Why Snake Embryos Curl Into Spiral Shapes Inside the Egg

An international team led by scientists at the Canadian Museum of Nature has found a likely explanation for why snake embryos curl into tight spirals before hatching — a shape that appears to help fit their exceptionally long bodies, which set snakes apart from other vertebrates. Findings published in Current Biology suggest the spiral forms as the embryo's body rapidly lengthens while its gut acts as a tether, forcing the body to buckle and twist into a right-handed coil.

Senior author Dr. Tetsuto Miyashita asked lead author Alexandra Weber — then at Carleton University, now at the University of British Columbia — and two undergraduates at the University of Ottawa to gather photographs of more than 900 embryos from 39 snake and other limbless squamate species, drawn from published research and museum records. A clear pattern emerged: in the first weeks after eggs were laid, before the embryos had muscles to move, they coiled only to the right, viewed head to tail.

A key clue came from collaborator Dr. Raul Diaz at California State University Los Angeles, whose CT scans revealed a structure never seen before: a pillar of gut tissue stretching through the coiling body, detached from the rest of the animal and surrounded by blood vessels from the yolk. Since the yolk always sits to the embryo's left, this tethering effect directs the coiling to the right.

The embryos do not stay locked into this shape. As development continues, the yolk shrinks, giving embryos more room to shift, and their muscles mature enough to move on their own. As a result, roughly half of near-hatching embryos remain right-handed while the rest recoil into left-handed coils.

The project began during the 2020 COVID lockdown, when Miyashita, working from home, sought a research question his students could pursue without lab or museum access, drawing on an interest in asymmetry in animal forms inherited from his PhD advisor.