Explained: How Narwhal the puppy grew a tail on its head

What are the theories behind Narwhal’s extra tail?

One theory is that the extra tail is Narwhal’s parasitic twin — a highly rare occurrence in nature. Parasitic twins are a rarely observed type of the phenomenon called conjoined twins, which in itself is highly uncommon.

Conjoined twins, as opposed to regular twins, are born when an embryo splits too late during pregnancy, and the halves do not fully separate. In the case of parasitic twins, the embryo is asymmetrically split. In such a case, one embryo develops at the expense of the other- the former being called the dominant twin and the latter the parasitic twin. The parasitic twin can be an extra body part– in Narwhal’s case, the additional tail.

While the parasitic twin theory is being cited as the reason for Narwhal’s extra tail, others believe that it is impossible to know the actual reason for sure. This is because the development of a body consists of a number of complex chemical processes, many of them still not understood by scientists.

How common are parasitic twins?

Although rare, cases of parasitic twins are seen in animals including dogs, snakes, cows, and even humans. Cases of two-headed calves and snakes have been reported, but most die in infancy.