How did sex pop up? a study of sperm swimming (Introduction)

by David Turell , Tuesday, October 11, 2022, 17:40 (773 days ago) @ David Turell

Against a mucus tide:

https://www.sciencenews.org/article/sperm-swimming-groups-effectiveness-fertility

"Bull sperm swim more effectively when in clusters, a new study shows, potentially offering insight into fertility in humans. In simulated reproductive tracts of animals like cattle and humans, the behavior increases the chances that groups of cooperative bovine sperm will outpace meandering loners as they race to fertilize a female egg cell, physicist Chih-kuan Tung and colleagues report September 22 in Frontiers in Cell and Developmental Biology.

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"On their own, sperm tend to follow curved paths — which is a problem, because the shortest distance between two points is a straight line. But when sperm gather in groups of two or more, they swim along straighter routes. It’s behavior that a couple of the same researchers noted in a previous study where they tracked sperm swimming in stationary fluids (SN: 3/17/16). Although that might give sperm clusters an advantage, it would only help if they happen to be going the right direction. Other benefits of sperm clustering weren’t clear until the researchers developed an experimental setup that introduced flowing fluid into their experiments.

"In creatures like humans and cattle, sperm make their way to the ovum by swimming against a current of mucus that streams through the cervix and away from the uterus. It’s difficult to study what benefits clustering might confer while swimming upstream inside living beings. So Tung and colleagues created an analog in their lab: a shallow, narrow, 4-centimeter-long channel filled with a thick fluid that mimics natural mucus and flows at rates the researchers could control.

"Whether alone or in groups, sperm naturally tend to swim upstream. However, clusters of sperm in the experiment did a better job heading upstream into the mucus flow, while individual sperm were more likely to head off in other directions. Despite the speedier travels of some individual sperm, a poorer ability to point upstream hampered the progress of sperm loners compared with slower moving clusters.

"Clusters also stayed the course in the face of rapidly flowing mucus. When the researchers turned up the flow in their apparatus, many individual sperm were washed away. Sperm clusters were much less likely to get swept downstream.

"While sperm in the study were bovine, the advantages of clustering should also apply to human sperm, Tung says. Sperm of both species have similar dimensions. The swimmers typically compete to fertilize a single ovum. And unlike pigs or other animals where semen is deposited directly in the uterus, both human and bovine sperm start out in the vagina and travel through the cervix to get to the uterus."

Comment: this shows how complex sexual reproduction actually is. The sperm are programmed to swim upstream. The mucus is produced to flow in an outward direction to protect against invasive bacteria. Why did evolution choose such a complex way of reproduction? If natural evolution seeks simple solutions and adaptations, this is not one. A designer can create this easily.