Ever since the agricultural revolution, humans have lived cheek by jowl with cows, enjoying their meat, milk and cream. However, there is one unfortunate by-product from this relationship: methane. Dairy cows can each produce up to 200 l of methane per day and, with an estimated world population of 1.4 billion animals, that quickly adds up to a serious greenhouse gas problem. However, one family of foregut fermenters seemed to buck the flatulence trend: measurements in the 1970s and '80s of kangaroo and wallaby methane production suggested that they produced much less gas than ruminants. Explaining that the gas is produced by specialised microbes -- known as Archaea -- in the animals' foreguts, Adam Munn from the University of Wollongong, Australia, says, 'The idea that kangaroos have unique gut microbes has been floating around for some time and a great deal of research has gone into discovering these apparently unique microbes'. However, Munn adds that the kangaroo's seemingly low methane production had never been confirmed. Intrigued by the mystery, Munn and his long-time collaborator Marcus Clauss from the University of Zurich, Switzerland, decided to measure everything that went into and came out of kangaroos to get to the bottom of the problem. They publish their discovery that kangaroo methane production is no lower than that of many other herbivores in Journal of Experimental Biology at http://jeb. biologists. org .

Knowing that methane production is dramatically affected by the length of time that a meal takes to pass through an animal and that food passes through well-fed kangaroos faster than through hungry animals, Munn and Clauss decided to feed alfalfa to the animals at two different levels (a restricted diet versus all they could eat) to find out how that affected the animals' methane production. However, Clauss had little experience of working with marsupials, despite years of measuring methane production in other species, whereas Munn had worked with kangaroos for over 20 years. So, Clauss's student Catharina Vendl travelled to the University of New South Wales' Fowlers Gap Research Station to work with Munn, Matthew Stewart and Keith Leggett to measure the animals' methane production and metabolic rates while also collecting their faeces. Then, back at the ETH Zurich in Switzerland, Michael Kreuzer analysed the nutrient content of the kangaroo's feed and faeces to find out just how much food the kangaroos had digested in relation to the amount of methane they produced.

After months of patiently pulling together the data, the team could see that their kangaroos produced similar quantities of methane to other kangaroos for their body size. However, when the team investigated the kangaroos' methane production relative to their metabolism, it was essentially the same as that of horses; so the kangaroos are no less flatulent than other herbivores, although they still produce less methane than cows. And when the team calculated the amount of methane produced relative to the animals' food intake, they were impressed to see that instead of producing more methane, the kangaroos that were free to eat their fill produced less gas. Just like other species that rely on fermentation as part of the digestion process, the well-fed animals produced less methane because food was passing through the gut faster, leaving less time for the microbes to break down the tough plant material and produce the gas.

'Kangaroos are not mysteriously low methane-producing creatures, but herbivores with an active methane-producing microbe community', says Clauss, adding that he and Jürgen Hummel suspect that the environment in the marsupial's foregut may naturally reduce Archaea methane production. And Munn is keen to find out how the animals' methane production will be affected by changes in diet quality as climate change takes hold and the frequency of droughts increases.

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REFERENCE: Vendl, C., Clauss, M., Stewart, M., Leggett, K., Hummel, J., Kreuzer, M. and Munn, A. (2015). Decreasing methane yield with increasing food intake keeps daily methane emissions constant in two foregut fermenting marsupials, the western grey kangaroo and red kangaroo. J. Exp. Biol. 218, doi: 10.1242/jeb.128165.

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