To Scientists' Surprise, Even Nonvenomous Snakes Can Strike at Ridiculous Speeds
The Texas rat snake was just as much of a speed demon as deadly vipers, challenging long-held notions about snake adaptations
image:
http://thumbs.media.smithsonianmag.com//filer/a3/3f/a33f8ee0-bfee-4cce-9a13-f9388c5323c0/42-55375529.jpg__420x240_q85_crop_upscale.jpgTexas Rat Snake
A nonvenomous Texas rat snake coils up in a defensive posture. (Erich Schlegel/Corbis)
By Marcus Woo
SMITHSONIAN.COM
MARCH 15, 2016
When a snake strikes, it literally moves faster than the blink of an eye, whipping its head forward so quickly that it can experience accelerations of more than 20 Gs. Such stats come from studies of how a snake lunges, bites and kills, which have focused mostly on vipers, in part because these snakes rely so heavily on their venomous chomps.
"It's the lynchpin of their strategy as predators," says Rulon Clark at San Diego State University. "Natural selection has optimized a series of adaptations around striking and using venom that really helps them be effective predators."
That means scientists have long assumed vipers must have the speediest strikes in town. "There's this kind of preemptive discussion that [vipers] are faster," says David Penning at the University of Louisiana, Lafayette.
Not so fast: When Penning and his colleagues compared strike speeds in three types of snakes, they found that at least one nonvenomous species was just as quick as the vipers. The results hint that serpents' need for speed may be much more widespread than thought, which raises questions about snake evolution and physiology.
Penning and his colleagues started by digging into the existing literature on snake strikes. They found fewer than three dozen papers describing the physics and kinematics of snake strikes, most of which focused on vipers. That means the research community could be ignoring the thousands of other snake species that populate the world.
"As sexy as the topic sounds," Penning says, "there's not that much research on it."
So the team set out to compare three species: the western cottonmouth and the western diamond-backed rattlesnake, which are both vipers, and the nonvenomous Texas rat snake. They put each snake inside a container and inserted a stuffed glove on the end of a stick. They waved the glove around until the animal struck, recording the whole thing with a high-speed camera. The team tested 14 rat snakes, 6 cottonmouths and 12 rattlesnakes, recording several strikes for each individual.
Slowed-down recordings from a lab experiment show two snake species striking at a test glove. (Video clips courtesy of David Penning)
All the snakes turned out to be speed demons, the team reports this week in Biology Letters. The rattlesnake scored the highest measured acceleration, at 279 meters per second squared. But to their surprise, the nonvenomous rat snake came in a close second at 274 meters per second squared. That's lightning-quick, considering that a Formula One race car accelerates at less than 27 meters per second squared to go from 0 to 60 in just one second.
"I was really surprised, because this comparison hadn't been made before," Clark says. "It's not that the vipers are slow, it's that this very high-speed striking ability is something that seems common to a lot of snake species—or a wider array than people might've expected."
And that makes a lot of sense when you think about it, Penning says: "Every snake has to eat," he says. "That means every snake has to close the distance between itself and what it's got to eat or when it defends itself."
Snakes probably evolved such quick strikes to compete with the reaction times of prey, Penning adds. Depending on the species and the situation, mammals can react and make an observable motion anywhere between 60 and 395 milliseconds. An average human eye blink, for instance, spans a leisurely 200 milliseconds.
According to the new measurements, a snake can lunge forward about half a foot in only 70 milliseconds. Faced with such speed, a lot of prey won't stand a chance—although some animals are closely matched, such as the kangaroo rat. But even if an animal can react in time, they might not be able to accelerate enough to escape.
"Snakes are evolved to be very fast," Clark says. "The prey is also under corresponding selection pressure to be very fast. I'm sure the snakes and the things they rely on to eat are locked in this co-evolutionary arms race."
Future studies of strike speeds across species may help solve another puzzle: How can snakes endure such high accelerations? Many animals can move quickly, but they're usually launching a tongue or a limb—not their whole heads. By comparison, a human pilot can lose consciousness when experiencing less than a quarter of the acceleration that snakes undergo.
"There are a lot of remaining questions that need to be addressed," Penning says. After all, the researchers have only compared vipers with one other species, and there are thousands more. "Based on what we've seen," he says, "my guess is that there are faster ones."
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