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.

But once the clusters get too big, their members start swimming against