An international research team led by scientists in Canada has uncovered a likely explanation for one of the more unusual features of snake development: why embryos curl into tight spirals before hatching. This coiling behavior appears to help snakes accommodate the exceptionally long bodies that distinguish them from every other vertebrate.
The findings, published in Current Biology, suggest 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.
Why Spirals Form in Nature
The discovery adds snake embryos to a long list of naturally occurring spiral structures that scientists are still working to understand.
“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,” says lead author Alexandra Weber, now a graduate student in zoology at the University of British Columbia.
Adds Miyashita: “These puzzles beckon our curiosity. After all, spiral forms in nature have inspired human creations ranging from rotini pasta, to a barber’s pole or even portrayals of the biblical ‘Tower of Babel’.”
The project itself 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. I had inherited from my PhD advisor this fascination with asymmetries in animal forms. So every time I saw images of snake embryos in papers, I wondered whether they are right- or left-handed in their coiling.”
That question became the foundation of the study.
More Than 900 Snake Embryos Examined
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.
“We 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,” Weber explains. “But we 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.
CT Scans Reveal a Hidden Gut Structure
A crucial 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.
“Raul’s CT scan of a snake embryo revealed a structure we had never seen before ¾ 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,” Miyashita explains. “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 Coiling Direction Later Changes
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,” Weber says. “So half of these near-hatching embryos are right-handed and the other half left-handed.”
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,” he says. “These are key discoveries. But here, out of the COVID lockdown, we 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.”
A New Model for Spirals in Biology
The researchers believe the same general model could eventually help scientists investigate other spiral-shaped structures found in living organisms.
“We are now opening the possibility to develop this model further to explain other spiral forms in nature,” he adds.
Weber agrees: “This all started out with a curiosity to see if snakes are ‘handed’. It was exciting to follow it to deep insights about their evolution.”
The study team consists of scientists, students, and professors from the Canadian Museum of Nature, the University of British Columbia, Carleton University, the University of Ottawa, California State University Los Angeles, and the University of Helsinki.
