A golden retriever puppy with tail on his face has gained fame online. Thousands of Twitter users commented on his pictures and retweeted them. The rescued puppy was named for a marine mammal with a single tusk that sticks out of its face. But instead of a tusk, Narwhal puppy has a tail flopping between his eyes. Scientists do not agree on how the puppy's condition came to exist.
Labeled as the "unicorn puppy"
Mac's Mission, a Missouri shelter, which specializes in what it calls "janky" dogs, took in the abandoned puppy. Staff was disappointed that Narwhal's extra tail did not wag. But the appendage did not seem to bother the otherwise normal, healthy puppy, and a veterinarian said that there was no need to remove the tail. An x-ray showed on bones.
After getting so much attention from social media, Narwhal is now known as the "Unicorn puppy."
Case of a parasitic twin
The most common explanation for how Narwhal got his face tail is not adorable, said Margret Casal, a professor at the University of Pennsylvania School of Veterinary Medicine. The tail is Narwhal's parasitic twin.
Regular twins form when an embryo splits in half soon after fertilization. Sometimes, this split happens too late in a pregnancy that the halves do not separate, leading to conjoined twins. Dr. Casal said that the late split is asymmetrical, which is even rarer. This means that one side of the embryo grows into a fully formed individual, and the other becomes an extra body part.
Dr. Casal also stated that the little Mohawk of backward-growing fur above Narwhal's face tail is similar to the crest on a dog like the Rhodesian Ridgeback. She said this could suggest a twin's rear end on Narwhal's face.
David Kilroy, who specializes in head anatomy and development at the University College Dublin School of Veterinary Medicine, has a different first impression. He said that at first, he thought that it was a bit of clever computer work and that it is not real. But after he took a long look at the pictures and X-ray, it looks like some weird outgrowth of skin, although something so massive and strange would be unusual.
Dr. Casal said that the bottom of a spine can't develop bones without signals from the top. So if the puppy's appendage is a parasitic twin, it might make sense that it never grew any bones. Unlike in humans, identical twins are rare in dogs, because they are typically born in litters. Dr. Casal said that dogs with a parasitic twin are very rare.
But this phenomenon is not unheard-of. There was a case of a puppy that had an extra pair of hind legs growing from its belly. Parasitic twins, like conjoined twins, can happen in humans too. Animals are sometimes born with more extreme spare parts, like a second head. Two-headed calves occasionally show up in headlines, though they die soon after birth.
This event does not only happen to dogs. Snakes can hatch with two heads too. In 2007, Van Wallach, a herpetologist, summarized almost a thousand reported cases of two-headed snakes. The two heads are almost always next to each other, but they occasionally stacked. A lot of factors can lead to two-headed snakes, including cold temperatures when eggs are incubating. Most two-headed snakes die immediately, but a few of them live to adulthood.
