Head to head
Tuatara vs Green Iguana
Put a tuatara next to a green iguana and most people would call them both lizards. Only one is. The tuatara is the last survivor of an entire reptile order that split from the lizard and snake line around 250 million years ago, and almost everything about how it lives is unusual. The green iguana is a large, ordinary, tropical lizard, and a very common exotic pet.
| Tuatara | Green Iguana | |
|---|---|---|
| Animal group | Rhynchocephalia (its own order; not a lizard) | Squamata (a true lizard) |
| Split from the other line | About 250 million years ago | Is part of the main lizard radiation |
| Native range | Islands and sanctuaries of New Zealand | Central and South American rainforest |
| Preferred body temperature | About 5 to 11 C, lowest of any reptile | Around 30 C |
| Teeth | Fused to the jaw; double upper row, single lower row | Ordinary replaceable lizard teeth |
| Growth and lifespan | Grows ~35 years; lives past 100 | Matures in a few years; lives about 20 |
| Diet | Insects, spiders, lizards, seabird eggs and chicks | Leaves, flowers, and fruit |
| Adult size | Up to about 60 cm and 1.3 kg | Up to nearly 2 m including the tail |
Tuatara vs Iguana: common questions
Is a tuatara a lizard?
No, although it looks like one. The tuatara belongs to the order Rhynchocephalia, or beak-heads, which separated from the lineage leading to lizards and snakes about 250 million years ago, before the dinosaurs. That group was diverse and successful for a long time and then dwindled, until only the tuatara was left, surviving on isolated New Zealand islands that had no land mammals. So a tuatara is roughly as distantly related to a lizard as it is to a snake. Its lizard-like shape is inherited from the common ancestor of both groups and has simply changed very little.
Why can a tuatara live in the cold when most reptiles cannot?
It evolved on cool, windy, temperate islands and its whole physiology is tuned to low temperatures. Its preferred body temperature, around 16 to 21 degrees Celsius, and its lowest active temperature, near 5 degrees, are both far below what a tropical lizard could tolerate, and it remains alert and can hunt on nights that would leave an iguana torpid. The trade-off is that everything happens slowly: it has a low metabolic rate, a slow heart, slow growth over decades, a very long incubation, and an exceptionally long life. Warmth, not cold, is the bigger threat to it now, because rising nest temperatures skew its hatchlings heavily toward males.
What is the third eye that people mention?
On the top of a young tuatara's head, between the two normal eyes, there is a small patch that is a genuine light-sensing structure, complete with a simple lens and retina and a nerve to the brain, though it becomes covered by scales and pigment as the animal ages. This parietal eye is not used for forming images; it detects light levels and is thought to help regulate daily and seasonal rhythms and time spent basking. Many lizards, including iguanas, and some frogs and fish have a parietal eye too, but the tuatara's is unusually well developed, which is why it is so often mentioned as one of its odd features.