Why Some Fish Build Sand Castles to Attract Mates

A male cichlid, left, and the mound it created to woo a female, right. York et al

Throughout Lake Malawi, an enormous body of freshwater in southeastern Africa, there are types of fish, called cichlids, that engage in a very peculiar ritual. The males of these species build a series of strange formations on the lake bottom that look vaguely like castles, pit-like depressions or some combination thereof.

These small architects don't have opposable digits, though—or, you know, hands—so they move around sediment with another body part: their mouths.

"They build these things one mouthful at a time...continually picking up and dropping sand," says Ryan York, a biology graduate student at Stanford University.

Some of these formations, also known as bowers, can stretch to a diameter of 10 feet. That's pretty incredible, considering these fish average about three to four inches in length.

But why on Earth would they go to such trouble?

"A tongue-in-cheek answer: that's what the ladies want," says Hans Hofmann, an evolutionary neuroscientist at the University of Texas at Austin.

Indeed, all these earthworks are just for show, to attract and impress mates. If the female of the species likes what she sees, she decides to breed with the male in question, which involves laying her eggs in the center of his castle/pit, York says. Moments later, the males fertilizes the eggs with his sperm, and the female engulfs them in her mouth. For the next few weeks, she will refrain from eating as the eggs develop and eventually hatch in her mouth.

Dozens of species of cichlids make a whole range of different structures. York and his colleagues set about to try to better understand what may be driving such variation. Their findings appeared in a study published in the journal Frontiers in Ecology and Evolution.

For one, they found that castles were more common in species that live in shallower waters. That may be because these fish tend to be able to better distinguish between light and shadow, and may be able to make out the finer features of these structures, York says. Likewise, pits are more common in deeper-living species, perhaps because they are easier to make out in low-light conditions, he adds.

It's also worth noting that the pit structures, which also sometimes appear more like a raised, bowl-like mound, likely first evolved as a way of demarcating territory. It's possible that they then developed into a "signal" for genetic quality and thus were then used for mating rather than territorial purposes, York says.

The mounds are genetic "signals" in that they show off a fish's ability to execute a complicated task, which takes strength and perhaps even something akin to creativity. Moreover, bigger structures—which are often preferred by females—take longer; a fish that is better and more efficient at capturing food may be able to free himself up to spend more time wooing the ladies, so to speak. And all of these things—strength, "creativity" or executive function, and hunting ability—are often correlated with "better" genetics, found in a male that is more likely to produce fecund and healthy offspring.

This study is "perhaps the most comprehensive analysis of bower building to date, as far as number of species and associated traits analyzed," says Reade Roberts, a biologist at North Carolina State University who wasn't involved in the project. One point that jumped out for Roberts was that pit-building species are more likely to display—or "dance"—to attract mates, while the castle-builders "spend more time tending their castles," he says.

"Perhaps pits are less impressive than castles, and the pit-building males have to compensate with dancing," he says.

These cichlids are not the only fish that create bowers to attract mates. Male pufferfish in Japan are also known to craft elaborate patterns to woo females, circular formations that divers found many years before their true (fishy) creators were uncovered.

These circles are made by male pufferfish to attract mates. Kawase et al