Michigan’s only rattlesnake is inbreeding

While sexual reversal, genetic gambling, or evolving to reproduce later into life make the animal kingdom incredibly resilient, it’s difficult to outrun inbreeding. Or out-slither it, in the case of the only rattlesnake species found in the state of Michigan. 

A 15-year study found that farms, buildings, and roads are making it difficult for the Eastern Massasauga rattlesnake (Sistrurus catenatus) to find a mate. The most inbred snakes had some serious problems not only producing viable offspring, but in simply surviving year-to-year.. The findings are detailed in a study published August 18 in the journal Proceedings of the National Academy of Sciences (PNAS).

“These are fairly large and stable populations of Eastern Massasaugas,” study co-author and Michigan State University conservation biologist Sarah Fitzpatrick said in a statement. “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.”

Checking fitness

As venomous snakes, Eastern Massasauga rattlesnakes aren’t necessarily the most popular animal. Yet the reptiles are a keystone species of wetland food webs throughout the Midwest. The snakes hunt mice and rats that could run rampant in nearby homes and barns if not kept in check. According to study co-author Meaghan Clark, if the rattlesnakes disappeared, the entire balance of the ecosystem would be disrupted.

These rattlesnakes are considered timid, and don’t stray too far from the wetlands where they were born. They will typically only wander to explore a nearby habitat and find a mate before they head home. However, increasing human development is likely keeping Eastern Massasaugas even more homebound. When it’s eventually time for the snakes to choose a mate, they are more likely to end up with a snake that is related to them instead of one from a distant population. 

[ Related: The last woolly mammoths were impressively inbred—but that’s not what killed them.  ]

“They’re very vulnerable to even minor disturbances to their habitat,” said Fitzpatrick. “Even a single road can isolate populations.” 

When animals reproduce with their relatives, it typically harms their babies’ fitness. In biology, “fitness” describes how successful an animal is at surviving, producing babies, and continuing on the species.  

Inbreeding can decrease fitness in what conservationists call inbreeding depression. However, it can be quite difficult to prove in wild populations, especially snakes with a venomous bite. 

Rattlesnake family trees

In the study, Fitzpatrick’s lab joined forces with long-term snake monitoring projects, partnering with researchers at Grand Valley State University, West Virginia University, and the Association of Zoos and Aquariums. Every summer since 2009, scientists armed with snake tongs have trekked through wetlands to capture the elusive reptile. 

The team noted each snake’s length, weight, and pregnancy status. They also drew blood so that they could extract DNA and sequence their genomes. That genetic information helped them track how many babies were born and survive into adulthood, but also allowed the team to reconstruct pedigrees and determine how closely any two individual snakes were related. The team ultimately traced the family histories of over 1,000 Eastern Massasauga rattlesnakes.

Before being released into the wild, each snake was marked with a PIT tag, similar to a microchip that a pet might have. This tracking means that the team can track the snakes’ survival, based on whether they were recaptured.

They found that the most inbred snakes were 13 percent less likely to have surviving offspring and their annual survival rate was nearly 12 percent lower than the less inbred rattlesnakes. Numbers this high even surprised the researchers. 

“This long-term field monitoring is the backbone of the study,” Clark said. “Having people out each season catching these snakes made all of this possible.” 

The team hopes that this data informs conservation efforts that could help Eastern Massasaugas find mates outside their families. Some small changes, including road underpasses or habitat restoration in wetlands, could promote more connectivity between snakes that would boost the gene pool. Conservationists could also experiment with moving the reptiles to new habitats with more options for finding suitable rattlesnake mates. 

 

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