Bunnies eat toxic leaves to conquer Australia’s snowy peaks

Snow problem: when there is no grass rabbits eat eucalyptus

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By Alice Klein

Nothing will stand in their way. After devastating Australia’s low-lying regions, European rabbits are now muscling in on snowy mountainous areas by adapting to survive on toxic snow gum leaves.

Rabbits were introduced to Australia in the 19th century and rapidly spread across the continent, creating huge problems for native wildlife and farmers. The only areas they have failed to colonise are those with snow cover in winter, because the grass they eat is buried.

But in 2011, Ken Green at Australia’s National Parks and Wildlife Service began to notice rabbits living above the winter snowline in the Snowy Mountains of New South Wales.

To understand how they are surviving, he collected their faecal pellets for three years and sent them to the University of Melbourne for dietary analysis.

Gum leaf gastronomy

The results showed that the leaves of alpine eucalyptus trees, also known as snow gums, form the biggest part of the rabbits’ winter diet.

It is astonishing that the rabbits can eat such high quantities of eucalyptus leaves, says Green. The tough leaves are difficult to digest, low in nutrients and contain toxins like tannins, terpenes and phenolics.

Native animals like koalas can survive on gum leaves because they have evolved special digestive mechanisms – such as hindgut fermentation – that allow them to extract nutrients and detoxify the chemicals. But koalas are mostly sedentary, conserving the limited energy they can extract.

Getting by

“Rabbits of course are quite different – they are very energetic – so it’s amazing that they’re getting by and not having major digestive issues,” Green says.

How they are managing this is not clear. In theory, the rabbits might have acquired gut microbes that help them digest eucalyptus leaves, or evolved physical adaptations. Or it could just be a behavioural change, which means the rabbits must already have had some ability to digest the leaves.

The health of the rabbits was not directly monitored, but they continued breeding from year to year, suggesting they were surviving well, he says. This may be because they only need to eat the gum leaves for three to four months of the year while there is snow cover, says Green.

The leaves that the rabbits are eating are those that have recently regenerated after a bushfire ripped through the area in the summer of 2003. These may be gentler on the rabbits’ stomachs than older leaves, says David Lee at the University of the Sunshine Coast in Queensland.

The regenerating trees also have leaves low enough to the snow for the rabbits to reach them. How the rabbits will fare as the trees grow taller is not clear.

This species of rabbit, Oryctolagus cuniculus, has not colonised snowy regions in Europe because European trees lose their leaves during winter, Green says. “It just goes to show that if you take animals out of their native range and put them in novel environments, strange things will happen.”

Journal reference: Australian Mammalogy; DOI: 10.1071/AM16015