggests that, as specialist leaf-eaters, ringtails should be better placed to cope with all leaf compounds, so perhaps ringtail possums aren't the `specialised' leaf-eaters we thought they were. And finally, could the possum's differing digestive abilities reduce competition over limited food resources between the species?
As with most good studies, Marsh's work has generated almost as many questions as answers. But, from the Eucalyptus perspective, respite from plundering possums isn't likely.