On Sable Island, a thin crescent of windswept sand about 300 kilometers off the coast of Nova Scotia, several hundred wild horses live without predators, without veterinary care and without any human control over who breeds with whom. Researchers at the University of Calgary, the University of Saskatchewan and the University of Exeter have spent years collecting fecal samples from these horses as part of a long-running monitoring program, and their most recent analysis has found that closely related horses carry communities of gut bacteria that are much more similar than would be expected by chance and, even more novel, that this heritability is linked to survival and is outweighed by a social effect stronger than genetics itself. The finding comes from a study led by Mason Stothart, published in late July as a preprint on bioRxiv under the title “Host genetics and social relationships jointly shape fitness-associated microbiome variation in a wild horse population.” It has not yet completed peer review, which is important here because the claim it makes is genuinely new: that the microbiome of a wild population may be heritable enough and important enough for survival to act as a lever on which evolution can act. What the data really shows The team relied on 2,394 fecal samples collected from 794 individually known horses, animals that have been tracked since 2007 as part of Parks Canada’s monitoring of the island’s population. Because Sable Island horses have detailed pedigree records, the researchers were able to separate the extent to which variation in gut bacteria was explained by shared genetics, by a shared environment, and by social relationships—that is, which horses spend time near which other horses, regardless of whether they are related. The combined genetic relationship, a horse’s own stable environment and its social associations accounted for 47 percent of the variation in specific microbiome traits related to survival, and 38 percent of the variation in the microbial gene families carried by those bacteria. The social effects, in particular, were two to four times stronger than the direct genetic effects. In layman’s terms: Horses that spend time together end up with more similar gut bacteria than horses that are related but rarely interact, and that social exchange was a bigger factor than heredity from one parent. No maternal effect was detected, the boost that a foal might receive from its mother’s specific microbial community early in its life. Why “heritable” does not mean “genetic” It would be easy to misinterpret “heritable” here to mean that bacteria themselves are inherited in the same way as eye color, and are passed on directly in DNA. That’s not what the study found. Gut bacteria colonize a horse’s intestine after birth, picking up from the environment, other animals, and everything the animal eats. What the researchers are describing is that a horse’s genetic background appears to shape the bacterial communities that take hold and thrive in its gut, along with a separate and apparently larger effect on the environment of other horses it spends its life with. That distinction is why the researchers frame this as evidence of “microbiome-mediated adaptive evolution,” in the study’s own wording, rather than as evidence that gut bacteria are simply programmed. A trait does not need to be directly encoded in DNA for natural selection to shape it; it just needs to be transmitted consistently, either genetically or socially, and affect survival and reproduction. This study is a first attempt, in a wild population, to demonstrate that a microbiome plausibly meets both conditions at the same time. What’s at stake in a mediocre gut The reason this matters beyond horses dates back to earlier work from the same research group, published in 2024 in Nature Communications, which found that horses whose gut microbes produced less methane during digestion, a byproduct of inefficient fermentation, had significantly better odds of surviving Sable Island’s harsh winters. Methane production is essentially wasted energy: the bacteria that generate more are extracting less usable energy from the same bite of marram. On an island with no supplemental feed and a landscape that offers little shelter, that efficiency gap can be the difference between a horse that makes it to spring and one that doesn’t. Putting the two studies together, the picture becomes more specific: The composition of the gut microbiome affects a horse’s chances of survival, and the microbiome a horse ends up with is determined, in part, by its genes and social world, both of which, in principle, can be passed on or reinforced from generation to generation. That’s the mechanism by which a microbiome could, at least in theory, become a target of natural selection rather than simply a passive byproduct of it. What this doesn’t show The authors are careful and the scope of the preprint must be taken into account. This is a population, on an island, studied by a group and has not yet undergone peer review. Correlational heritability estimates in a single wild population are not proof that horse gut microbiomes have actually changed the evolutionary trajectory of the species; They show that the ingredients for that type of process, a trait that is partly transmissible and that affects survival, are present and measurable. The Sable Island horses are also an unusual population in some relevant ways: They descend from a small founding group, are isolated for generations, and live somewhere without natural predators, so what happens there may not clearly generalize to more typical wild horse populations, let alone other species. What the researchers have shown is more limited and even notable: that in this specific population, a microbiome linked to survival is neither noise nor simply a byproduct of each horse eating from the same patch of grass. Some travel with lineages and a greater part travel with company. The next natural question, which the preprint does not resolve, is whether these differences in the microbiome persist throughout a horse’s life or change as its social bonds change, which would help separate how much of the effect is established early from how much of it continues to be renegotiated. The Sable Island horses, tracked individually for nearly two decades, are one of the few wild populations detailed enough that that question can even be answered.
