It's become common for vegetarians and environmental activists to criticize animals. Cows, for example, used to be criticized for carbon dioxide production, with manufactured claims like "it takes a gallon of gas to make a pound of beef" but that has long been debunked and CO2 is dropping thanks to natural gas, so anti-science groups have turned toward methane.

While it's true that methane has 23X the warming effect of CO2, it is so short-lived as to be silly from a physics point of view, but so are the anti-vaccine and anti-GMO movements, and those still raise a lot of money for groups. 

Why single out cows, though? Foregut fermenters like kangaroo and wallaby produce lots of methane also, thanks to specialized microbes -- known as Archaea -- in their bodies.

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."

It was believed but never confirmed that kangaroos produced lower methane. Munn and 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 found that kangaroo methane production is no lower than that of many other herbivores.

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.

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 analyzed the nutrient content of the kangaroo's feed and feces to find out just how much food the kangaroos had digested in relation to the amount of methane they produced.

After 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. 

 Published in Journal of Experimental Biology.