A mathematical model based on observing yellow anacondas in the lab shows that if they are light and muscular enough, they can jump away from danger in an S-shape motion similar to sidewinding of much smaller snakes that live in the sand.
Yellow anacondas are able to leap into an unusual S-shape to escape danger. However, only those that are light and muscular enough can actually do it.
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| Some yellow anacondas can leap in an S-shape Shutterstock/cellistka |
Many snakes that live in sandy environments can move sideways or sidewind, rippling their bodies while also partially lifting off the ground. Anacondas, which typically live near rivers and in marshes, aren’t normally considered to be in this group and have only been captured on film doing something like sidewinding once, in 2014.
Nicholas Charles at Harvard University and his colleagues have now found when and how yellow anacondas (Eunectes notaeus) are able to perform lunges similar to sidewinding after systematically observing them doing so in the lab.
They studied how 10 yellow anacondas of different ages and sizes moved when scared and noticed that some of them could quickly jump away in a move they dubbed the S-start. Similar to sidewinding snakes, a scared anaconda would contort itself in a shape like a squished letter S.
It would keep its middle segment still and flat to the ground but raise the curved parts of its body and slide sideways along the top and bottom of the S-shape. This combination of intricate yet quick moves would produce a ripple along the snake’s body, causing it to jump sideways.
Among the 10 snakes, the two oldest and largest anacondas never moved this way, while three newborns and five juveniles did. This motivated the researchers to determine the rules for when the S-start is possible based on the gravity and friction the snake must overcome.
Through computer simulations, they found that the ratio of a snake’s weight and the strength of its muscles must be just right for the S-start to be possible. Simulated snakes that were too heavy unravelled into a flat line instead of moving, while those that were too light flailed in place.
For instance, when a snake weighed 20 times the torque that its muscles could produce, it could S-start, but when that ratio was 40, it couldn’t. This explained why adult anacondas couldn’t perform the S-start, as older snakes have less muscle and more bone mass, so they are too heavy.
“Different parts of a snake’s body grow at different rates and the torque the snake’s muscles can produce doesn’t always keep up with its growing volume,” says Jessica Tingle at the University of Akron in Ohio. The team suggests that the S-start may have led to the evolution of sidewinding, but Tingle says that more detailed studies are needed to establish that.
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