An international team of researchers has discovered that snake embryos form right-handed spirals during the first few weeks of development because a slow-growing gut detaches from the yolk and tethers the lengthening body. Published in Current Biology, the findings explain a mystery of animal asymmetry.
Snakes emerge from their shells boasting the longest bodies of any vertebrate animal. That remarkable evolutionary achievement relies on limber gestational contortions, with embryos developing inside a tight spiral. For years, biologists knew that these early-stage coils always formed dextrally, or towards the right, but the physical mechanism behind the pattern remained a persistent puzzle. There is a touch of mystery to spirals,
explained University of British Columbia zoologist Alexandra Weber. 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.
An international team of scientists set out to investigate the phenomenon during the COVID-19 pandemic lockdowns in 2020. Tetsuto Miyashita, an evolutionary biologist and study co-author at the Canadian Museum of Nature in Ottawa, was working from his home office trying to dream up remote research projects for his students. Recalling his doctoral advisor’s deep fascination with asymmetries in animal forms, Miyashita began wondering about the handedness of snake embryos depicted in scientific literature. Every time I saw images of snake embryos in papers, I wondered whether they are right- or left-handed in their coiling,
said Miyashita.
Museum Collections and a Statistically Robust Sample
To investigate the pattern, Miyashita instructed Alexandra Weber—then at Carleton University and now a graduate student in zoology at the University of British Columbia—along with two undergraduate students at the University of Ottawa, to scour literature and museum databases for photographic evidence. The team amassed pictures for more than 900 embryos representing 39 snake and other limbless squamate species, creating what researchers described as a statistically robust sample.
A meticulous review of the images revealed a striking biological pattern. Across all 39 species, the reptiles began their right-handed coiling within the first few weeks of embryonic development, moving from head to tail. Yet this consistent coiling occurred long before the embryos developed functional muscle tissue. At these stages, the embryos don’t have muscles to move with, so different forces are making them coil right-handed, Weber explained, noting that the underlying driver was initially unknown. Different forces are making them coil…but we didn’t know what’s making them do that,
said Weber.
CT Scans Reveal a Hidden Pillar of Gut
To solve the mechanical mystery, collaborator Dr. Raul Diaz of California State University Los Angeles performed CT scans on snake embryos. The imaging exposed a previously undocumented anatomical structure: an intestine located outside the embryo’s body, enveloped by tendrils of blood vessels stretching outward from the yolk. 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.
The team realized that rapid body growth creates a mechanical conflict when the internal organs cannot keep pace. So they detach the slow-growing gut, which is now tethering the lengthening body, Miyashita explained. The body buckles and twists into coiling. Because the yolks reside on the left side of the developing snake embryo, the resulting coiling force is directed opposite the yolk, forcing the embryo to start its spiral toward the right.
From Rigid Spirals to Flexible Wriggling
Miyashita likened the gestational dynamics to adjusting a strap’s length so that the longer, buckling side of the loop twists. As the embryos continue to grow and the yolk diminishes in size, the physical constraints shift. Their muscles mature, granting them the ability to wiggle. 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.
By the time the reptiles approach hatching, about half of the spiraled bodies have recoiled back to the left side, leaving a roughly even split of right-handed and left-handed near-hatching embryos. While many evolutionary biologists historically focused on sophisticated genetic research involving Hox genes and enhancers to explain snake anatomy, the team emphasized that simple observation and anatomy imaging unlocked a fundamental mechanical truth. Out of the COVID lockdown, we uncovered a snake’s secret with a startlingly simple approach,
Miyashita said. Just scroll through an album of snake embryos and record which way they are coiled, and take a good look at their anatomy.
