Media playback is unsupported on your device Media caption Different fluids changed the way the sperm behaved

Physicists studying the motion of sperm have found that the little swimmers flock together in tight groups if the surrounding fluid is "viscoelastic".

Such fluids - including the mucus which sperm have to negotiate - are not only thick but have some bounce and stretch; they spring back when disturbed.

Watching bull sperm swim in different fluids, the scientists found that this gloopiness caused them to bunch up.

If the liquid was thick or thin but not elastic, they tended to swim solo.

The team presented their preliminary findings on Thursday at the March Meeting of the American Physical Society in Baltimore.

Viscoelastic fluids - like nasal mucus, silly putty or melted mozzarella - are common in biology.

And they include the mucus found in the reproductive tract, explained Dr Chih-kuan Tung from North Carolina A&T State University.

"What we are talking about is not just a physical curiosity; this is a real environment that they have to overcome."

Despite their fierce competition to reach and fertilise eggs, some degree of cooperation between swimming sperm is known to take place in several species - including humans. Certain rodent sperm even have hook-shaped heads which can help them join together into sperm "trains".

But Dr Tung and his colleagues, including Susan Suarez and Mingming Wu at Cornell University, are interested in what drives free-swimming sperm to flock together - or fly apart.

Image copyright Science Photo Library Image caption Bull sperm - like those of humans - have simple paddle-shaped heads

"If you look carefully, you can see that it's a very dynamic process," he explained. "There will be new cells joining in to the group, and there will be cells leaving the group at the same time."

His team is even comparing the statistics of this packing and dispersing with the way molecules behave at the interface of a liquid and a gas - with the clumped sperm corresponding to molecules sliding around each other in a liquid, while the free swimmers are more like molecules of gas.

"We are trying to use the liquid-gas phase coexistence to understand this process," Dr Tung said.

Swimmers not thinkers

By placing samples of bull sperm in different fluids, with varying amounts of an elastic polymer stirred in, the researchers discovered that this consistency is important to the way the single-cell swimmers behave.

"With a higher concentration of polymer, it's more viscoelastic, and you see much larger groups," said Dr Tung.

Interestingly, if they added a polymer that made the fluid thicker - more viscous - but not more elastic, the same tight, coordinated clumps did not form.

"Occasionally they might collide and you briefly see a group, but that dissolves pretty fast."

The team does not yet know why viscoelasticity is important for the sperm; it may relate to how the fluid moves around their beating tails, which appear to be largely - but not entirely - synchronised within the groups. They are studying those flow patterns with high-speed cameras to learn more.

And if stretchy, springy fluid makes a really big difference to their movement, Dr Tung suggested, it might be worth considering in IVF procedures.

"Right now... they just mix sperm and egg in a tube and hope they meet each other," he said - adding that one proposal for improving IVF outcomes has been to give the sperm an obstacle course more like what they face naturally.

For now, however, his research is concentrated on "the fundamental science of the problem".

Image copyright Science Photo Library Image caption The team is interested in using sperm to study collective movement more generally

Thinking more broadly, Dr Tung and his colleagues believe they have found a valuable tool for studying collective behaviour - principally because the head-plus-wiggling-tail structure of a sperm is marvellously simple.

"When you think about collective dynamics in biology, the more obvious choice is probably to look at animals," Dr Tung said. But a flock of birds or a school of fish is made up of complicated, variable critters.

"They look around, they think about it, then they move. That thinking is hard for physicists to model."

Sperm, on the other hand, are relatively uniform - and brainless.

"It's a better system for us to understand the underlying physics."

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