Marine biologists have documented the first-ever “in the wild” observation of endangered leopard sharks copulating, and were surprised to catch not just two sharks in action, but three.

The intimate encounter, captured in an aquamarine sex video complete with a romantic soundtrack provided by researchers at the University of the Sunshine Coast in Australia, comprised two adult males and a female.

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The ménage à trois between members of the globally endangered species is outlined in a paper just published in the Journal of Ethology.

“It’s rare to witness sharks mating in the wild, but to see it with an endangered species — and film the event — was so exciting that we just started cheering,” marine biologist Hugo Lassauce said.

Lassauce took weekly snorkeling excursions for over a year to monitor leopard sharks living just over nine miles off the New Caledonia coast. In all that time, he hadn’t witnessed the animals “in the act,” Popular Science reports.

Dr. Christine Dudgeon, a USC researcher and co-author of the study with Lassauce, called the remarkable “throupling” a “surprising and fascinating” development, and said it raises several interesting questions, including how multiple potential fathers factor into the species’ mating habits.

“From a genetic diversity perspective, we want to find out how many fathers contribute to the batches of eggs laid each year by females,” she said.

Also known as a zebra shark, the leopard shark inhabits the coral reefs and sandy ocean shelves of the tropical Indo-Pacific, ranging from the east coast of Africa to the South Pacific islands. The shark pups are born with banded markings that fade into their recognizable, leopard-like spots by the time they reach adulthood, explaining the interchangeable names.

A full-grown leopard shark averages about eight feet long and dines on mostly shelled mollusks, crustaceans, and small fish.

“I’d seen males swimming fast after females before and I’d arrived ‘on the scene’ just after a male and female separated, but I’d never seen the whole sequence,” Lassauce explained in a statement accompanying the study.

Then one day, his luck changed, as Lassauce caught sight of the unfamiliar grouping. 

“While I was surveying this particular aggregation of leopard sharks, I spotted a female with two males grasping her pectoral fins on the sand below me,” he said.

To ensure nothing accidentally disturbed the trio, Lassauce asked the research vessel to depart to give the sharks some space.

And then he waited.

“I waited an hour, freezing in the water, but finally they started swimming up,” he said.

“The mating behavior followed a structured sequence,” according to the study, “including prolonged pre-copulation positioning, male grasping of the female’s fins and tail, conspicuous siphon sac, copulatory thrusting, and clasper use consistent with previous reports from captive settings.”

Lassauce finally witnessed and recorded the two males quickly mating in succession with the female, one for 63 seconds and the second for 47 seconds.

Then, like human males in the “refractory period” after sex, they both lost their drive for more.

“The males lost all their energy and lay immobile on the bottom while the female swam away actively,” Lassauce explained.

Dr. Dudgeon, a marine ecology and evolution expert renowned for her work with leopard sharks over two decades, said the sighting offered great insights into the mostly solitary animals. 

The fact that “two males were involved sequentially on this occasion” was fascinating, she said, and could aid artificial insemination research aimed at helping “rewild” the species, currently underway between countries including Australia.

The remarkable threeway could “help us understand population dynamics and reproductive behaviors more widely,” she said.

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