Iguanas are large, ancient, herbivorous lizards with a stocky trunk, long, slender tail, scaly skin, and a single row of spines from the nape of the neck to the tip of the tail. On either side of the head is an eye with a round pupil and with moveable lids. The well-defined snout has two nostrils, the mouth houses a short, thick tongue, and dangling beneath the chin is a “dewlap,” or throat fan. Iguanas are well equipped for speed and climbing with four short, thick, powerful legs, each with five long thin toes tipped with strong claws. Iguanas are found in warm, temperate, and tropical zones and, depending on the species, live in trees, holes, burrows, and among rocks. Iguanas are oviparous (egg-laying), diurnal (active during the day), and ectothermic (cold-blooded), thermoregulating by basking in the sun or sheltering in the shade. Iguanas are found only in the New World, and were completely unknown in the Old World until European explorers discovered the Americas.
More than 30 species of iguanas belong to the ubfamily Iguaninae, of the family Iguanidae. Iguanas are assigned to seven genera, their common names being banded iguanas (Brachylophus), land iguanas (Conolophus), spiny-tailed iguanas (Ctenosaura), ground iguanas (Cyclura), desert iguanas (Dipsosaurus), green iguanas (Iguana), marine iguanas (Amblyncus), and chuckwallas (Sauromalus).
Size, weight, and longevity vary between species. Large land iguanas of the Galapagos Islands range in weight from a hefty 26.5 lb (12 kg) to less than 11 lb (5 kg), while the tiny ground iguana of the Bahamas and West Indies weighs scarcely 2 lb (1 kg).
Iguanas are believed to be monophyletic, that is, they have evolved from a single ancestral type dating back to the extinction of the dinosaurs. Except for the banded iguana of the Fijian islands, all species are found exclusively in the Western Hemisphere. Marine iguanas are the only living lizard that spends time in the ocean, exclusively existing on algae gathered from rocks either by diving or foraging on tidally exposed reefs. Only the banded iguana and the green iguana of South America are found in wet tropics, while all other species inhabit dry environments.
Iguanas are strictly herbivorous—with the exception of the spiny-tailed iguana whose young eat insects and a few species which occasionally eat readily available meat. Iguanas are selective about their diet, preferring easily digested fruit, flower buds, and tender young leaves.
After reaching sexual maturity, iguanas reproduce annually until death. Green iguanas mature during their second or third year and live to be 10 or 12 years old, while the large land iguana attains adulthood around 10 years, and may live to age 40. Adult males establish mating territories and are selected by females who prefer larger males. Females may court several males before choosing a mate, and one male may be chosen by several females, all of which take up residence in the male’s territory.
Several weeks after mating, the female selects a nesting site where she digs a burrow, creates a special chamber, and lays her single clutch of eggs. Seven to 12 weeks after mating, the green iguana lays 20 to 30 eggs, each about 1.5 in (4 cm) long. The banded iguana lays three to six eggs, each about 1 in (3 cm) long, approximately six weeks after mating.
After laying her eggs, the female exits and fills in her burrow, leaving an air pocket in the chamber for the hatchlings, which appear three to four months later at the onset of the rainy season, when food is abundant. The banded iguana is unique in that egg incubation takes an unusually long five to eight months. The young hatch simultaneously and dig to the surface. In most species, only a small percentage of hatch-lings reach maturity.
Males of most species use head bobbing, pushups, and expansion of the dewlap to attract a mate. More threatening postures, such as opening the mouth, tongue-flicking, and snorting, are added when defending territories or warding off rivals. Banded iguanas also puff up their torso and their green bands become much darker, increasing the contrast with their pale blue-green bands. Physical aggression is rare, and the occasional clash results in head-thrashing, tail-swinging, and sometimes biting, with the loser creeping quietly away. Females usually only show aggression when contesting for, or defending, nesting sites. Each species has a distinct display pattern which seems to aid in recognition.
Dewlap —A loose fold of skin that hangs from beneath the chin.
Diurnal —Refers to animals that are mainly active in the daylight hours.
Ectothermic —A cold-blooded animal, whose internal body temperature is similar to that of its environment. Ectotherms produce little body heat, and are dependent on external sources (such as the sun) to keep their body temperature high enough to function efficiently.
Monophyletic —Evolving from a single ancestral type.
Thermoregulate —Regulate and control body temperature.
The green iguana is the largest, most prolific, and best-known species in the Americas, and is in great demand in the United States where proud owners can be seen parading this gentle green lizard on their shoulders, restrained in specially designed harnesses. This arboreal (tree-dwelling) lizard naturally inhabits the periphery of rainforests from Mexico to the tip of South America. Green iguanas live in groups near rivers and water holes, and lie along tree limbs high above the water, basking in the sun as still as statues, prepared to plunge if danger approaches. Green iguanas are excellent swimmers, and can remain submerged for 30 minutes, often surfacing in a safer location.
Few iguanas escape the skilled, professional human hunter, however, for apart from their value in the pet trade, their eggs are dietary delicacies, as is their flesh, which is often called “gallina de palo,” or “tree chicken.” Iguana meat is credited with medicinal properties that supposedly cure such conditions as impotency.
While all iguanas have natural predators such as snakes, carnivorous birds, and wild canines, most species are in danger of extinction from human actions— direct capture, habitat destruction, introduction of domestic and feral mammals, pesticides, and firearms. Fortunately, green iguanas are now being successfully bred in captivity for both the food and pet trades. Some conservation efforts for this and other species have been implemented in the form of protective legislation, wildlife reserves, and public awareness campaigns. However, much effort is still necessary to prevent the rapidly increasing destruction of these ancient, docile herbivores.
See also Herbivore.
Alberts, Allison, et al., eds. Iguanas: Biology and Conservation. Berkeley: University of California Press, 2004.
Burghardt, Gordon M., and A. Stanley Rand, eds. Iguanas of the World, Their Behavior, Ecology, and Conservation. Park Ridge, NJ: Noyes Data Corp, 1982.
Harris, Jack C. A Step-by-Step Book About Iguanas. Neptune, NJ: T.F.H. Publications, 1990.
Marie L. Thompson