Why did the rattlesnake cross the road? It didn't, and that's a problem, say conservationists
- Rattlesnakes appear to avoid mating with snakes living across even minor roads.
- This may be leading to isolated, inbred populations, which are more vulnerable to disease.
- Roads are especially perilous to rattlesnakes since they freeze when they sense threats and vibrations.
Why did the rattlesnake cross the road? It didn't, says new research, and that may be a problem.
New DNA analyses of rattlesnakes in New York State finds that even minor roadways deter snakes from crossing the road to breed. Populations are becoming isolated from each other by roadways, which may threaten their future.
"We worry for the health of these populations because connectivity is so key to responding to environmental pressures and avoiding inbreeding," said Rulon Clark of San Diego State University, who led the study published in Conservation Biology. The reduced genetic diversity that results from inbreeding makes populations less resistant to diseases or other disturbances.
Groups of up to dozens of rattlesnakes hibernate together in common dens, which serve as a home base. Come spring, they leave to hunt, and males strike out for neighboring dens to mate with females before returning to their home dens for winter.
The males' visits to nearby dens provide the genetic mixing necessary to keep a population from becoming inbred.
Clark and colleagues wanted to test whether roads were affecting the males' mating journeys. They collected blood samples from timber rattlesnakes in 19 different dens in different regions of New York State, including dens separated from each other by roads and others with uninterrupted forest between them.
The researchers analyzed selected gene sequences that mutate quickly from generation to generation to determine how related different snakes are to each other and whether certain dens are mixing with others.
The team found that dens without roads between them acted as a single, connected population, while populations separated by roads showed signs of significantly reduced mixing.
"These roads have been in place for maybe nine or 10 timber rattlesnake generations," Clark said. "That's not a long time, but even in that relatively short time frame, we found some very strong patterns. It's somewhat disturbing to see how quickly the populations lose genetic diversity when they become isolated by these roads."
Other studies have found that species including bobcats, coyotes and bighorn sheep also change their behavior to avoid roads, with consequences for gene flow. But rattlesnakes may be particularly susceptible, because they avoid roads, and when they do try to cross, the consequences are often fatal, Clark said.
"When they do venture across, they move extremely slowly," he said. "If they're disturbed by noise or vibration, their natural response is to freeze and rely on their natural camouflage to hope they won't be detected. With cars, that's exactly the wrong response."
It may be possible to help male rattlesnakes complete their conjugal journeys by building underpasses with surrounding fences that help shepherd the snakes under the roads, the researchers note. Appropriately timed road closures during the migratory season could also help.
Kenneth Dodd of the University of Florida in Gainesville agreed that roads are a threat to wildlife, especially snakes, and that methods to help animals cross roads safely should be implemented.
At the same time, Dodd thinks other forces are at work in explaining the genetic differences among rattlesnakes in this area. Dodd was part of research that analyzed some of the same dens as the new study but found less dramatic effects.
"It's more complicated," he said. Small population sizes and localized differences in habitat and topography may also be contributing to the separation of populations, not just roads, he explained.
"We believe that the roadway contributes to genetic structuring of the population, but it is not the sole driving force."