When it comes to sperm, only the fiercest swimmers can wriggle past the bottlenecks of the female reproductive tract, new research shows. The study could help one day improve tests for male fertility. But, in the meantime, it brings us delightful footage of sperm desperately, fruitlessly swimming into a fake fallopian tube. Sorry, little guys.

With a single ejaculate, 100 million sperm can swim up the female reproductive tract — and the theory goes that only solid swimmers can make it to the finish line: the egg. But for years, laboratory sperm have only had to endure simple obstacle courses. That isn’t actually a great analog for the twists and turns of the female reproductive tract. There are bottlenecks, for example, that change how genital mucus flows, which in turn can change how sperm swim and steer upstream. In a new study, published today in the journal Science Advances, scientists decided to really throw down the gauntlet.

Study author Alireza Abbaspourrad and his team at Cornell University developed a device to better mimic the real thing, and watched how both bull and human sperm swam up it. They discovered that sperm congregate beneath the opening of the bottleneck with the fastest swimmers at the tip of the pack, and the slower sperm farther away. That means the fastest wrigglers are mainly competing with each other, and not the slower sperm, for a clear shot to the egg.

The findings could eventually help with designing new and improved screens for speedier sperm, Abbaspourrad tells The Verge. But that will take more research in the clinic to confirm if these speedy sperm actually create better embryos. For now, it’s an interesting new way to watch how sperm wriggle up a faux junction between the uterus and fallopian tube.

John Amory, a professor and fertility expert at the University of Washington who was not involved in the study, calls the device “a fallopian tube on a chip.” For him, the findings weigh in on a key question about the sperm that ultimately fertilizes an egg: “Is it random, or does it have to be the best sperm?” The results back up what fertility doctors have long thought: “Clinically, we’ve always suspected that it’s the highly motile sperm that are responsible for fertilization, and this paper suggests that’s the correct way of looking at things,” he says. “It’s a very cool little model.”