Scientists have discovered why snake embryos twist into tight spirals before hatching, a behavior that helps the animals accommodate their long bodies. An international research team led by scientists in Canada found that the spiral forms as the embryo’s body rapidly lengthens while its gut acts as a tether. That physical constraint causes the growing body to buckle and twist into a right-handed coil.
“It’s like when you adjust the length of a strap and the longer, buckling side of the loop twists,” explains senior author and team leader Dr. Tetsuto Miyashita, evolutionary biologist at the Canadian Museum of Nature.
The project grew out of an unusual circumstance. During the COVID lockdown in 2020, Miyashita was working from home and looking for a research question his students could investigate without access to laboratories or museum collections. Then the lightbulb turned on. He had inherited from his PhD advisor this fascination with asymmetries in animal forms. So every time he saw images of snake embryos in papers, he wondered whether they are right- or left-handed in their coiling.
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That question became the foundation of the study. Miyashita asked Weber, who was then at Carleton University, along with two undergraduate students at the University of Ottawa, to search published research and museum databases for photographs of developing snakes. They obtained pictures for more than 900 embryos from 39 snake and other limbless squamate species. That’s a statistically robust sample.
A clear pattern emerged. During the first several weeks after the eggs were laid, the embryos appeared to coil only dextrally, meaning to the right, as viewed from head to tail. At these stages, the embryos don’t have muscles to move with, so different forces are making them coil right-handed. But they didn’t know what’s making them do that. Because the embryos were not yet capable of actively moving their bodies into position, the researchers suspected that some physical feature of their development was creating the twist.
A key clue came from Dr. Raul Diaz, a collaborator at California State University Los Angeles. Diaz used CT imaging to examine snake embryos in greater anatomical detail. The scans revealed an unexpected arrangement inside the developing animal.
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“Raul’s CT scan of a snake embryo revealed a structure we had never seen before—it was a pillar of gut stretching through the spiral of the coiling body,” Miyashita says. “There’s an intestine detached from the rest of the body, surrounded by tendrils of blood vessels from the yolk.”
That observation gave the team the mechanism they had been searching for. Snake embryos must lengthen rapidly to produce their unusually elongated bodies, but the gut does not grow at the same rate. The mismatch in growth creates a mechanical constraint that helps force the body into a spiral. So they detach the slow-growing gut, which is now tethering the lengthening body. The body buckles and twists into coiling. This coiling force is directed so the embryos grow to the opposite side of the yolk. And the yolk is always to the left side of the embryo, hence the embryo will always start coiling right-handed.
Why the direction changes later
The embryos do not remain locked into this right-handed arrangement throughout development. As they grow, the yolk becomes smaller, and the embryos gain more room to shift position. Their muscles also mature, allowing them to begin moving on their own. Some remain in right-handed coils, but some recoil to the left side. So half of these near-hatching embryos are right-handed and the other half left-handed.
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The results suggest that the earliest direction of coiling is imposed by developmental anatomy and physical forces rather than by deliberate muscular movement. For Miyashita, the discovery also illustrates how a simple observational question can lead to a broader biological insight. Scientists have long been fascinated with how and why snakes evolved their strange body form. To answer that question, they tended to take a deep dive into sophisticated genetic research, looking at Hox genes, enhancers, and so on. These are key discoveries. But here, out of the COVID lockdown, they uncovered a snake’s secret with a startlingly simple approach—just scroll through an album of snake embryos and record which way they are coiled, and take a good look at their anatomy.
The researchers believe the same general model could eventually help scientists investigate other spiral-shaped structures found in living organisms. They are now opening the possibility to develop this model further to explain other spiral forms in nature.
