Rodents' "brother sperm" recognize one another and form "trains." Dozens of cells latch onto one another via small hooks on their heads, forming a DNA-carrying conga line that swims 50% faster than any lone-wolf mouse sperm could on its own if you'll allow us the pleasure of mixing animal analogies.
Sperm-trains are especially effective for promiscuous species, like deer mice. By teaming up, sperm try to out-compete their rivals in a microscopic race that would be infinitely more captivating than either NASCAR or cycling if only someone could secure licensing rights. The sperm recognize themselves even when pitted against sperm from a sibling mouse, showing a high level of discrimination among the sperm and a low level of discrimination among the mice.
Oddly enough, or maybe not at all considering convergent evolution, diving beetles use a similar insemination method. Scientists knew bugs had insane sex organs. But the reproductive tract of diving beetles wasn't really studied until about 10 years ago because looking at beetle vaginas all day is a tough ask.
It turns out that diving beetles and TLC share a “No Scrubs” policy. And the former's reproductive tracts have evolved into tiny labyrinths to ensure that only the most potent sperm-givers knock her up. This female-driven sexual evolution kicked off a procreational arms race, with beetle sperm becoming more cunning in response. So the sperm cells latch onto each other to aid in navigation. And a single cell becomes a gigantic, writhing, worm-like aggregation of hundreds or thousands of members, with their haphazardly thrashing tails slowly pushing the jumbled mass toward the egg.
The lady beetles' excessively tortuous tracts may also be advantageous for another reason. Females sometimes have little choice in picking their mates, who may hold them underwater, pressuring them into mating by the threat of drowning.