Sidewinding instead depends on lifting large chunks of the body into the air as the animal moves. Scales that create strong directional friction, Dr. Rieser said, do very badly with this kind of movement. But if scale friction is uniform in all directions, it makes sidewinding significantly easier.
The Saharan horned viper and the sidewinding adder of the Namib desert which are closely related have belly scales with uniform pits and no spikes. But the sidewinding rattlesnake, which comes from a different branch of the viper family tree, still has a few vestigial belly spikes as well as pits.
One possible explanation for the difference is that the deserts of the North American southwest are only 15,000 to 20,000 years old, compared with the North African deserts, which are seven million to 10 million years old.
So maybe theres been less time for American sidewinders to evolve structures that might help this type of movement, Dr. Rieser said.
While the teams hypothesis about the precise function of the microscopic pits will require additional study, the loss or reduction of these belly spikes in distantly-related sidewinders suggests that these changes are a direct adaptation to sideways movement, they suggest.
Given that movement is so crucial to survival, its reasonable to think thats part of the reason this change has occurred, Dr. Rieser said.