The Reptile With Three Eyes

The tuatara (“spiny back” in Maori) is a reptile found only in New Zealand. As old as the dinosaurs, the tuatara has many unique features by today’s standards. Among them is a well-developed third eye on the top of the head, although skin slowly grows over it as the tuatara ages.

Though similar in appearance to lizards, the tuatara is actually the last surviving remnant of a group of reptiles that lived alongside the dinosaurs. Found only in New Zealand, the tuatara consists of two extant species, their differences primarily due to geographical variance.

Both species—the Cook’s Strait and Brother’s Island tuatara—have a third eye, a parietal eye, located on the top of their heads. This eye is visible in youths, but as the reptile matures, a patch of skin makes it all but invisible (thus the reason you can’t see it in the image above). The tuatara’s parietal eye is particularly well-developed, as it includes a lens and retina. But a more primitive parietal eye is present in other species, including some lizards, fish, and amphibians. In these animals, the parietal eye is usually photoreceptive: It is helpful only in the perception of light. The eye is also thought to help with an animal’s circadian rhythm.

The tuatara also has a diapsid skull—a feature it shares only with crocodilians. The diapsid structure features two openings in the skull, above and below the eye, which allows the jaw to open wider and bite down harder. The tuatara also operates at lower temperatures than other lizards.

This third eye hasn’t helped the tuatara much in competition with rats. The Brother’s Island tuatara is vulnerable to extinction. Recently, some specimens were relocated to rat-free islands around Wellington Harbour. The Cook’s Strait tuatara, however, is listed as “least concern” by the IUCN. The total tuatara population is estimated to be between 50,000 and 100,000.

The tuatara isn’t doing itself any favors by waiting over a dozen years to reach sexual maturity (or by being cannibalistic). Females are ready to mate every two to five years, and pregnancies last about eight or nine months. But a tuatara can live to be over 100 years old, and will be sexually viable well into its golden years. The temperature of the soil around the egg plays a factor in the hatchling’s sex. Warmer temperatures produce males; climate change could eventually result in generations of entirely male tuataras.