Scientists in the Netherlands have reported the first known sighting of a conjoined twin porpoise. The animal was caught in a net by fisherman in the North Sea, and was dead when they found it.
The porpoise was a male baby, its age clear by its not-yet-firm dorsal fin, small hairs on its upper jaw and an open umbilicus from where it had been attached to its mother. His peculiar anatomy was unmistakable: the porpoise had two fully formed heads. The heads were connected to a single body with the usual two pectoral fins. He was about two feet long and weighed about 13 pounds. He was found about 15 nautical miles (about 17 regular miles) west of Hook of Holland, a small town in the southwestern corner of the country.
According to a report about the porpoise by Erwin J.O. Kompanje, a scientist at the Natural History Museum of Rotterdam, and colleagues, the fishermen threw him back into the water because they feared keeping the dead animal was illegal. They did, however, take a few photographs to document his existence.
Normal twinning is extremely rare among cetaceans, the authors of the report note. “There is simply not enough room in the body of the female to give room to more than one fetus, Kompanje told New Scientist. Adult harbor porpoises give birth to one baby every one to two years, on average. The first known instance of normal harbor porpoise twins was reported in 2014.
This report of a conjoined twin porpoise is the 10th known instance of symmetrical conjoined twins among cetaceans, the scientific name for the marine animal group including whales, dolphins and porpoises. Such twinning also has been found among baleen whales and toothed whales.
Scientists believe that symmetrical conjoined twins, such as this North Sea porpoise, are the result of either embryonic cells that had been separated fusing together or the incomplete separation of cells from a fertilized ovum. But what causes conjoined twins, “remains enigmatic,” the scientists write in Deinsea.