Inbreeding threatens Michigan’s only rattlesnake – What it means for conservation

The post Inbreeding Threatens Michigan’s Only Rattlesnake – What It Means for Conservation appeared first on AZ Animals.

Quick pick up

• Michiganonly him rattlesnake species, the eastern massasauga, shows evidence of inbreeding depression.

• More inbred individuals from two eastern massasauga populations have fewer reproductive output and survival.

• Measures for restoring genetic fitness of these populations may be necessary to prevent local extinctions.

• maintaining habitat corridors for dispersal could also 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 (Chained sistrum), the only venomous snake in the state. They are relatively small for rattlesnakes, averaging two to three feet in length as adults. You’d be lucky to see one, as eastern massasaugas spend most of their time hiding under logs or brush near moist habitats and tend to run away when disturbed. So, although massasaugas have a venomous bite, humans are rarely bitten.

Although they have a wide geographic range in several US states, from Michigan to Missouri, populations of eastern massasaugas have declined sharply since the mid-1970s, according to monitoring by the US Fish and Wildlife Service. They were listed as threatened under the Endangered Species Act in 2016, with the causes of their decline believed to be illegal collection and fragmentation of their habitats over the past 200 years.

As the development of roads and human settlement continued, the eastern massasauga became restricted to smaller and smaller areas of habitat. In a recent paper, conservation biologists found that habitat fragmentation makes it difficult for massasaugas to successfully reproduce and therefore reduces their chances of future survival.

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

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

“This long-term follow-up is the backbone of the study,” lead author then graduate student Meaghan Clark said in a news release. “Every season, people catching these snakes made it all possible.”

Analysis of the data showed that massasauga inbreeding in these two populations results in reproductive problems. By non-random mating with more closely related snakes, these massasaugas reduced their chance of producing viable offspring. Inbreeding reduces fitness by amplifying the expression of deleterious gene combinations. The most inbred Massasaugas were 3.5% less likely to have surviving offspring and had an 11.6% lower annual survival rate. 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 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 native populations. “In our data set, we found that the distance between snakes and their parents was shorter than the distance between pairs of unrelated individuals,” adds Clark.

So Michigan’s massasaugas, already domesticated, are being squeezed even closer to their relatives by habitat modification and loss. Genetic relatedness of parents causes “inbreeding depression” – known decreases in fitness and survival. Over time, you would expect inbreeding depression to reduce the massasauga population size in Michigan, leading to more inbreeding with close relatives in a vicious cycle.

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, which results from habitat fragmentation, on the fitness of eastern massasaugas.

“These are fairly large and stable populations in the eastern Massasaugas,” lead author Sarah Fitzpatrick said in a news release. “The fact that we detect inbreeding problems in these populations is concerning, given that many other populations in the Midwest are much smaller and even more fragmented.”

Once a population of organisms becomes small and isolated, its chance of local extinction increases dramatically. A single natural event could wipe out these massasauga populations, which lack the buffer of a nearby feeding population to restore both genetic diversity and individuals.

Clark says: “The way to counteract this decline is to increase population size and facilitate gene flow with other populations. [of massasaugas]. There is currently ongoing research into how best to do this.” Saving genetic diversity to limit harmful gene combinations may be critical to protecting Michigan populations of eastern massasaugas and may even justify moving snakes from other populations.

The study authors also recommend long-term monitoring of other snake populations to learn whether they successfully maintain 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 thereby increase gene flow between populations.

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