Why Do Blood-Sucking Fish Find Bile Sexy?

Move over, Chanel No. 5. You’ve got a new competitor: The scent of bile is driving female sea lampreys crazy, a new study says.

Photo of a sea lamprey.
The suction-cup mouth of a sea lamprey bristles with teeth. Photograph by James. L. Amos, National Geographic

Bile wasn’t always an alluring scent—not even to sea lampreys, a type of parasitic fish. Millions of years ago it started out as a digestive aid, breaking down the fats in lampreys’ diet. Over time, however, bile became a sexual signal, encouraging lady lampreys to mate with the males releasing the bile.

Recently, researchers at the United States Geological Survey and Michigan State University teamed up to study how bile made this strange evolutionary transition. (Hint: It may have to do with the females’ ability to detect a chemical in bile.)

The team took advantage of the fact that the Great Lakes are currently home to two species of lamprey: the native silver lamprey (Ichthyomyzon unicuspis) and the invasive sea lamprey (Petromyzon marinus). (Read more about marine invasive species.)

Both of these lampreys shared a common ancestor at some point in the past, although researchers believe the silver lamprey is more closely related to this ancestor, while the sea lamprey is a relative newcomer.

Both species have an oral disk that they use to attach to the side of their fish prey. Then, using their strong, sharp teeth, they cut and chew their way through the flesh to access the fish’s blood.

Evolution of a Sex Signal

About ten years ago, Weiming Li, a professor of fisheries and wildlife at MSU, and his colleagues showed that sea lampreys use a chemical found in bile as a pheromone. When males release a compound known as 3kPZS, it causes females to swim over and look to mate.

Since sea lampreys are a major invasive species, some scientists thought that studying this compound might help them learn how to disrupt mating and reduce the lampreys’ numbers.

The immediate question that Li and colleagues wanted to answer was whether 3kPZS was a sexual signal in silver lampreys. If it was, then using 3kPZS to control sea lampreys would also harm the native silver lampreys, and the strategy wouldn’t work. So researchers needed to know how silver lampreys responded to bile. But Li was also thinking in broader evolutionary terms.

“Bile has nothing to do with sex—so how could it have evolved as a sex signal?” Li asked. “It would be more fun to look into how these pheromone signals actually evolved, which would have a greater scientific significance.”

He thought that the secret to understanding why sea lampreys found bile so irresistibly sexy could be explained by something known as receiver bias. (Read more about animal attraction.)

This occurs when a male trait (in this case, the release of 3kPZS right before mating) evolves through a bias in the female sensory system (her ability to detect this compound). Although bile salts didn’t start out as a sexual signal, over time, the females who were better at detecting 3kPZS had more offspring than those who were less sensitive. This would lead to the transformation of bile from a simple digestive aid to a pheromone.

Bile No. 5?

It’s similar to how our own use of perfume has changed over the centuries. Initially, perfume was used to help people who didn’t wash often conceal their body odor.

Although our hygiene has generally improved, we still use perfume. But now it’s often for a sexual purpose—a way to entice potential partners by giving them a whiff of desire. (Also see “Big Cats Wild for Calvin Klein Cologne?”)

Li’s experiments supported the idea that receiver bias could explain how 3kPZS became a sexual signal in sea lampreys. Since the silver lampreys are evolutionarily less developed than sea lampreys are, it’s no surprise that the females responded to 3kPZS but weren’t triggered to mate. As expected, this compound did trigger mating behavior in sea lampreys, fish that have evolved more recently.

“We need to be cautious in how we interpret these results, since there are still so many unknowns,” said Li, whose study appears in the current issue of the Proceedings of the Royal Society B.

Still, his results are convincing enough that some of the fisheries management organizations in the Great Lakes are developing programs to use 3kPZS in these waters to try and reduce the numbers of invasive sea lampreys and protect native fish populations, Li said.

Although bile does the trick for female sea lampreys, I have my doubts that we’ll ever see bile on the perfume counter.

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Meet the Author
Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at http://www.carriearnold.com