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upstart writes in with an IRC submission for nutherguy:

Human sperm swim more like otters than eels, study finds:

For more than 300 years, most scientists have assumed that sperm "swim" through fluids by wriggling their tails back and forth like eels to propel themselves forward. But according to a new paper in Science Advances, this is actually an optical illusion—the result of viewing the creatures from above with 2D microscopes. New observations with 3D microscopy have revealed that human sperm actually roll as they swim, like otters, essentially corkscrewing themselves forward.

"With over half of infertility caused by male factors, understanding the human sperm tail is fundamental to developing future diagnostic tools to identify unhealthy sperm," said co-author Hermes Gadelha from the University of Bristol.

[...] Back in 1977, physicist Edward Purcell did some calculations that showed how animals of different sizes would swim at different Reynolds numbers. The number would be very high for a whale, for instance, which is able to coast a good distance with a single flap of its tail. According to Purcell's calculations, however, bacteria swim at low Reynolds numbers, so they can barely coast any distance at all if you push them to set them in motion. It's akin to a human trying to swim in molasses and moving their arms at slow speeds on par with the movement of a clock's hands. So eels and sperm (or bacteria) would adopt very different swimming strategies by necessity because they are dealing with different Reynolds numbers.

[...] One such strategy might be to break the symmetry of the stroke to create more drag on the power stroke than on the recovery stroke. A creature could do this by changing the shape of the "paddle"—for example, the cilia that cells use to propel themselves forward. Bacteria and sperm, in contrast, have helical tails they can use as a corkscrew-like propeller. This latest study sheds some intriguing light on precisely how this works for human sperm. (Of course, as Bhatia pointed out, "Don't expect to see human swimmers doing 'the corkscrew' anytime soon. This strategy works only at low Reynolds number, where water 'feels' as thick as cork, so you can push against it effectively.")

Fertility clinics today still rely on 2D views when examining sperm movement, so this new work provides a better understanding of how the sperm tail moves, which could in turn lead to better diagnostic tools. "This discovery will revolutionize our understanding of sperm motility and its impact on natural fertilization," said co-author Alberto Darszon from the Universidad Nacional Autonoma de Mexico, who pioneered the 3D microscopy technique with his colleague and co-author Gabriel Corkidi. "So little is known about the intricate environment inside the female reproductive tract and how sperm swimming impinge on fertilization. These new tools open our eyes to the amazing capabilities sperm have."

Journal Reference:
Osborne Reynolds. An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous, and of the law of resistance in parallel channels, Philosophical Transactions of the Royal Society of London (DOI: https://royalsocietypublishing.org/doi/10.1098/rstl.1883.0029)

E. M. Purcell. Life at low Reynolds number, American Journal of Physics (DOI: 10.1119/1.10903)

Hermes Gadêlha, Paul Hernández-Herrera, Fernando Montoya, et al. Human sperm uses asymmetric and anisotropic flagellar controls to regulate swimming symmetry and cell steering [open], Science Advances (DOI: 10.1126/sciadv.aba5168)

Original Submission