An international team of researchers, led by scientists in Canada, has finally cracked the mystery of why snake embryos coil into tight spirals before hatching. The answer, published in Current Biology, is not some existential dread about the length of their bodies - though that's close. It turns out the coiling is a physical necessity for accommodating their exceptionally long bodies, which set them apart from every other vertebrate.
The spiral forms as the embryo's body rapidly lengthens while its gut acts as a tether, causing 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 Dr. Tetsuto Miyashita, evolutionary biologist at the Canadian Museum of Nature and senior author of the study.
The discovery adds snake embryos to the growing list of natural spiral structures that continue to baffle scientists. "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.
Miyashita adds: "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 was born out of a rather mundane circumstance: the COVID lockdown in 2020. Miyashita, working from home, needed a research question his students could tackle without labs 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. Miyashita asked Weber, then at Carleton University, along with two undergraduate students at the University of Ottawa, to scour 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 eggs were laid, embryos coiled only dextrally - 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."
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, revealing something unexpected: "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 sought. Snake embryos must lengthen rapidly, but the gut doesn't grow at the same rate. The mismatch creates a mechanical constraint that forces 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."
The embryos don't stay locked in this right-handed arrangement forever. As they grow, the yolk shrinks, giving them more room to shift, and their muscles mature, allowing them to move 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 coiling direction is imposed by developmental anatomy and physical forces, not deliberate muscular movement.
For Miyashita, the discovery also illustrates how a simple observational question can lead to deeper 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."
The researchers believe the same model could eventually help investigate other spiral-shaped structures 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 includes 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.