Kangaroo farts may not be so eco-friendly after all

Amount of methane emitted is linked to how fast food moves through the grazers’ guts, researchers suspect

HOT AIR  Kangaroos like these western grays (Macropus fuliginosus) release the greenhouse gas methane, but microbes in the animal’s gut keep emissions relatively low.

Adam Munn

Kangaroos do blow farts spiked with methane.

More methane is escaping the behinds of kangaroos than previously thought, an international group of researchers reports online November 4 in the Journal of Experimental Biology. It’s still less than many other grazing animals, though, the researchers say. They think that microbe populations haven’t reached a stage of their life cycle during which they produce a lot of methane, helping to keep the animal’s gassy emissions down.

Knowledge about how methane — a greenhouse gas contributing to global warming — is produced in kangaroos could have implications for curbing the significant amount of methane emitted each year by farm animals like cattle.

Kangaroo toots have been considered easy on the environment because they’ve been thought to contain little to no methane. But Adam Munn, a wildlife biologist at the University of Wollongong in Australia, and colleagues show that a single kangaroo can produce nearly 1,000 liters of methane per year. The researchers aren’t saying that kangaroos are a major source of methane — cows can let out between 250 and 500 liters a day. “What we found is that kangaroo’s methane is low, but it’s not as low as some studies have suggested,” Munn says.

Over a 12-day period, Munn and colleagues studied western gray kangaroos (Macropus fuliginosus) and red kangaroos (Macropus rufus) that were both fed the same diets of alfalfa hay. At the start, the kangaroos were put on a restricted diet, getting slightly less than they usually eat. Then the researchers switched the amount, letting the kangaroos eat as much as they wanted.

On the restricted diet, both species of kangaroos produced more methane than when they ate the free-rein diet. That could be because a higher food intake moves food and microbes through the digestive tract faster, giving the microbes too little time to establish a steady population and start producing methane.   

When “food moves through the gut more slowly, it’s giving the bacteria more time to go from a growth phase to a maintenance phase of the life cycle,” says Munn. It’s the maintenance phase that produces methane, he says.  

Not everyone agrees, though.

The researchers failed to determine the mix of all the microbe species in the kangaroos’ guts, says Athol Klieve, a microbiologist at the University of Queensland in Australia. “They haven’t looked at the microbiome at all,” he says. Klieve isn’t sure how Munn and his colleagues could draw conclusions about methane production without thoroughly looking at the methane-producing microbes. Klieve and his team reported last year that the composition of microbes in kangaroo guts is responsible for low methane emissions in kangaroos (SN: 4/19/14, p. 10).

Klieve says there are issues with studying animals in captivity, too. Because the kangaroos were fed a different diet than they would eat in the wild, he questions how normal the kangaroos’ gut microbe communities were. “There are heaps of other papers that show that very little methane is produced,” says Klieve.

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