In humans, it’s every sperm for itself: sperm cells race to reach an egg and the first one there gets to fertilise it. But in many other animals, sperm can clump together to form cooperative bundles that outswim any solo cells.
Take the deer mouse. The sperm of this common North American rodent have heads that are flattened paddles with small hooks, rather than the usual round teardrops. These heads can stick to each other, forming clusters of up to 35 sperm. Scientists have reasonably assumed that the sperm swim better as a team, but that’s not always the case. Sometimes, the groups are faster; sometimes, they barely move.
Heidi Fisher from Harvard University knows why. Her collaborators Luca Giomi and L Mahadevan created a mathematical model that simulated the swimming sperm. It showed that while groups don’t swim any faster, they do swim straighter because each cell cancels out the wobbling movements of its neighbours. Their speed stays the same but their velocity—their speed in a straight line—goes up. “The aggregate gets to the finish faster,” says Fisher.
But once the clusters get too big, their members start swimming against each another and their velocity falls. The optimal number is seven. A seven-strong sperm swim-team will get to an egg faster than either a smaller or a bigger group.
When Fisher then timed the sperm of actual mice under a microscope, she found that they behaved exactly as the model predicted: groups of seven really do have the highest velocity. And the mice are very good at meeting this ideal number. Fisher compared the sperm of two species—the deer mouse and the oldfield mouse—and found that in both, the average number of sperm per cluster is 6 or 7.
But Fisher also found one critical difference between the two species: the deer mice somehow keep tighter control over the size of their sperm clusters. Look at the graph below. The red diamonds represent the average size of a sperm cluster in different male deer mice; they range from 5 to 7. The blue diamonds represent the average cluster sizes in male oldfield mice; they range from 4 to 9.
Why? Because the two species, though closely related, have very different sex lives. The oldfield mouse is strictly monogamous: females only ever mate with one male at a time. The deer mouse is promiscuous: females mate with many males in quick succession, and often have the sperm of many suitors in their bodies. These cells must then compete to fertilise the female’s eggs.
So, relative to the easy-going oldfield mice, the deer mice are under intense evolutionary pressure to have more competitive sperm. That’s why their sperm are better at gathering in exactly the right numbers for straighter swimming.
They’re also better at recognising each other. In 2010, Fisher showed that the sperm of deer mice prefer to stick with sperm from the same species, and especially to sperm from the same male. Oldfield mouse sperm cells are less fussy; they’ll just stick to whatever’s nearby.
The researchers still don’t understand how the deer mouse keeps a tighter rein on the size of its sperm teams. “That’s the next step,” says Hopi Hoekstra, who led the study. They also don’t know how the cells stick together; contrary to what they used to think, the hooks are not involved. “How do they actually clump and recognise each other? We have no idea.”
Similar evolutionary pressures have shaped the sperm of other animals. In the desert ant, Cataglyphis savignyi, females also mate with many males, sperm cells also gather in groups (of 50 to 100 cells!), and the groups also swim faster than individual cells. In diving beetles, hundreds or even thousands of sperm can unite into long worm-like trains that navigate the females’ maze-like reproductive tracts—the more convoluted her anatomy, the more cooperative his sperm. The sperm of the promiscuous wood mouse also forms similarly spectacular trains.
In all these creatures, the sperm hook up after ejaculation, but opossum sperm come pre-paired. The cells form paired clusters within the male’s body and once they are shot into the female, their tails start beating in sync like a pair of frog’s legs. These clusters are far smaller than those of the diving beetle, desert ant, or mice, but they are perhaps the ultimate example of cooperative sperm. Alone, these cells swim in a futile circle. They can only create the next generation of opossums as a team.
Reference: Fisher, Giomi, Hoekstra & Mahadevan. 2014. The dynamics of sperm cooperation in a competitive environment. bioRxiv http://dx.doi.org/10.1101/006890
More on sperm competition:
- Indiscriminate squid just implanting everyone with sperm
- Hermaphrodite insects fertilise daughters with parasitic sperm
- Sperm war – the sperm of ants and bees do battle inside the queens
- Why do female seed beetles prefer the sperm of inferior males?
- The sexual battles of flatworms: barbed sperm, mating rings, traumatic insemination, and going down on yourself
- The rubbish sperm of the naked mole rat