Snake embryos develop in tight, clockwise spirals long before they have muscles to move, driven by a mismatch in growth rates between the spine and a detached gut structure known as a visceral pillar, according to a study published in the journal Current Biology.
How COVID-19 Lockdowns Sparked a Serendipitous Discovery
Most recently, biologists sought to answer how baby snakes grow such long bodies while cooped up inside an egg. While researchers have long known that snake embryos spiral as they develop and typically exhibit a right-handed preference, the exact mechanical cause remained a mystery. The investigation kicked off during the COVID-19 pandemic lockdowns of 2020, when senior author and team leader Dr. Tetsuto Miyashita, an evolutionary biologist at the Canadian Museum of Nature in Ottawa, sought a remote research project for students stuck indoors.
Recalling his PhD advisor’s deep interest in evolutionary asymmetries among reptiles, Miyashita wondered whether snake embryos coiled in distinctly right- or left-handed patterns. To find out, the team requested images from museums, published papers, and lab colonies across North America and Europe, amassing a database covering over 900 embryos across 39 species of snakes and other limbless reptiles.
The Visceral Pillar: A Previously Unknown Anatomical Structure
Meticulous review of the collected photographs revealed a striking biological pattern. In the African house snake, 146 early embryos coiled clockwise and none turned the other way, while corn snakes came close with only nine counterclockwise embryos among more than 200 scored early specimens. This early stage covers roughly the first two weeks after laying, before trunk muscles have developed enough for the embryo to move.
To uncover the physical forces driving the phenomenon, Raul Diaz of California State University Los Angeles put embryos through CT scanners. The scans revealed a previously unidentified biological structure: an intestine located outside the embryo’s body but enveloped by blood vessels stretching from the yolk.
According to Dr. Tetsuto Miyashita of the Canadian Museum of Nature, this pillar of gut slowly detaches from the yolk while tethering the elongating body.
The digestive tract grows on its own schedule, running under 20 percent of the body’s length in specimens examined six to seven days after laying. Anchored at the stomach in front and near the tail behind, this detached section of gut and vitelline vessels hangs from a thin sheet of tissue called mesentery, running straight up the middle of the spiral as a column.
Yolk Position and the Mechanics of Clockwise Buckling
The coiling action stems directly from the mismatch between the growth rates of different body parts. Because the gut does not grow as fast as the spine early on, it acts like a tether, causing the elongating body to buckle and twist into a spiral.
Alexandra Weber, a graduate student in zoology at the University of British Columbia who started the project at Carleton University, noted the consistency of the pattern. When you see a trait that consistently, you can’t help but be excited and curious about the possible mechanisms behind it,
Weber told Earth.com.
The direction of the buckle is determined by the position of the yolk. In snakes and their relatives, the yolk mass is fixed to the left side of the embryo. As the embryo grows and sinks into that mass, the head slumps until the left side of the face presses against it, forcing the trunk to coil away to the right.
Maturation, Muscle Development, and Late-Stage Recoiling
As the snake embryo continues to mature, its internal anatomy undergoes dramatic catch-up growth. The gut triples in length between roughly six and twelve days after laying, eventually growing long enough for the coiling body to pull it back via the mesentery and incorporate it into its proper anatomical position under the axial column by 18 or 19 days.
Muscle development follows a similar schedule. By 11 to 12 days, fibers form along the trunk and the backbone takes shape, granting the embryo the ability to move. With the yolk shrinking and muscle control established, late-stage embryos demonstrate a much more even split.
Among late-stage snake embryos, 200 coiled clockwise and 160 counterclockwise, a distribution even enough that researchers treat late direction as random. While many evolutionary biologists previously focused on detailed genetic analysis of Hox genes to explain snake anatomies, this observational approach highlights how basic morphological development can create complex structural forms.
