Five-Year Poop Study Reveals Evolutionary History of Animal's Gut Microbes

A thorough (and stinky) study of poop from large, plant-eating mammals has revealed that microscopic organisms inside the animals' guts have evolved at the same rate as the animals themselves.

By sifting through the excrement of cows, horses, elephants, goats, sheep and other herbivorous mammals, researchers were able to analyze and compare the animals' gut ciliates - one-celled microorganisms that are especially important to the digestion process.

A five-year study chronicling the excrement and gut fluids of many plant-eating mammals appears in the European Journal of Protistology, where a team of evolutionary biologists, microbiologists and computer scientists from Radboud University Nijmegen in the Netherlands reveal details of a wide range of animal droppings.

The researchers make the distinction between herbivores known as "foregut fermenters" and "hindgut fermenters," the former of which digests food in the front part of its gut (the rumen) and the latter digests in the posterior part of the gut, the appendix and large intestine.

Ruminant animals such as cows, goats and sheep are foregut fermenters, while horses, elephants and zebras, for example, are hindgut fermenters.

The poop used in this study came from around the world, notably from goats, deer and sheep from around Europe, an Asian elephant, and zebras and an elephant from Africa.

The researchers preformed a genetic analysis of the ciliates present in the animals' excrement.

"Our most important discovery is that ciliates are extremely diverse," said evolutionary biologist Johannes Hackstein. "We hadn't expected that, given their appearance."

The analysis revealed that the the ciliates of hindgut fermenters were "very species-specific and originated early on in evolution, just like the hosts themselves," the researchers report.

Foregut fermenters, on the other hand, do not have species-specific ciliates, which the researchers say, from an evolutionary point of view, means the oldest animals have the oldest ciliates in their droppings.

Hackstein said the difference may be due to how the ciliates are transmitted.

"In hindgut fermenters transmission occurs via coprophagy [eating excrement], and that happens almost exclusively in extremely young offspring," he said. "Ruminants, on the other hand, can be infected with ciliates throughout their lives. However, this still does not explain how the huge ciliate diversity arose and how this can be sustained."