Snakes in an egg all coil in the same direction. Scientists think they’ve learned why.

A new study suggests this twist arises from how the rapidly growing spine develops around the gut, offering fresh insight into one of nature’s most unusual body plans.

Long before they slither into your nightmares, baby snakes perform a curious feat: early in the egg, they begin to coil, forming a tight spiral that is almost always clockwise.

Researchers led by Dr. Tetsuto Miyashita of the Canadian Museum of Nature discovered the directional phenomenon after examining images of more than 800 embryos. 
To find out why, they utilized a CT scanner and made an interesting observation: a detached pillar of gut stretching through the spiral of the coiling embryo’s body.

Twist and sprout

Turns out, snake embryos likely coil due to a developmental mismatch.

The snake’s spine grows so quickly its gut can’t keep up. The rapidly lengthening body effectively buckles around that shorter digestive tract, forcing the embryo into a tight spiral. Because the yolk sits on the embryo’s left side, the growing snake almost always coils away from it, producing a clockwise twist, the authors say.

“It’s a surprisingly simple solution to a surprisingly difficult problem,” said Alexandra Weber, a UBC zoology master’s student and lead author of the study published in Current Biology.

Nature’s packing hack

The finding helps explain how snakes build one of the most extreme body plans in the animal kingdom.

A developing snake must squeeze hundreds of future vertebrae into the egg’s tight confines. Coiling is not just a consequence of becoming long, the researchers argue. It’s one of the tricks that makes becoming long possible in the first place.

As muscles develop and the gut catches up in length, embryos can move and reposition themselves. Coiling directions become more variable.

COVID collaborations

The team investigated the phenomenon during the 2020 COVID lockdown, engineering a research topic that could be conducted without travel. They trawled through literature and reached out to colleagues. UBC dentistry professor Dr. Joy Richman was a big help, providing images of snake embryos she had been studying and carefully photographed to investigate tooth replacement and skull development.

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This research was funded by the Canadian Museum of Nature and Natural Sciences and Engineering Research Council.