If the ocean were an Olympic pool, swordfish would currently be in danger of disqualification. Researchers, who published their findings in the Journal of Experimental Biologysay a large gland in the swordfish’s skull produces lubricating oil that may boost the fish’s already enviable speed.

Able to reach speeds of more than 60 miles per hour, swordfish may be the fastest swimmers on the planet. One reason for that is readily apparent: their impressive, sea-piercing, sword-like bills. But while they may seem fearsome, these toothless fish have their soft spots. Last summer, researchers looking at swordfish skulls found a surprisingly weak area at the base of the fish’s rigid bill.

This structural oddity intrigued John Videler, a bionics expert at Groningen University in the Netherlands. Twenty years ago, fishermen friends of Videler had given him two freshly caught swordfish. Another person might have eaten the fish or had them mounted by a taxidermist. Videler raced to get the fish into an MRI machine in the middle of the night, cleaning up and spraying copious amounts of air freshener to ready the machine for the next morning’s patients. He also dissected the two specimens, along with the heads of two more swordfish.  

Reading about the swordfish skulls last summer, Videler decided to take a second look at his original MRI scans. Almost immediately, he discovered the reason for the swordfish’s apparent Achilles’ heel: a sizeable gland, right at the base of the fish’s sword. “It was so big there was hardly any room for bony structure,” he said in a press release, “and the bone around it was very thin.”

But what was this gland for, exactly? To find out, Videler and his colleagues used high-powered microscopes to examine the skin above and around the gland, but it wasn’t until one of the researchers accidentally dropped a light bulb onto the fish skin that they found their answer: tiny pores on the fish’s head, all connected to the mysterious gland. When they heated the gland, oil began to ooze from the fish’s pores.

If you or I wanted to run faster, it wouldn’t make much sense for us to lubricate our heads. But since a swordfish cuts through the world face-first, an oily head might help reduce drag even further.

At this point, the greased-fish hypothesis is still just a hypothesis. Swordfish can’t be kept in captivity, and researchers would have a pretty hard time keeping up with them in the open ocean. So, for now, at least, we can’t be sure.

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