Today on a Moment of Science, we're live at the scene of a competitive wood mouse mating. The male has just released millions of sperm that are now making their way toward the egg.
There's a lot at stake for these sperm. The female wood mouse has multiple mates, so the sperm of this wood mouse may be competing with sperm from other males. Time is of the essence! Some of these sperm are hooking onto each other using hooks on their heads or by grabbing the tail of the sperm in front of them. They appear to be forming trains of hundreds of thousands of sperm!
Groups of sperm travel twice as fast as individuals, so collaboration makes sense when timing is crucial. Still, this speed comes with a cost. As these sperm join together, a chemical reaction occurs that takes away their ability to fertilize the egg. Only the sperm at the head of the train remains viable.
While this is the first known instance of cooperation among sperm in which individual sperm sacrifice their reproductive potential to increase the chances of success for a "sibling" sperm, it's also been observed that the sperm of most South American marsupials swim in pairs that split up right before reaching the egg. Anything to ensure survival.