Sometimes you have to go it alone, but when the going gets tough, you should probably stick together. That’s certainly what sperm seem to do, at least; when swimming in particularly thick, elastic liquids, they bunch up into little squadrons, but they make a break for it on their own if the fluid becomes less elastic.
Viscoelastic fluids, such as melted mozzarella cheese and silly putty, are both stretchable and gloopy. Within humans, cervical mucus – another viscoelastic fluid – is the first thing that sperm swim through after being “deposited” within the vagina.
Working out how they cooperate with or compete against each other is key to understanding the intricate workings of reproduction. To this end, a team of physicists placed different samples of bull sperm – not dissimilar in shape and behavior to those of humans – into a variety of viscoelastic fluids to examine how they behaved.
As they explained during the recent March Meeting of the American Physical Society, they added an elastic polymer to various fluids in order to control how viscoelastic they were. And the sperm tended to stick to themselves at low viscoelasticities, but herded together at high viscoelasticities.
The viscosity or “gloopiness” of the fluid alone did not seem to affect their behavior, although a few crowds temporarily formed in clumsy collisions. “Occasionally they might collide and you briefly see a group, but that dissolves pretty fast,” Dr. Chih-kuan Tung, an assistant professor of experimental physics at North Carolina’s A&T State University and the study’s lead author, told BBC News.
Sperm swimming as a group in a viscoelastic fluid. Tung et al./A&T State University
The team, at present, do not know why the fluid has to be both elastic and viscous in order for sperm to join forces. It could be related to how the fluid flows around their tails, and that swimming conditions might be more optimal in viscoelastic fluids when they’re swimming together.
The sperm didn't quite act like synchronized swimmers, though. “We observed sperm swimming collectively instead of synchronized,” Tung told IFLScience. In any case, if viscoelastic fluid is important to sperm in some way, it should be taken into account during in vitro fertilization procedures, Tung noted.
It may seem strange that physicists, not biologists, are studying the motions of sperm, but this all falls under the realm of fluid dynamics – the behavior and characteristics of various fluids. Physicists have already unraveled the secrets behind sneezes and phlegm – yet another viscoelastic fluid – so observing the migration of sperm within various fluids isn’t actually that unusual.
Sperm swimming in a non-elastic fluid. Tung et al./A&T State University
In fact, the team even compared the way that these sperm gather together and disperse again with the way molecules behave on the boundary between a gas and a liquid. The clumped-up sperm are similar to molecules sliding and slipping around each other in a liquid, whereas those making a solo run are akin to gaseous molecules.
So there you have it: like birds of a feather, sperm flock together.