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| Sperm swim in teams when fluid is gloopy | Sperm swim in teams when fluid is gloopy |
| (about 20 hours later) | |
| Physicists studying the motion of sperm have found that the little swimmers flock together in tight groups if the surrounding fluid is "viscoelastic". | 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. | 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. | 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. | 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. | 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. | 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. | 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." | "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. | 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. |
| "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." | "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. | 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. | "We are trying to use the liquid-gas phase coexistence to understand this process," Dr Tung said. |
| Swimmers not thinkers | 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. | 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. | "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. | 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." | "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. | 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. | 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. | "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". | For now, however, his research is concentrated on "the fundamental science of the problem". |
| 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. | 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. | "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." | "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. | Sperm, on the other hand, are relatively uniform - and brainless. |
| "It's a better system for us to understand the underlying physics." | "It's a better system for us to understand the underlying physics." |
| Follow Jonathan on Twitter | Follow Jonathan on Twitter |