Splashes at the surface give an indication of the commotion below. Beneath the waves, a tornado of hundreds of devil rays swirl in a dizzying dance that goes on for hours.
This is a “courtship vortex”: a previously undescribed behavior among munk’s devil rays (Mobula munkiana) that has now been captured on film in a study conducted by the conservation nonprofit Manta Trust. New drone footage shows 122 individuals taking part in this five-hour display in Mexico’s Baja California, with different mating groups joining and leaving the main formation over time.
“It’s amazing to see a vortex of mobulas,” says Marta D. Palacios, co-founder of Mobula Conservation, whose study examines courtship and mating behaviors in three mobula species: spinetail, bentfin, and munk’s devil rays.
Mobulas are known to create vortex formations when feeding or resting, but this is the first time vortexes have been reported in a mating context. The circling motion was more relaxed: the rays were touching each other, and they weren’t unfurling their cephalic fins—the horn-like appendages in front of their face—to eat. The researchers also saw courtship trains—when male mobula rays chase a female to try to mate with her—weaving in and out of the larger vortex.
This behavior gives the rays a faster way to select a mate, explains Palacios: “If you’re in a courtship train, you need to chase the same female for hours or even days”—but in the vortex males can choose their best match from 30 or 40 females.
“We know so little about mobula rays,” says Stephanie Venables, senior scientist from Marine Megafauna Foundation, who was not involved in the study: “Reproductive behavior is one of the most important aspects of their life history to understand so these findings will be really important for protection.”
The study also describes another new behavior, called piggyback leaping, when the male leading the chase in a courtship train leaps on top of the female, lifts his tail, and starts rapidly thrusting his clasper, or reproductive organ.
This happened to one female over 135 times. “Imagine you’re just swimming at the surface and someone jumps on you from behind again, and again, and again,” says Palacios.
It was previously believed that males need to bite a female’s pectoral fin before trying to copulate, but that wasn’t seen here.
Piggyback leaping leaves a rash on the female’s back, Palacios says—a finding that future researchers may be able to use to determine whether a female is sexually mature.
First, however, Venables points out that researchers must be able to distinguish these rashes from other abrasions—and understand how long they last on the body—before they can be effectively used as a sign of maturity.
Devil rays, which are incredibly shy, typically don’t tend to approach humans, and can be easily frightened away.
But, during this encounter, “the animals didn’t care about [the researchers’] presence, they were even bumping into them,” says Palacios. The mating frenzy “makes them almost blind to the exterior world,” she adds, making them vulnerable to threats, such as tourism, fishing, and boat traffic.
When rays get together in big groups like this it leaves them all more vulnerable, says Palacios, because one threat won’t just affect one ray: “It’s going to be 10 or even hundreds of devil rays that are caught in the same net or in the same trawler.”
“Think how easy it would be for a fisherman to put a net around that handy little cyclone and catch them all,” says Venables.
The study also discovered that these waters off the southwestern Gulf of California are reproductive grounds for three mobula species, all of which are threatened: the International Union for the Conservation of Nature categorizes munk’s devil rays as vulnerable to extinction, while spinetail (Mobula mobular) and bentfin (Mobula thurstoni) devil rays are both endangered.
“When you’re putting a net around the mature mating population, you’re not just taking away animals but you’re taking away the animals that are creating babies,” says Venables, adding that these findings highlight why it’s important to regulate fishing in these areas, particularly during mating season.
Local success story
This area was once one of the main fisheries for manta rays and devil rays, and Palacios recalls days when you could see “hundreds, thousands of [dead] mobulas laying on the beach.”
After the introduction of a new law preventing fishing, the area has instead become a tourism hotspot. Rather than trying to catch rays, communities now take tourists to swim with them.
However, without proper regulations, this tourism can also pose a threat. Conservationists are teaching guides best practices so tourists can see these animals in a responsible way without influencing their natural activities—particularly reproductive behaviors.
“Disturbing reproduction directly impacts population health,” says Venables. “Courtship is the worst thing to disturb.”