Group Sperm Superiority Beats Loner in Mating Race

Research that group sperm beat loners in mating races are a solution to human infertility.

Group Sperm Superiority Beats Loner in Mating Race
Image of Sperm

In recent research, group swimming helps sperm stay in the reproductive tract. After ejaculation, sperm can swim in different directions. Sperm grouped in groups of two or more swims straighter and more directly than individual sperm on its way to fertilize a female's egg. In this case, even the sperm must stick together to win in the mating race.

A new study suggests sperm swim more effectively in groups, potentially providing better human fertility. In simulations of the reproductive tracts of animals such as cows and humans, the behavior increases the likelihood that groups of sperm swimming in groups will surpass individual sperm as they race to fertilize female eggs, as physicist Chih-Kuan Tung of North Carolina Agricultural and Technical University in Greensboro reported Sept. 22 in the journal Frontiers in Cell.

The benefits of group swimming put them ahead of speed. In terms of speed, they are comparable to or slower than sperm walking alone. Like a herd of turtles racing with individual rabbits, the winner is not necessarily the fastest but the one who can stay on track.

By itself, sperm tend to follow a curved path—which is a problem since the shortest distance between two points is a straight line. But when sperm gather in groups of two or more, they swim along a straighter route. This is a behavior noted by some of the same researchers in previous studies in which they tracked sperm swimming in a stationary fluid.

Although it may give the sperm group an advantage, it will only help if they are heading in the right direction. Other benefits of sperm grouping were not apparent until the researchers developed an experimental arrangement incorporating fluid flow into their experiments.

In living things such as humans and livestock, sperm head towards the egg by swimming against the current of mucus that flows through the cervix and away from the uterus. Learning what benefits grouping may provide when swimming upstream inside living beings is difficult.

Tung, in his research, made an analogy in their lab: a shallow, narrow, 4-centimeter-long channel filled with a viscous liquid that mimics natural mucus and flows at speed researchers can control.

Whether alone or in groups, sperm naturally tend to swim upstream. However, the sperm group in the experiment did a better job while heading upstream to the mucous stream, while individual sperm were more likely to run in the other direction. Although the passage of some individual sperm is faster, the poorer ability to lead upstream hinders the progress of sperm loners compared to slower-moving groups.

In a viscous-like liquid flowing from left to right, sperm groups move more consistently against the flow than individual sperm do themselves.

When the researchers raised the flow in their test kits, many of the individual's sperm were washed away. Sperm groups are much less likely to be swept downstream.

Swimming sperm usually compete to fertilize a single egg. And, unlike other animals where semen is stored directly in the uterus, both human and animal sperm begin to come out of the vagina and travel through the cervix to get to the uterus.

Studying sperm in a fluid similar to mucus flowing in the reproductive tract can uncover problems that do not arise in conventional observations of sperm swimming in a stationary fluid.

This knowledge can help researchers perform better diagnostics to provide clues to understanding infertility in humans and provide practical assistance for people with difficulty conceiving.

How sperm cells respond to their environment and how it can change their behavior are critical topics. This technology can be used or adapted to select better sperm quality for people who need fertility assistance.