Those of you who are chronic users of TikTok (welcome to the gang) may have seen an unusual animal video doing the rounds this week: a two-headed calf. It’s a condition known as polycephaly, and although it’s widespread in mythology, it’s much rarer in reality.
What is polycephaly?
The term polycephaly stems from the Greek poly, meaning many or multiple; and kephalē, meaning head. As seen in the calf from the video, it is usually a case of an organism with a single body and two heads, known as bicephaly or dicephaly, although cases of tricephaly have been – perhaps more unreliably – reported.
How rare is it?
Cases of polycephaly tend to make the news or go viral because of just how rare a phenomenon it is – to the point where, back in 2006, an American aquarium attempted to auction off a two-headed snake for $150,000.
It’s difficult, however, to truly know the scale of this rarity. As vet Dr Alaina Macdonald told Huron County Museum, which is home to two taxidermied polycephalic calves, it’s a chance event that “would generally result in the early loss of pregnancy, and those pregnancies often are undetected. Indeed, it is exceptionally rare for the embryo to develop into a full-term fetus, and to survive the length of the gestation.”
That being said, the viral calf video demonstrates that the live birth of polycephalic animals is possible, and it’s been documented across a range of animals. It’s most commonly seen in snakes and turtles, but it’s also been reported in pigs, sheep, and even dolphins. The oldest known case was discovered in the Yixian Formation in China – a fossilized reptile believed to date back to the Early Cretaceous.
Polycephaly can also occur in humans. You may have heard of Abigail and Brittany Hensel, conjoined twins who share a body; in humans, this is known as dicephalus parapagus dipus.
What causes it?
Speaking of twins, scientists have looked to their development to figure out how polycephaly occurs. Identical twins occur when a single fertilized egg splits into two embryos. In the case of polycephaly, this split is incomplete, known as axial bifurcation.
As for why this might happen, though there are theories, the answers are unclear. “No one knows the exact mechanism behind the abnormal splitting of the embryo,” said Macdonald. “Trace mineral deficiencies and environmental factors such as increasing water temperatures and toxin exposure have been implicated in some species, but there are likely many causes. Anything that disrupts that delicate timing of gene expression (including random chance) can cause abnormalities in embryonic development.”
It’s also not something that’s considered to be inherited, as polycephalic animals often don’t live long enough to reproduce.
So, we probably shouldn’t get too attached to the now internet-famous two-headed calf, but you never know; another bicephalic cow born in the 1990s, the aptly named Gemini, managed to survive for nearly a year and a half.
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