Bumpus in the Road: New study suggests vehicle-induced selection reduces snake body size

Excited to share my recent paper on Road Ecology in Chihuahuan Desert snakes. After several years of road cruising and measuring all snakes encountered (AORs and DORs), I found some H.C. Bumpus-level surprises in Western Diamondback Rattlesnakes.

PeerJ (link to paper: https://peerj.com/articles/19871/)

Western Diamondback Rattlesnake in southern New Mexico

Background

Snakes are vital predators in ecosystems, but road mortality threatens their role. Understanding road use patterns is essential for mitigation.

Study Overview

A study in the Chihuahuan Desert, New Mexico, conducted nocturnal surveys from April to November 2017 along a 37-km road, resulting in 101 encounters across 10 species. The average was 4.2 snakes per survey, with a density of 0.057 snakes per kilometer. Peak activity was from August to October.

Findings

From 2014 to 2016, 34% of snakes were found dead on the road, with juveniles having a lower mortality rate than adults. The Western Diamondback Rattlesnake (Crotalus atrox) was most observed, with 33% DOR. Larger adult C. atrox had a higher collision risk; a 1 cm increase in size raised the probability of collision by 10%. Simulations indicate that selection against larger sizes could reduce average size by 12 cm in 50 generations.

Conclusion

Road mortality may drive evolutionary changes in snakes. As roads and traffic increase, mitigating impacts on snakes is crucial to avoid significant ecological and evolutionary consequences.

#1: Snakes are key predators in deserts

... but roads take a toll on snakes. SO, in the Chihuahuan Desert, I road cruised for 4 years to ask: how do roads impact snake communities, mortality, and perhaps even evolution?

#2: Roads were more than asphalt

... they’re selective landscapes

1) ~34% of all snakes were DOR

2) spatially, no DOR hotspots detected

3) Western Diamondback Rattlesnakes dominated—48% of all sightings—and one third of them were DOR

#3: Size mattered (for C. atrox)

Models showed that each extra 1 cm in rattlesnake length raised the odds of being killed on the road by ~10%

"Road-crossing" simulations showed that bigger rattlesnakes = bigger targets (easy to see, hard to avoid)

#4: What does that mean over time?

Evolutionary simulations suggested that sustained size-biased mortality could reduce the average body length of Western Diamondback Rattlesnakes by ~12 cm in 50 generations (~165 yrs) ...

#5: We just might be steering snake evolution

"Old Rattler, it is part of Nature's plan That I should grind you underneath my [w]heel -- The age-old feud between the snake and man -- As Adam felt in Eden, I should feel."

-Montgomery