Tetsuto Miyashita needed a research project his students could do from home. During the COVID-19 lockdown, the evolutionary biologist kept thinking about photographs he had seen of snake embryos, their long bodies curled tightly inside their eggs. One detail had caught his attention. Did they all curl in the same direction?
When Miyashita and his students began looking more closely, they noticed young snake embryos consistently started out curled to the right, even before their muscles were developed enough to move their bodies that way.
A study published in Current Biology now points to the reason. Differences in how quickly the body and digestive tract grow appear to physically force the embryo into its first spiral, helping snakes fit a long developing body inside an egg.
“There is a touch of mystery to spirals, and we are only beginning to understand how these shapes are produced in animals, such as our looping intestine, snail shells, and now these beautifully coiled snake embryos,” said lead author Alexandra Weber in a press release.
Why Snake Embryos Curl to the Right
(Image Credit: Raul Diaz, California State University Los Angeles)
Miyashita asked Weber and two other students to search scientific papers and museum collections for photographs of developing snakes.
The team collected images of more than 900 embryos from 39 species of snakes and other limbless squamates. Among the youngest embryos, they found a preference for right-handed, or dextral, coils running from head to tail.
Explore All Options“At these stages, the embryos don’t have muscles to move with, so different forces are making them coil right-handed,” Weber explained. “But we didn’t know what’s making them do that.”
To figure out what was forcing the embryos into that shape, researchers examined a snake embryo with a CT scan. The images showed a section of intestine running through the center of its coiled body, surrounded by blood vessels extending from the yolk.
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A Fast-Growing Body and a Slower Gut
CT scan of a snake embryo showing the gut.
(Image Credit: Raul Diaz, California State University Los Angeles)
Snake embryos lengthen quickly during development, but their digestive tract initially grows more slowly. The gut cannot initially keep pace with the rest of the body. It separates from the lengthening body axis and acts as a tether.
As the body continues to grow around that shorter structure, it cannot extend in a straight line. Instead, it bends and twists into a spiral.
The position of the yolk helps determine which way that first twist goes. It sits on the left side of the embryo, directing the growing body toward the opposite side and producing the initial right-handed coil.
That rightward preference eventually disappears. As the embryo grows larger and uses up more of its yolk, its muscles mature enough for it to move. Some snakes then shift into a left-handed coil. By the time the embryos are close to hatching, the researchers found roughly equal numbers curled in each direction.
How Snakes Fit Their Long Bodies Inside an Egg
The spiral may help explain how snakes manage a difficult part of development. Their bodies grow extraordinarily long while they are still confined inside an egg.
Coiling allows the growing body to fit within that limited space. The researchers think this interaction between the lengthening body, the slower-growing gut, and the yolk may have helped make the snake’s elongated body possible.
Research into snake evolution has often focused on the genes behind their elongated bodies and numerous vertebrae. The new findings show that growth itself also plays a role, with different parts of the embryo influencing the shape the body takes.
“We are now opening the possibility to develop this model further to explain other spiral forms in nature,” Miyashita said.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in Current Biology: How snake embryos coil
