The involvement of humans in the lives of horses may not be all that good for their gut health, the findings of a recent study suggest.
An international study team set out to see how domestication affected gut microorganisms in horses, using horses living in Mongolia for their research.
They focused on the gastrointestinal microbiome − that ‘s the sum of all the hosted microbes and their respective genes in their gut.
Ludovic Orlando and his colleagues, writing in the journal Scientific Reports, said the mammal gut microbiome was recognized as essential, providing critical functions for the host.
Human studies had shown that lifestyle strongly influenced the composition and diversity of the gastrointestinal microbiome, they noted.
The study team hypothesized that these trends in humans may be paralleled in mammals subjected to human forces such as domestication and captivity, in which diets and natural life histories were often greatly modified.
For their study, the scientists compared the fecal microbiomes of the primitive Przewalski’s horse − the only horses alive today not successfully domesticated by humans − to that of herded domestic horses living in adjacent natural grasslands.
The 44 Przewalski’s horses used in the study lived at large in a fenced 140-square-kilometre reserve in Seer, Mongolia. They had been born in several locations, including France, although roughly half had been born in the reserve itself.
The 28 herded domestic horses used for the research lived under the control of humans in adjacent grasslands outside the fenced area.
“We discovered Przewalski’s horse fecal microbiomes hosted a distinct and more diverse community of bacteria compared to domestic horses, which is likely partly explained by different plant diets as revealed by [testing]” the authors reported.
Among the Przewalski’s horses used in the study, four had been born in captivity in European zoos.
“[They] hosted a strikingly low diversity of fecal microbiota compared to individuals born in natural reserves in France and Mongolia,” they said.
The results suggested that human forces can dramatically reshape equid gastrointestinal microbiomes, a finding which they said had broader implications for the conservation management of endangered mammals.
The authors had hypothesized that the Przewalski’s horse and domesticated horse may differ in their gut microbiomes, reflecting their divergence time of approximately 45,000 years ago.
The individual fecal microbiomes of the Przewalski’s horses had a significantly more diverse range of bacteria than the domestic horses, they said.
“It is possible that the distinct Przewalski’s horse diet supports a higher diversity of gastrointestinal microorganisms. Another possibility is that gut diversity in horses was lost as a result of domestication, mirroring losses detected in humans transitioning to agricultural and urban lifestyles.”
They also found that variation between individual Przewalski’s horses was significantly lower than in the domestic horses tested, which was similar to a trend discovered by other researchers in the gut microbiomes of a small human hunter-gatherer population in South America.
“Whether this trend is due to a less diverse host genetic pool or due to similar diets within the small host population is unknown.”
The researchers said that the Przewalski’s horse fecal microbiomes were clearly influenced by life history.
“We discovered that place of birth was associated with significant although small differences in fecal microbiota.
“In particular, the four Przewalski’s horse individuals born in the European zoos appeared to harbor a distinct and less diverse community of microbes.
“In contrast, the animals born in Villaret, a nature reserve [in France], harbor a community that is more similar in composition to those born in Seer.
“We first considered that these trends in fecal microbiome diversity may be related to diet diversity. However, fecal microbial diversity was only weakly correlated with the plant diversity found in their feces, suggesting that current diet diversity may not necessarily equate to a diverse gut community.
“Furthermore, diversity of diet was not associated with place of birth. Therefore, we hypothesize that birth environment is another factor important in shaping gut microbiomes of Przewalski’s horses.
“We may,” they continued, “be witnessing potential lasting founder effects of colonization by microbes in early life. In a similar vein, human infants born in differing environments (cesarean vs vaginal delivery) assemble different starting gut microbiomes, with differences persisting for over a year in some cases.”
They also found that social structure has a significant effect on the composition of horse fecal microbiomes.
“Fecal microbiome similarities within social groups could be due to a shared similar diet as individuals belonging to the same social groups tend to graze together and tend to share a more similar diet profile than expected by chance.
“In this study, we cannot thus disentangle the effects of diet and social contacts.”
That said, it was likely that herd social structure was important for shaping the gastrointestinal microbiomes of Przewalski’s horses and other social mammals.
“Disruption of social interactions is one of several ways in which anthropogenic [human-related] forces such as captivity may influence mammal gut microbiomes.”
They continued: “We are only beginning to understand the consequences of disrupting relationships between mammals and their microbial partners that have evolved over tens of millions of years.
“Domestication, as already shown for captivity, may result in disruptions of gut microbial communities, especially in cases with diet shifts and restricted exposure to outdoor environments.
“This is particularly important for conservation managers as they urgently try to slow the Earth’s sixth major extinction in the wild.
“Therefore, it is critical to document the variables influencing the structure and diversity of microbiomes in wildlife populations with an eventual goal of considering microbiome health when managing endangered populations.
“Our findings also highlight the importance of life history in shaping the horse gastrointestinal microbiome. Importantly, a legacy of captivity may persist in zoo-born individuals, which we discovered harbored a much lower gut diversity than reserve-born individuals, with potential long-term survival consequences that may impact the outcome of reintroduction programs.”
The authors were Jessica Metcalf, Se Jin Song, James Morton, Sophie Weiss, Andaine Seguin-Orlando, Frédéric Joly, Claudia Feh, Pierre Taberlet, Eric Coissac, Amnon Amir, Eske Willerslev, Rob Knight, Valerie McKenzie & Ludovic Orlando. They came from a range of institutions, including Colorado State University, the University of California San Diego, the University of Copenhagen, the University of Colorado, and the Université Grenoble-Alpes, in France.
Evaluating the impact of domestication and captivity on the horse gut microbiome
Jessica L. Metcalf, Se Jin Song, James T. Morton, Sophie Weiss, Andaine Seguin-Orlando, Frédéric Joly, Claudia Feh, Pierre Taberlet, Eric Coissac, Amnon Amir, Eske Willerslev, Rob Knight, Valerie McKenzie & Ludovic Orlando
Scientific Reports 7, Article number: 15497 (2017) doi:10.1038/s41598-017-15375-9