'Every sperm for itself' is a myth, new research finds

Sperm cells swimming in a pack may help them to push upstream through thick vaginal mucus, according to new research published on Frontiers on Sept 23.

The discovery may provide new ways to aid in diagnosing unexplained infertility through future research that focuses on the quality of female reproductive fluids and the degree to which sperm congregate.

A long-held view of 'singular' swimming sperm, debunked

Sperm have often been represented as individuals racing against each other to fertilize an egg. The problem with this stems from looking in the wrong place — i.e., under a microscope where sperm don't really do much.

Most sperm action happens once inside the female reproductive system — which, while a very challenging place to observe, is where sperm thrive. Because of this, several persistent misconceptions have emerged, such as the notion that sperm competition is a constant aspect of reproduction. Now, new research proves that sperm embrace teamwork.

Chih-kuan Tung, a physicist at North Carolina Agricultural and Technical State University, and colleagues noticed this clustering in their laboratory in a previous study they had completed in 2017.

From this study, the group knew that bull sperm, similar to human sperm, formed clusters, but also that those clusters couldn’t swim faster than individuals. Therefore, there was no apparent advantage as to why this clustering was happening.

In a statement to New Scientist, Tung explains, "in biology, when [cells and structures] do something, they should probably get something out of it...So that became the question we were asking ourselves: what are these sperm getting out of it?"

100 million fresh bull sperm, a silicone tube, and 'melted cheese'-like fluid

To answer the enigma, the researchers put 100 million fresh bull sperm into a silicone tube filled with fluid that mimicked the cervical and uterine mucous of cows. According to Tung, this fluid had the consistency of melted cheese. The scientists then used a syringe pump to create two speeds of flow.

The scientists discovered that the clustered sperm swam in a straighter line than the individual sperm when there was no flow.

Additionally, individual sperm could not move upstream in an intermediate flow, but the clusters could. When the flow was high, the clustered sperm were much more effective at pushing through the approaching current than individual sperm, which typically got carried away by the heavy stream.

A 'no leader' mechanism like cyclists riding together on a peloton so they can encounter forces more efficiently

The team found that there was never a single 'leader' sperm merely backed by a cluster of 'other sperms' in any of these scenarios. Rather, the groups were highly dynamic, with sperm frequently entering and exiting their cluster and moving around within them.

In a statement, Tung describes this as an arrangement similar to how cyclists ride on a peloton. Like teamwork helps to encounter less air resistance, sperm clustering may indicate that protective dynamics are at play.

“In the longer term, our understanding may provide better selection of sperm used for intervention such as in vitro fertilization or other assisted-reproduction technologies,” explains Tung in a statement.

“This may be needed as [these methods] typically skip some or all of the selection mechanisms present in the female tract and yield less favorable results,” the scientists adds.