Inbreeding Threatens Michigan’s Only Rattlesnake — What It Means for Conservation

Inbreeding Threatens Michigan’s Only Rattlesnake — What It Means for Conservation

Devin Reese

Fri, January 30, 2026 at 2:02 PM UTC

5 min read

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Key takeaways

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• Inbreeding depression is threatening Michigan's only rattlesnake species, the eastern massasauga, leading to lower reproductive output and survival in two populations.
• Habitat fragmentation caused by development is making it difficult for eastern massasaugas to reproduce successfully, reducing their chances of survival into the future.
• Measures to restore genetic fitness to these populations, such as increasing population size and facilitating gene flow with other populations, may be needed to prevent local extinctions of the eastern massasauga in Michigan.

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The post Inbreeding Threatens Michigan’s Only Rattlesnake — What It Means for Conservation appeared first on A-Z Animals.

Quick Take

• Michigan‘s only rattlesnake species, the eastern massasauga, shows evidence of inbreeding depression.

• More inbred individuals in two populations of eastern massasaugas have lower reproductive output and survival.

• Measures to restore genetic fitness to these populations may be needed to prevent local extinctions.

• Maintaining habitat corridors for dispersal could benefit other snake species that may also be susceptible to inbreeding depression.

If you come across a rattlesnake in Michigan, it’s an eastern massasauga (Sistrurus catenatus), the only venomous snake in the state. They’re relatively small for rattlers, averaging two to three feet long as adults. You’d be lucky to spot one, since eastern massasaugas spend most of their time hidden under logs or brush near wetland habitats and tend to flee when disturbed. So, although massasaugas have a venomous bite, people are rarely bitten.

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While they have a geographically wide range across multiple U.S. states from Michigan to Missouri, populations of eastern massasaugas have declined precipitously since the mid-1970s, according to monitoring by the U.S. Fish and Wildlife Service. They were listed as threatened under the Endangered Species Act in 2016, with the causes of their decline considered illegal collection and fragmentation of their habitats over the past 200 years.

As development of roads and human settlements has proceeded, eastern massasaugas have become confined to increasingly smaller patches of habitat. In a recent paper, conservation biologists found that habitat fragmentation is making it difficult for massasaugas to reproduce successfully and, therefore, reducing their chance of survival into the future.

Biologists from Michigan State University used genetics to trace the family histories of snakes in two populations. By partnering with long-term monitoring projects run by Grand Valley State University, West Virginia University, and the Association of Zoos and Aquariums, the researchers were able to gain DNA samples from more than 1,000 individual snakes that had been captured and released in a project funded by the U.S. Fish and Wildlife Service.

Each captured snake was weighed, measured, assessed for sex and reproductive status, sampled for blood, and then released with a unique PIT tag for individual recognition. DNA from the blood sample served to construct family trees for the populations, showing the relatedness between individuals. Over 15 years, visits to the two populations yielded estimates of snake births and snake deaths.

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“This long-term field monitoring is the backbone of the study,” said lead author, then graduate student, Meaghan Clark, in a press release. “Having people out each season catching these snakes made all of this possible.”

The data analysis showed that massasauga inbreeding in these two populations is resulting in reproductive problems. By mating nonrandomly with more closely related snakes, these massasaugas reduced their chance of producing viable offspring. Inbreeding reduces fitness by amplifying the expression of harmful combinations of genes. The most inbred massasaugas were 3.5 percent less likely to have surviving offspring and had an 11.6 percent lower annual survival rate themselves. But why would these eastern massasaugas choose to mate with their relatives?

“Habitat loss and fragmentation prevent occasional migration between eastern massasauga populations, which over time increases the relatedness within each population,” lead author Clark explains via email. Additionally, massasaugas naturally tend to disperse short distances from their parents, limiting gene flow away from their natal populations. “In our dataset, we found that the distance between snakes and their parents was less than the distance between pairs of unrelated individuals,” Clark adds.

So, the Michigan massasaugas, already homebodies, are being further squeezed into proximity with their relatives by the alteration and loss of habitat. Genetic relatedness of parents causes “inbreeding depression”—known decreases in fitness and survival. Over time, you’d expect inbreeding depression to reduce Michigan massasauga population sizes, leading to more breeding with close relatives in a vicious cycle.

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Other eastern massasauga populations are often even smaller, and their movements are similarly restricted by roads, farms, and other barriers. This was the first study to directly quantify the impact of inbreeding, resulting from habitat fragmentation, on the fitness of eastern massasaugas.

“These are fairly large and stable populations of Eastern Massasaugas,” said senior author Sarah Fitzpatrick in a press release. “The fact that we’re detecting problems from inbreeding in these populations is concerning, given that many other populations throughout the Midwest are much smaller and even more fragmented.”

Once a population of organisms becomes small and isolated, its chance of going locally extinct rises dramatically. A single natural event could wipe out these massasauga populations, which lack the buffer of a nearby feeder population to replenish both genetic diversity and individuals.

Clark says, “The way to counter this decline is to increase population size and facilitate gene flow with other populations [of massasaugas]. There’s currently research underway as to how to best do this.” Rescuing genetic diversity to limit deleterious gene combinations may be key to protecting the Michigan populations of eastern massasaugas and may even warrant moving snakes from other populations.

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The study authors also recommend long-term monitoring of other snake populations to figure out whether they’re successfully maintaining their gene pools in the face of environmental changes that limit habitat connectivity. Implementing changes such as building underpasses for snakes to cross roads or creating openings in fences could improve connectivity and, therefore, increase gene flow between populations.

The post Inbreeding Threatens Michigan’s Only Rattlesnake — What It Means for Conservation appeared first on A-Z Animals.

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