When rattlesnakes marry their cousins

Roads, buildings and farms are preventing Michigan’s only rattlesnake from finding mates outside of their population. A 15-year study shows that fragmentation into smaller, more isolated patches is likely reducing the threatened snake’s chances of survival. 

Michigan State University conservation biologists traced the family histories of more than 1,000 Eastern Massasauga rattlesnakes caught and released in a U.S. Fish and Wildlife Service-funded project. The new findings, published in the Proceedings of the National Academy of Sciences, surprised even the researchers – the most inbred snakes were 13% less likely to have surviving offspring and had a nearly 12% lower annual survival rate. 

These results paint a striking picture of the need for rattlesnake habitat connectivity to be restored, MSU Professor Sarah Fitzpatrick said. 

“These are fairly large and stable populations of Eastern Massasaugas,” said Fitzpatrick, a senior author on the study. “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.” 

While Eastern Massasauga rattlesnakes aren’t the most popular animal, they’re keystone species of wetland food webs throughout the Midwest. They hunt prey like mice and rats that otherwise might run rampant in nearby homes and barns. If rattlesnakes disappeared, the entire balance of the ecosystem would be disrupted, said Meaghan Clark, lead author and former MSU graduate student in Fitzpatrick’s lab. 

These rattlesnakes are homebodies who typically don’t like to venture beyond the wetland where they were born. They only wander to explore a nearby habitat and find a mate before returning home. 

But an increasing human presence – and developments such as roads, farms and houses – are likely keeping Eastern Massasaugas even more homebound. That means when it’s time to choose a mate, they are more likely to end up with a relative instead of an out-of-towner. 

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

When animals have offspring with their relatives, there’s usually a negative impact to their babies’ fitness. In the evolutionary sense, an animal’s fitness doesn’t mean how many pushups they can do or how much they can bench press. Researchers use “fitness” to describe how successful an animal is at surviving, producing babies and continuing the species.  

A decrease in fitness resulting from inbreeding is known to conservationists as “inbreeding depression.” But that’s difficult to prove in wild populations, especially snakes with a venomous bite. 

Fitzpatrick’s lab joined forces with long-term monitoring projects in Cass and Barry counties, partnering with Jennifer Moore at Grand Valley State University, Eric Hileman at West Virginia University, and Lisa Faust from the Association of Zoos and Aquariums. Each summer since 2009 and 2011, respectively, their research teams have donned tall rubber boots and armed themselves with snake tongs to trek through wetlands and capture the elusive reptile.  

The teams checked each captured snake’s length, weight and pregnancy status and drew blood, which they used to extract DNA and sequence their genomes. These details were enough for researchers to reconstruct pedigrees and determine how closely any two individual snakes were related. Each snake was marked with a PIT tag, similar to a microchip for a pet, before being released back into the wild. 

By returning to the same wetlands year after year, researchers could track snakes’ survival based on whether they were eventually recaptured. Genomic sequencing made it possible to generate a family tree for each population, track how many snake babies were born and survived to adulthood, and determine who their parents were. 

“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.” 

Fitzpatrick hopes the study informs conservation efforts that help Eastern Massasaugas find mates outside their families, especially in declining populations beyond Michigan. Small changes, like habitat restoration or building road underpasses, could promote more connectivity that would boost the gene pool and give the snakes a better shot at survival. Conservationists could also explore moving imperiled rattlesnakes to new habitats with more options for finding mates. 

They may be feared and misunderstood, but the Eastern Massasauga is a silent ally. Ensuring their survival could keep Michigan’s wetlands balanced and thriving for generations.