A 15-year study has revealed that eastern massasauga rattlesnakes – the only rattlesnake species in the US state of Michigan – are suffering the effects of inbreeding, as roads, farms and housing developments prevent them from reaching mates outside their local population.
The findings, published in the journal Proceedings of the National Academy of Sciences, raise concerns about the survival of this threatened species and highlight the need for better habitat connectivity in the snake's range.
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Eastern massasauga rattlesnake – clues in the DNA
Eastern massasaugas are creatures of habit. These small, venomous pit vipers rarely leave the wetland where they are born, typically venturing out only to find food or a mate before returning home.
However, as human presence increases across their range, the snakes are even less likely to travel between habitats. Barriers such as roads or farmland can cut off populations completely.
“They’re very vulnerable to even minor disturbances to their habitat,” says Sarah Fitzpatrick, Michigan State University (MSU) professor and one of the senior authors on the study. “Even a single road can isolate populations.”
To see whether this isolation is causing problems for the rattlesnakes, researchers at MSU analysed 1,056 individuals collected over 15-year period across two counties in south-west Michigan: Cass and Barry.
The team looked at each captured snake’s length, weight and pregnancy status. They also used blood samples taken at the time of capture to extract DNA and sequence their genomes, which then allowed the researchers to work out how closely any two individual snakes were related.
They found that the most inbred individuals were 13% less likely to have offspring that survived, and had a nearly 12% lower annual survival rate than healthy, less inbred snakes.
“These are fairly large and stable populations of eastern massasaugas,” says Fitzpatrick. “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.”
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Why is inbreeding bad?
The findings paint a troubling picture for the future of these snakes, says Meaghan Clark, the study's lead author and a former MSU graduate student.
Inbreeding reduces the ability for an animal to survive, reproduce and pass on its genes. Biologists have long suspected this so-called 'inbreeding depression' takes place in the wild, but it is difficult to prove, especially in elusive species such as rattlesnakes.
Eastern massasaugas play a vital role in wetland ecosystems across the Midwest. They are skilled hunters of rodents such as mice and rats, keeping numbers under control and preventing infestations in nearby barns and homes, says Clark. “If rattlesnakes disappeared, the entire balance of the ecosystem would be disrupted.”
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What's next?
The researchers say this study provides rare evidence from natural populations that the consequences of inbreeding, caused by isolation, are both measurable and significant.
Fitzpatrick hopes the findings will guide conservation action, such as creating wildlife underpasses, restoring wetlands or even relocating snakes to increase genetic diversity. Such measures could help prevent population decline in Michigan and across the Midwest, say the researchers.
Top image: Michigan wetlands. Credit: Getty
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