Why snake embryos coil right before some later curl left

Snake embryos begin life with a distinctive twist: during their early development, they coil from head to tail in a right-handed spiral. Researchers now say they have identified the physical feature that helps produce the unusual shape. The finding comes from an international team led by scientists at the Canadian Museum of Nature. Their study, published in Current Biology, examined images of more than 900 embryos representing 39 species of snakes and other limbless squamates. The researchers found that embryos initially form right-handed coils even though they do not yet have developed muscles capable of moving them. CT scans provided by collaborator Raul Diaz of California State University Los Angeles revealed what the team describes as a previously unseen structure: a pillar of gut running through the spiral formed by the body. The intestine is temporarily separated from the main body axis and surrounded by blood vessels connected to the yolk. According to the researchers, the snake’s body grows rapidly as it lengthens, while the gut grows more slowly. The gut therefore acts like a tether, causing the expanding body to buckle and twist. Because the yolk is positioned on the embryo’s left side, the resulting force initially directs the body into a right-handed coil. The embryos do not necessarily keep that orientation. As they grow larger and the yolk becomes smaller, they reposition themselves. At the same time, their muscles mature and allow them to move. The study team reported that some embryos remain coiled to the right, while others recoil toward the left; among embryos close to hatching, the researchers observed roughly equal numbers of right- and left-handed coils. The project began during the COVID-19 lockdown in 2020, when lead researcher Tetsuto Miyashita sought a study that students could conduct without access to laboratories or museum facilities. Students collected embryo images from scientific literature and museum databases, allowing the team to compare coiling direction across a broad sample. The researchers say the work offers a relatively simple physical model for studying how spiral forms develop in animals. It also provides new information about the early development of snakes, whose bodies become exceptionally long while still developing inside eggs.