Iguana iguana
common green iguana
(Also: green iguana)

Green iguanas, Iguana iguana, occur throughout Central and South America, from Sinaloa and Veracruz, Mexico, south to the Tropic of Capricorn in Paraguay and southeast Brazil. This large lizard also inhabits many islands throughout the Caribbean region and the coastal eastern Pacific, and has been introduced into southern Florida and in Hawaii. This is the largest known lizard to occur within the borders of the United States (Conant and Collins, 1998; Campbell, 1998). (Campbell, 1998; Conant and Collins, 1998)

Green Iguanas are arboreal lizards that live high in the tree canopy. Juveniles establish areas lower in the canopies while older mature iguanas reside higher up. This tree dwelling habit allows them to bask in the sun, rarely coming down except when females dig burrows to lay eggs. Although preferring an arboreal (forested) environment, they can adjust well to a more open area. No matter where they inhabit, they prefer to have water around as they are excellent swimmers and will dive beneath the water to avoid predators (Conant and Collins 1998). (Alberts et al., 2004; Campbell, 1998; Conant and Collins, 1998)

Within three years, a young, 12 gram hatchling iguana can become a 1 kg adult (de Vosjoli, 1992). Upon hatching, the length of green iguanas ranges from 17 to 25 cm. Most mature iguanas weigh between 4 and 6 kg, but some in South America, with proper diet can reach up to 8 kg. These large lizards can reach head to tail lengths of around 2 m.

Although called green iguanas, these animals are actually variable in color. The adults become more uniform in color with age, whereas the young may appear more blotchy or banded between green and brown. Color of an individual may also vary based upon its mood, temperature, health, or social status. Such color alteration may aide these animals in thermoregulation. In the morning, while body temperature is low, skin color will be darker, helping the lizard to absorb heat from sunlight. However, as the hot mid-day sun radiates upon them, these animals become lighter or paler, helping to reflect the sun rays and minimizing the heat absorbed. Active dominant iguanas usually have a darker color than lower-ranked iguanas living the same environment (Frye, 1995). Most color variation seen in this species is exhibited by males, and may be attributed in part to sex steroids. Six to eight weeks prior to and during courtship, males may acquire a bright orange or gold hue, although coloration is still related to dominance status (Frye, 1995). Mature females, for the most part, retain their green coloring.

Other distinguishing features of this species include a pendulous dewlap under the throat, a dorsal crest made up of dermal spines that run from the mid neck to the tail base, and a long tapering tail. The dewlap is more developed in adult males than females. Extensions of the hyoid bones stiffen and support the leading edge of this structure, which is used in territorial defense or when the animal is frightened. This fleshy structure also serves in heat absorption and dissipation when it is extended.

The laterally situated eyes are protected mainly by a immovable eyelid and freely mobile lower eyelid (Oldham and Smith, 1975). On the dorsal midline of the skull behind the eyes is a parietal eye. This sense organ, although not a true "eye," serves as a meter for solar energy, and aids in the maturation of sex organs, thyroid gland, and endocrine glands (Frye, 1995). The visual effect of this "eye" is mostly limited to the detection of predatory shadows from above.

The scales or plates on the head are larger and more irregular than the scales on the rest of the body. Below the tympanum there is a large rounded scale called the subtympanic plate. (De Vosjoli, 1992; Frye, 1995; Oldham and Smith, 1975)

Approximately 65 days after mating, a female is ready to lay her eggs. The size and number of eggs produced varies depending upon her size, her nutritional status, and her maturity. Eggs measure around 15.4 mm in diameter, and 35 to 40 mm in length (Frye, 1995). Over a three day period, an average of 10 to 30 leathery white or pale-cream colored eggs are deposited into a nest. Nests are located 45 cm to more than a meter deep, and may be shared with other females if nesting areas are limited. After laying the eggs, females may return to the nest several times but do not stay to guard it.

Incubation lasts from 90 to 120 days. Temperature should range from 85 to 91 degrees Fahrenheit. The hatchlings pip the egg open using a special egg tooth, called the caruncle, that falls off shortly after hatching. Absorbed yolk provides most of the nourishment for the first week or two of an iguana's life.

There are no major morphological changes in these animals as they age, except that they grow. However, diet is related to age. The young, with higher need for protein, are more likely to consume insects and eggs than are mature individuals. (Frye, 1995; Kaplan, 2002)

Iguanas can live for more than 20 years in captivity, although wild iguanas are thought to live only about 8 years. Proper nutrition for growth is a concern for captive management of these animals. Improper housing and nutrition can shorten a captive iguana's lifespan. (De Vosjoli, 1992; Frye, 1995)

In the wild, most disputes between iguanas take place over basking sites. There is usually adequate food for these herbivorous lizards, but good perches are limited. Basking is important for increasing body temperature and aiding digestion.

During the breeding season, males become territorial and display head bobbing, dewlap extension, and color changes. They will bite at each other. Injuries in the wild are rare, as there is ample space for males to retreat when threatened. However, in captivity where space is limited, injuries are more common. Females may also display some of these behaviors when nesting sites are limited.

Green iguanas may travel considerable distances in several cases. Females migrate to the same nesting site for several years in a row, then travel back to their home territory once their eggs are laid. Hatchlings may disperse over large distances as well (Alberts et. al., 2004).

When frightened, an iguana will usually freeze or hide. If caught, twisting and rotating around or tail whipping may occur. Like many other lizards, iguanas can autotomatize, or drop of part of their tail. This gives them a chance to escape before their predator figures out what is going on. A new tail will sprout from the autotomatized spot and regrow with in a year, though not to the length it was before. (Alberts et al., 2004)

These animals are known to use visual signals, such as head bobbing and dewlap extension, as means of communicating with rivals. In extreme cases, physical contact is involved in altercations. In addition, males scent mark females as well as branches. Hissing, which is a form of auditory communication, sometimes occurs.

Green iguanas are primarily herbivorous. They occasionally eat a small amount of carrion or invertebrates. Green leafy plants or ripe fruits are their preferred foods.

Green iguanas use their tongues to help manipulate the food and bite small enough pieces to swallow, with little or no chewing. The food mixes with enzymes in the stomach before moving to the small intestine where pancreatic enzymes and bile are mixed with it. Most digestion occurs in the sacculated colon, where microflora break down the cellulose (Frye, 1995). Microflora are essential for hind-gut digestion of the hard to digest diet of this species. Hatchling iguanas are inclined to eat feces from adults, which may be an adaptation for acquiring this much need microflora (Alberts et.al., 2004). This microflora breaks the food down and makes it available for absorption.

Iguanas require a high amount of dietary protein in their first two to three years for adequately fast growth. During this time period, young iguanas may consume insects and spiders. Older iguanas that have reached close to maximum growth consume a low phosphorous, high calcium, leafy diet for their maintenance requirements.

Iguanas are exothermic. Their body temperature is mainly dependent upon the environmental temperature. Low environmental temperatures inhibit an iguana's appetite and digestive enzymes. Active eating usually occurs when the environmental temperatures are between 77 and 95 degrees farenheit (Frye, 1995). Basking is an important aid to digestion. Iguanas may cease eating prior to or during skin shedding. Females may refuse to eat during later stages of egg development. Individuals who are overly stressed or in a new environment may also refuse to eat. (Alberts et al., 2004; Frye, 1995)

One of the best methods for iguanas to avoid predation is their cryptic coloration. Because they look like so much of their green environment, they can remain immobile when a predator has been spotted, and go unnoticed themselves. Young iguanas may be found in small groups, and use the "selfish-herd" or "more eyes are better" strategy to avoid predators. Iguanas prefer to bask in tree limbs that over-hang water so when threatened by a predator they can dive into the water and swim swiftly away. In addition to these strategies for avoiding predation, green iguanas are able to shed a large portion of their tail, thus distracting predators and allowing the "rest" of the animal to escape.

Hawks and other large birds are potential predators of juvenile iguanas. Humans are another one of major predators of green iguanas. Humans eat both iguanas and their eggs. Humans also use these reptiles for crocodile bait, and poach them for the pet trade.

Like many other animals, green iguanas also suffer from habitat destruction.

In addition to helping disperse seeds, iguanas provide a source of food for larger predatory animals, including humans. Like other amphibians and reptiles, iguanas can be indicators of environmental changes (Kaplan, 2002). Reptiles are more sensitive to environmental changes than are humans, and by watching their responses, we can be alerted to possible problems before they are large enough for us to detect with our own senses. (Kaplan, 2002; Phillips, 1990)

The most adverse effect green iguanas have on humans would be eating exotic tropical foliage in gardens. They do not pose any major problems for humans.

Iguanas are farmed in some countries as a source of food and leather, as well as for the pet trade. Due to their large size, iguana hides provide a source of luxury leather that can be made into boots, belts or purses. The pet industry also prizes iguanas; most are sold in the United States, Europe, and Japan. Iguanas also make an interesting tourist attraction in resort areas.

Exploitation of iguanas has resulted in marked declines in their numbers in some parts of their range. (Campbell, 1998). (Campbell, 1998)

Although some populations have suffered from poaching and collection for the pet trade, green iguanas are not considered a conservation risk at this time. All Iguana species are listed under CITES Appendix II.