“Our results confirm that donkeys are better adapted to using high-fiber forages by having more specialized fiber-degrading microbes,” said Joan Edwards, PhD, of the Laboratory of Microbiology at Wageningen University & Research, in the Netherlands.

“These results support donkeys’ previously documented ability to degrade fiber more efficiently,” Edwards said.

Scientists have suspected that donkeys—which evolved in dry environments with generally poor-quality forage—digest fibers better than horses and ponies because they digest it longer. This longer digestion time would theoretically let the digestive enzymes work more efficiently on the fiber, she said.

But Edwards wondered something else was going on, as well. So she and her fellow collaborators (The Donkey Sanctuary & Utrecht University) collected fecal samples from eight ponies, eight donkeys, and eight donkey/pony mules, all healthy and living at the same farm. They were all fed the same straw and haylage, provided in rations according to their weights. The scientists ran sequencing on the feces to determine the makeup of each animal’s hindgut microbiota.

They found that, overall, the donkeys, ponies, and mules had similar total concentrations of microbiota in the hindgut, Edwards said. However, the mix of bacteria and fungi was different.

With regard to bacteria, donkeys had more of two groups of Lachnospiraceae—Lachnoclostridium 10 and ‘probable genus 10’—than ponies and mules.

Lachnospiraceae bacteria are already known to play an important role in energy production, she said. The two groups of Lachnospiraceae they identified include species that appear to help degrade and digest the fiber in hay and other forages. So it’s possible, said Edwards, that the greater abundance of these two Lachnospiraceae groups helps donkeys digest forage better, allowing them to gain more energy from it compared to other equids.

As far as fungi goes, the scientists noted more species of beneficial anaerobic (not needing oxygen in the air to live) fungi in the donkeys’ feces, said Edwards. In the hindgut, this would probably also help them break down plant fibers more efficiently, she said.

The findings make sense, given what we know about the histories of donkeys and horses, Edwards explained.

“Horses originate from open grassland plains, and donkeys from semiarid, often mountainous areas—environments containing very different types of forages,” she said. “As such, compared to horses, donkeys have evolved a different feeding strategy, physiological traits, and behavior. All of these factors play a role in shaping the community of microbes living in their gut.”

Because horses and ponies are the same species, Edwards did not test horses. However, a recent study has suggested horses and ponies might have different microbiotas, so follow-up studies comparing them along with donkeys could be useful, she said.

Future studies might also investigate how microbiota in different equids could be adjusted to help them better digest fibers and gain more energy from them, she said.