Polycephaly, from the Greek roots poly meaning “many” and kephalē meaning “head”, is the term for when animals are born with more than one head. Polycephaly does happen in the animal kingdom, but it can be tricky to estimate how often it happens as it usually results in the early loss of pregnancy. It is most common in snakes and turtles, but has been documented in cows, sheep, pigs, cats, dogs, and fish. In human beings, polycephaly can be seen in cases of conjoined twins.  Many cultures include polycephalic beings in their mythology, such as the Hydra in Greek mythology or the Naga from Hindu and Buddhist myths. In real life, two-headed animals do occur in nature. They are known as either bicephalic or dicephalic. 

How does it happen? 

Much like twins, polycephanic organisms form from a monozygotic embryo. This means they come from a single fertilized egg – just like identical twins. But early in pregnancy, something causes the twinning process to go wrong and it does not split entirely, a process called “axial bifurcation.” Instead, the embryo develops two heads, but does not develop fully-separated body systems. The way the body systems interact is different in each case. In some cases, the two heads may have separate brains but share other critical organs and body systems. They may have only one heart, one pair of lungs, and one digestive system. This makes it nearly impossible for the embryo to function effectively and it often does not survive pregnancy.  Scientists aren’t sure exactly what causes the disruption in the twinning process. Few polycephalic animals live long enough to reproduce, but there have been a few exceptions. When it does happen, two-headed animals have given birth to single-headed anatomically typical offspring. This suggests that the cause of polycephaly is not genetic. Environmental factors or random chance may play a role. Because it is so difficult to study, no one knows for sure.

A Two-headed Life

Unfortunately, animals with polycephaly do not typically live long, even if they do survive pregnancy and birth. They may have trouble moving or eating because the communication between brains and other organ systems are severely disrupted. However, there are some instances of these unique animals defying the odds! Janus, a Greek tortoise born in 1997, is the oldest living two-headed animal. He has two heads, two hearts, two sets of lungs, and two stomachs. Janus resides at the Natural History Museum in Geneva, Switzerland.

Our specimen

A two-headed calf is featured in the museum’s collection and will soon be on display in the museum. The calf was sourced from a local farmer and it likely lived for only a few hours after birth. The specimen offers a great example of one potential skeleton structure found in polycephalic calves. You will be able to see it on the second floor near the classroom, so keep an eye out for it on any future visits.