The belief that kangaroos have special microbes in their digestive system that don't produce methane like other grass-eating species has been refuted. It turns out that kangaroos produce just as much methane for their bodyweight as horses, but most comes out the rear end. The discovery is more than an excuse for scientists to make fart jokes; it carries possible implications for how we tackle the meat industry's huge levels of methane production.

Ruminants such as cows and sheep produce enormous amounts of methane. Their widespread use for meat, dairy, and wool is a major contributing factor to global warming, since methane is a far more powerful greenhouse gas than carbon dioxide. This has prompted suggestions from those who don't want to go vegetarian that we should seek less gassy animals.

Studies in the 1970s suggested that kangaroos and wallabies produced very little methane, leading to speculation their gut bacteria might be fundamentally different from those that populate the digestive systems of animals that evolved on other continents. However, Dr Adam Munn of the University of Wollongong in Australia says these discussions were based on errors.

“It was good research in its day,” Munn told IFLScience, but measuring methane from living animals is hard. Some previous attempts strapped masks on kangaroos to measure how much they burped, since this is the main way eutherian herbivores release methane. Munn kept his animals in fully enclosed spaces to track the gasses emitted and fed them two different sorts of food, providing a much more accurate picture.

In the Journal of Experimental Biology Munn, and the University of Zurich's Professor Marcus Clauss and Catharina Vendl, report that while red and grey kangaroos release substantially less methane than ruminants, their production is similar to other grazing animals, allowing for weight. However, most of it comes out of their rears, something Munn can personally attest to from having worked with kangaroos. This is quite surprising, since kangaroos ferment food in a forestomach, while perissodactuls use a hindgut.

Despite the different locations and millions of years of evolutionary separation, Munn told IFLScience the architecture of the kangaroo's forestomach and a horse's hindgut are very similar, and the organisms that occupy them probably are as well.

Munn thinks ruminants' exceptional methane emissions may have to do with the state of their microbial ecosystem. “In ruminants the community is more established, a bit like an old growth forest, with populations slowly reproducing,” he said to IFLScience. “In a horse's hindgut or kangaroo's forestomach microbes are constantly washed out and the community is always in a growth phase, with the organisms devoting all their energy to growth and development.”

The finding challenges campaigns to replace cows with kangaroos, but Munn said: “There are a lot of plusses to having mixed grazing environments with a variety of herbivores. They eat different grass types, defacate differently, which helps preserve diversity and the soil.”

Whether Munn's theory, if proven correct, could provide a path to less gassy cows is unclear, but Munn also noted that well-fed kangaroos produced less methane than those on restricted diets.