The green iguana is a large lizard and is probably the largest species in the iguana family, though a few in the genus Cyclura may match or exceed it in weight. Adults typically grow to 1.2 to 1.7 m (3.9 to 5.6 ft) in length from head to tail. As in all iguanas, the tail comprises much of this length, and the snout-to-vent length of most green iguanas is 30 to 42 cm (12 to 17 in). A typical adult male weighs around 4 kg (8.8 lb) while the smaller adult female typically weighs 1.2 to 3 kg (2.6 to 6.6 lb).[31][better source needed] A few large males can reach or exceed 8 kg (18 lb) in weight and 2 m (6.6 ft) long. Some specimens have even reportedly been measured at a body weight of greater than 9.1 kg (20 lb).
Despite their name, green iguanas occur in different colours and types. In southern countries of their range, such as Peru, green iguanas appear bluish in colour, with bold blue markings. On islands such as Bonaire, Curaçao, Aruba, and Grenada, a green iguana's colour may vary from green to lavender, black, and even reddish brown. Green iguanas from the western region of Costa Rica are red, and animals of the northern ranges, such as Mexico, appear orange. Juvenile green iguanas from El Salvador are often bright blue, but lose this color as they get older.
Adult iguanas found on most of St. Lucia, mainly on the northeastern coast, Louvette, and Grand Anse, have many differences from other green iguana populations. They are light green with predominant black stripes. Instead of the typical orange dewlap, the iguanas of St. Lucia have a black dewlap. When compared to the common green iguana, females lay about half the number of eggs, 25 instead of 50. Scales to the back of their head, near the jawbone, are smaller. Their irises are white or cream, whereas other green iguanas have yellow irises.
Green iguanas possess a row of spines along their backs and tails, which helps to protect them from predators.Their whip-like tails can be used to deliver painful strikes, and like many other lizards, when grabbed by the tail, iguanas can allow it to break, so they can escape and eventually regenerate a new one. In addition, iguanas have a well-developed dewlap, which helps regulate their body temperature. This dewlap is used in courtships and territorial displays.
Green iguanas have excellent vision, enabling them to detect shapes and motions at long distances. As green iguanas have only a few rod cells, they have poor vision in low-light conditions. At the same time, they have cells called double-cone cells that give them sharp color vision and enable them to see ultraviolet wavelengths. This ability is highly useful when basking so they can ensure they absorb enough sunlight to produce vitamin D.
Green iguanas have a white photosensory organ on the top of their heads called the parietal eye (also called the third eye, pineal eye, or pineal gland), in contrast to most other lizards that have lost this primitive feature. This "eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movement. This helps the iguana detect predators stalking it from above.
Green iguanas have very sharp teeth that are capable of shredding leaves and even human skin. These teeth are shaped like a leaf, broad and flat, with serrations on the edge. The similarity of these teeth to those of one of the first dinosaurs discovered led to the dinosaur being named Iguanodon, meaning "iguana tooth", and the incorrect assumption that it had resembled a gigantic iguana. The teeth are situated on the inner sides of the jawbones, which is why they are hard to see in smaller specimens.
Primarily herbivorous, green iguanas are presented with a special problem for osmoregulation; plant matter contains more potassium and as it has less dense nutritional content, more must be eaten to meet metabolic needs. As green iguanas are not capable of creating liquid urine more concentrated than their bodily fluids, like birds they excrete nitrogenous wastes as urate salts through a salt gland. As a result, green iguanas have developed a lateral nasal gland to supplement renal salt secretion by expelling excess potassium and sodium chlorides.
Green iguanas from Guatemala and southern Mexico (which may belong to the distinct species I. rhinolopha) predominantly have small horns on their snouts between their eyes and their nostrils, whereas others do not.
Male green iguanas have highly developed femoral pores on the underside of their thighs, which secrete a scent (females have femoral pores, but they are smaller in comparison to those of the males). In addition, the dorsal spines that run along a green iguana's back are noticeably longer and thicker in males than they are in females, making the animals somewhat sexually dimorphic.
Male green iguanas tend to display more dominant behaviors, such as head bobbing and tail whipping. They also tend to develop a taller dorsal crest than females, as well as taller dorsal spines (or spikes). Large, round, very pronounced jowls are generally a male characteristic. Jowls are located under the jaw and are protected by the subtympanic plate, which is a large, green, circular-shaped scale.
Green iguanas are oviparous, with females laying clutches of 20 to 71 eggs once per year during a synchronized nesting period. The female green iguana gives no parental protection after egg laying, apart from defending the nesting burrow during excavation. In Panama, the green iguana has been observed sharing nest sites with American crocodiles, and in Honduras with spectacled caimans.
The hatchlings emerge from the nest after 10–15 weeks of incubation. Once hatched, the young iguanas look similar to the adults in color and shape, resembling adult females more so than males and lacking dorsal spines.
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