Rare Two-Headed Porpoise Caught in North Sea

A pair of conjoined harbour porpoise twins were caught by fishermen off the coast of the Netherlands, reports New Scientist, in the 10th known case of conjoined twins in cetaceans.

This case of a single body and two fully grown heads, also known as partial twinning or parapagus dicephalus, is extremely rare -- only 9 other cases of conjoined twins in cetaceans, the group of animals which includes whales and dolphins, are known. Twins are extremely rare in cetaceans as there is usually not enough room in the body of female for more than one fetus.

Two-headed animals, called bicelphalic or dicephalic, and three-headed animals, or tricephalic, are the forms of polycelphaly, or the condition of having more than one head that exist in the real world, as opposed to mythical creatures like the Hydra. A polycephalic organism may be thought of as two or more beings with a shared body, and are formed by the same process as conjoined twins from monozygotic twin embryos.

Scientists believe the twins probably died shortly after birth because their tail had not stiffened, which is necessary for newborn dolphins to be able to swim. The fishermen who made the discovery unfortunately returned the twins to the ocean because they believed it would be illegal to keep such a specimen.

Although researchers will not be able to study the body of the twins, they can learn from photographs taken by the fishermen. "The anatomy of cetaceans is strikingly different from terrestrial mammals with adaptations for living in the sea as a mammal. Much is unknown," says Erwin Kompanje, a researcher at the Erasmus MC University Medical Center in Rotterdam and one of the authors of the paper describing the find.

He also remarked that although there are only 10 known cases, most instances of conjoined twins in cetaceans are never found because they born and die at sea.