A Two-Headed Turtle Hatchling: A Rare Occurrence Of Polycephaly

Polycephalic turtle hatchling in the egg. Image.
Figure 1. Two-headed turtle hatchling still curled around in the egg.

Sometimes nature doesn’t always get everything right! The ORP team at Coco Palm Dhuni Kohlu recently discovered a green sea turtle hatchling with a very unusual abnormality during a routine excavation of a hatched turtle nest: a two-headed hatchling. The hatchling had sadly died within its egg and never hatched, but that didn’t stop it from giving us all a huge surprise. The experience also served as an interesting reminder of the intrigues of the natural world.

We carry out nest excavations to determine how many eggs were laid versus how many hatched in a nest. This helps us to understand what might have happened to the ones that didn’t hatch and informs conservation efforts and the protection of future clutches. On this occasion, when we gently opened the unhatched egg, we found a fully formed hatchling with two heads!

Polycephaly – Having More Than One Head

The technical term for this mutation is ‘polycephaly’. In this case, it was two heads joined in the centre, with an adjoined eyelid. The hatchling had two fully formed beaks but shared a neck. In figure 2 you can see the hatchling removed from his egg and uncurled. You can see their prominent yolk sac in the middle of their belly area – this gets absorbed into their abdomen after they have hatched but before they emerge out of the nest. The yolk sac provides the hatchlings nutrients and energy for the long journey ahead! Figure 3 shows a normal hatchling in an uncurled position.

Polycephaly turtle hatchling removed from the egg. Image.
Figure 2. The two-headed turtle hatchling removed from the egg and uncurled.
A normal turtle hatchling. Image.
Figure 3. A normal turtle hatchling uncurled.

How Often And Why Does Polycephaly Occur?

Polycephalic turtle hatchling. Image.
Figure 4. Polycephalic (two-headed) turtle hatchling.

Polycephaly is a very rare birth defect and one we haven’t seen during any of our nest surveys so far. There are isolated anecdotal reports of two-headed sea turtle hatchlings; some that had even gone on to hatch.

However, concrete data of the prevalence is hard to come by. Reptiles are more commonly found with polycephaly than other animals but it is still very unusual.

It occurs due to either one embryo that incompletely splits, or two embryos that incompletely fuse, early in development. (Figure 4). This can cause a number of other abnormalities, such as extra organs, incompletely fused parts of the body or even the absence of some body parts or organs.

What Was The Two-Headed Turtle Hatchling’s Anatomy?

Polycephalic turtle hatchling on x-ray plate. Image.
Figure 5. The two-headed turtle hatchling on the x-ray plate.

Dr. Minnie took x-rays of the hatchling to have a peek at its bones and shell. (Figure 5 – the hatchling on the x-ray plate). We weren’t looking for anything in particular, but it was very interesting to see what their anatomy looked like. Figure 6, 7 and 8 show their x-rays from 3 angles, firstly from from the side, secondly above with the hatchling resting on its front, and then thirdly on its back. It is hard to see fine detail due to his tiny size, but we can see where the spine of his neck divides into two. Plus we can make out the bones of their limbs very well, including the bits that are tucked within their shell, such as their shoulder bones. The bright white circular structure in the middle is their yolk sac, which shows up well on x-rays due to its density.

Lateral x-ray view of polycephalic turtle hatchling side x-ray. Image.
Figure 6. Side x-ray.
Dorsoventral x-ray view of polycephalic turtle hatchling. Image.
Figure 7. Front x-ray.
Ventrodorsal x-ray view polycephalic turtle hatchling. Image.
Figure 8. Top x-ray.

Why Didn’t This Two-Headed Turtle Hatchling Survive?

Two-headed turtle hatchling with joined eye lids. Image.
Figure 9. Two-headed turtle hatchling with joined eye lids.

Although there are examples of living polycephalic animals such as snakes and tortoises, it is a problematic mutation. Dr. Minnie believes this hatchling didn’t make it due to the mutation also affecting its brain and spinal cord development. There was a little area on the back of its neck where the tissues hadn’t formed properly. It also had a malformed carapace near its neck, so this was all likely part of the same problem.

However, the hatchling had a totally normal set of organs, along with one windpipe that split into two, one for each head, and the same for the oesophagus. And although the two heads shared a central eyelid, they each had a normal pair of eyes. (Figure 9 – showing their joined eyelids).

Hatchling Survival Rates

Even if this hatchling had survived to hatching point, it is highly unlikely that it would have survived due to the difficulty it would have had feeding with two mouths. However, that is sometimes the way it goes in the natural world. The survival of sea turtle hatchlings is already naturally very low; it is estimated that only one in one thousand sea turtle hatchlings survive until adulthood. When the normal ecosystems are balanced, this is not a concern for sea turtle populations – it is the way they have been doing things for 110 million years! It is when human threats such as climate change, beach development or fishing nets, artificially place pressure on the survival of hatchlings, that we see even greater problems with falling sea turtle populations.  

It was certainly a very interesting learning experience for all involved and makes for a very interesting look into the wonders, and sometimes mistakes, of nature.