The newborn male harbour porpoises were caught as accidental bycatch and thrown back into the North Sea by Dutch fishermen fearing it would be illegal to keep them. (PHOTOGRAPH BY HENK TANIS)

The first known case of a two-headed harbour porpoise was documented last month when Dutch fishermen in the North Sea accidentally caught the anomalous creature as bycatch in a beamtrawl net.

Fearing it would be illegal to keep the deceased marine mammal, they threw it back into the ocean, but not before taking photos of their bizarre find and alerting researchers.

While case studies of conjoined twins have been studied in humans, reptiles, and domestic animals, they are rarely seen in wild mammals. In fact, before this find, only nine other instances of two-headed cetaceans of any kind had ever been reliably documented.

A study recently published by the online Journal of the Natural History Museum Rotterdam contextualized the rarity of the find.

“Descriptions of conjoined twins in whales and dolphins are extremely rare,” Erwin Kompanje, from the National History Museum Rotterdam, noted in his study. “We were aware of only nine [other] published cases.”

Though researchers lamented not being able to physically examine the porpoise, they could glean details from the fisher’s photos.

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SOURCE: National Geographic, Sarah Gibbens