How to Tell if a Giant Manta Is Pregnant? Give Her an Ultrasound.

Giant manta rays are otherworldly leviathans gliding through the world’s seas on massive wings. But even though they are the largest ray species in the world, a fundamental part of their lives remains obscure: where they give birth.

“The big unknown for us now is where females go to pup and where these pups hang out,” said Joshua Stewart, a marine ecologist at Oregon State University.

Now, a team of scientists in Mexico has reported the first use of underwater ultrasound scans to detect pregnancies in these animals, a strategy that may help answer that question.

An endangered species, giant oceanic mantas are sparsely distributed and travel vast distances, making many aspects of their reproduction difficult to study. So Madalena Cabral, a marine ecologist at Mares de México Giant Manta Project, led two expeditions, in January and March, in Revillagigedo National Park, about 250 miles off Baja California in Mexico.

On dives to locate female manta rays, Dr. Cabral let each animal grow comfortable with her presence before holding an ultrasound probe near its body — without touching it. She recorded the resulting sonograms on an iPhone 15 in a waterproof housing.

It took time to learn how to interpret the resulting images. But one night, while reviewing recent footage, Dr. Cabral realized she was looking at a manta fetus.

“Once you see it, you can’t unsee it,” she said.

Soon, Dr. Cabral was able to recognize a pregnancy underwater while scanning a manta during a dive. “Watching the fetus moving was incredibly emotional,” she said.

She confirmed pregnancies in three giant mantas, named Timshel, Ophelia and Grecia. The mantas were given tags so that their journeys could be tracked by satellite. She and her colleagues published their work as a preprint in June.

The ultrasound images, showing fetuses in various developmental stages, were “extremely cool,” said James Ketchum, a marine ecologist at Pelagios Kakunjá, a scientific nonprofit organization, who was not involved in the research.

“This is the first time this is really captured clearly,” he added.

In the past, scientists have identified pregnant mantas by a bulge at a late stage of pregnancy. But one of the study’s three pregnant mantas, Timshel, showed no outward sign of pregnancy.

“Externally visible signs of pregnancy can be absent even when pregnancy is confirmed by ultrasound,” the authors concluded. Dr. Cabral hopes that by identifying more pregnant females, scientists will be able to tag and follow their journeys during this critical stage.

While the same touchless ultrasound technology has been used on a smaller species of reef manta rays in the past, this is the first time scientists have used it on giant mantas.

Reef mantas, which live in shallower, coastal habitats, are less transient, Dr. Stewart said, and their reproductive habits are better documented.

“It’s much harder for us to know that for the big guys, the giant mantas,” he said.

Giant mantas have a slow reproductive cycle: They give birth to only one pup every four to five years, in a pregnancy that researchers believe lasts about a year. For giant mantas, each pregnancy “is incredibly valuable,” Dr. Cabral said.