Quick Answer
- Iguanas have a typical lizard skeleton with a long, flexible spine and a tail longer than the body, with built-in break points for tail dropping.
- Their skull has an opening for the light-sensing parietal eye, and their teeth are fused to the inner side of the jaw.
- Inside, iguanas have a three-chambered heart, a large hindgut for fermenting plants and salt glands in the nose.
- Outside, they are covered in scales, with a dewlap, dorsal crest, cheek shield, femoral pores and sharp climbing claws.
External Anatomy
| Body part | What it does |
|---|---|
| Scales | Protect the body and limit water loss; shed in patches. See shedding. |
| Dorsal crest | Row of soft spines along the neck, back and tail; larger in males; used in display |
| Dewlap | Flap of skin under the chin, supported by the hyoid bones; used for display and heat exchange |
| Subtympanic shield | Large, round cheek scale below the ear on green iguanas |
| Tympanum (eardrum) | Visible ear opening behind the eye |
| Parietal eye | Light-sensing "third eye" on top of the head. See third eye. |
| Femoral pores | Pores on the inner thighs that secrete waxy scent markers; larger in males |
| Claws | Five toes on each foot with sharp, curved claws for climbing |
| Vent (cloaca) | Single opening for waste and reproduction |
| Tail | Balance, swimming, defense; more than half of total length |
Skeleton
An iguana's skeleton follows the basic lizard plan: a skull, a long backbone, ribs, a shoulder and hip girdle and four limbs that sprawl out to the sides. The spine is long and flexible, allowing the side-to-side movements iguanas use when walking and swimming.
The Tail
The tail is supported by many small vertebrae. In green iguanas, several of these have fracture planes, natural weak points where the tail can break off when grabbed. A regrown tail is supported by a cartilage rod rather than bone. See do iguana tails grow back?
Limbs and Claws
Iguanas have five long toes on each foot, each tipped with a sharp, curved claw. These help them grip bark and climb high into trees. The claws are also why handling an iguana without long sleeves can leave scratches.
The Skull
The iguana skull is lightweight with large openings that save weight and anchor jaw muscles. Notable features include:
- Parietal foramen: a small opening in the top of the skull beneath the parietal eye.
- Pleurodont teeth: rows of small, serrated teeth attached to the inner side of the jaw and replaced throughout life. See iguana teeth.
- Large eye sockets: iguanas rely heavily on sight.
- A flexible skull: like many lizards, iguanas have some movement between skull bones, which helps when feeding.
Anatomists have described the skull bones of the green iguana and of rock iguanas such as the Turks and Caicos rock iguana in detail, partly to help understand how iguana species are related.
Internal Organs
| Organ | Notes |
|---|---|
| Heart | Three chambers (two atria, one ventricle), typical of lizards |
| Lungs | Paired lungs; iguanas have no diaphragm and breathe using rib muscles |
| Stomach and small intestine | Begin digestion of plant matter |
| Hindgut (colon) | Large chamber where microbes ferment fibrous plants; key to the iguana's herbivorous diet |
| Liver | Large; stores fat and processes nutrients |
| Kidneys | Remove waste mainly as uric acid, seen as white urates in droppings. See iguana poop. |
| Fat bodies | Energy stores in the abdomen, used during breeding and food shortages |
| Reproductive organs | Males have paired hemipenes in the tail base; females have paired ovaries. See male vs. female. |
Digestive System
Green iguanas are hindgut fermenters. They bite and swallow plant material with little chewing, and the real work happens in the large intestine, where microbes break down tough fiber. Young iguanas likely acquire these microbes from their environment. Digestion depends heavily on body temperature, which is why a cold iguana cannot process its food properly. See what iguanas eat.
Salt Glands
Iguanas have salt glands in the nasal area that remove excess salt from the body. They "sneeze" out a salty spray, which can leave a white crust around the nostrils. This is normal. In the sea-feeding marine iguana, these glands are especially important.
Senses
- Vision: sharp, with color vision including ultraviolet.
- Parietal eye: detects light, dark and overhead shadows.
- Hearing: eardrums behind the eyes detect sound.
- Smell and taste: the tongue collects chemical cues for an organ in the roof of the mouth.
Frequently Asked Questions
How Many Chambers Does an Iguana Heart Have?
Three: two atria and one ventricle, as in most lizards.
Do Iguanas Have Ears?
Yes. Iguanas have visible eardrums (tympana) behind their eyes and can hear.
What Is the Round Scale on an Iguana's Cheek?
The subtympanic shield, a large scale below the ear on green iguanas. Its purpose is not fully understood, though it may play a role in display.
Why Do Iguanas Have Such Long Tails?
The tail helps with balance when climbing, propulsion when swimming and defense by whipping, and it can be dropped to escape predators.
Sources: PeerJ, The Skull of the Turks and Caicos Rock Iguana (2024); Bioscience Journal, The Skull Bones of the Iguana iguana; Melissa Kaplan, Tail Loss.
