Iguana

Iguanas can range from 1.5 to 1.8 metres (5 to 6 ft) in length, including their tail. The two species of lizard within the genus Iguana possess a dewlap and a row of elongated scales running from the midline of their necks down to their tails. Iguanas have varying types of scales covering different areas of their body, for example, there are some large round tuberculate scales scattered around the lateral region of the neck among smaller, overlapping scales.[5] The scales on the dorsal trunk of their body are also thicker and more tightly packed than those on the ventral side.[5] These scales may be a variety of colors and are not always visible from close distances. They have a large round scale on their cheeks known as a sub-tympanic shield.[6]

Iguanas have keen vision and can see shapes, shadows, colors, and movement at long distances. Their visual acuity enables them to navigate through crowded forests and to locate food. They employ visual signals to communicate with other members of the same species.[6]

The tympanum, the iguana's eardrum, is located above the sub-tympanic shield (or "ear shield") behind each eye. Iguanas are often hard to spot, as they tend to blend into their surroundings and their coloration enables them to hide from larger predators.[6]

Like most reptiles, an iguana has a three-chambered heart with two atria, one ventricle, and two aortae with a systemic circulation.[7] The muscles of an iguana are very light in color, this is due to the high proportion of fast glycolytic muscle fibers. These fibers are not very vascularized and are low in myoglobin, giving them their pale look. This high density of fast glycolytic muscle fibers allows iguanas to move very quickly for a short period of time, which facilitates short bursts of movement but is inefficient for long duration movement, since cellular respiration in fast glycolytic muscle fibers is anaerobic.

Skull morphology and diet

Iguanas have an exclusively herbivorous diet,[9] as illustrated above by a green iguana eating a mango in Venezuela

Iguanas have developed an herbivorous lifestyle, foraging exclusively on vegetation and foliage.[9] In order to acquire, process and digest plant matter, herbivorous lizards must have a higher bite force relative to their size in comparison to carnivorous or omnivorous reptiles. The skull of the iguana has undergone modifications resulting in a strong bite force and efficient processing of vegetation, according to one study.[10]

In order to accomplish this biomechanically, herbivorous lizards (such as the iguana) have taller and wider skulls, shorter snouts, and larger bodies relative to carnivorous and omnivorous reptiles.[10] Increasing the robusticity of the skull allows for increased muscle presence and increases the ability of the skull to withstand stronger forces.[11]

Green iguana skull and teeth. The teeth of the green iguana sit on the surface of the jawbone, known as acrodontal placement.[12]

Furthermore, the teeth of the iguana are acrodontal, meaning that their teeth sit on top of the surface of the jaw bone[12] and project upwards. The teeth themselves are small and serrated - designed to grasp and shear food.[13]

A phylogeny based on nuclear protein-coding genes, reviewed by Vidal and Hedges (2009) suggested that the subclade Iguania is in a group with snakes and anguimorphs (lizards). These groups share an oral gland capable of secreting toxins (a derived trait).[14] On the other hand, the phylogeny based on whole mitochondrial genomes, proposed by Rest et al. (2003), places the green iguana as the closest relative of the mole skink (Plestiodon egregius).[15] Lepidosaurs are reptiles with overlapping scales, and within this group both iguanians and tuataras (Sphenodons) project their tongue to seize prey items instead of using their jaw, which is called tongue prehension. However, iguanians are the only lineage within Squamata that displays this trait, meaning that it was gained independently in both iguanians and tuataras.[14] Iguanians are also the only squamates that primarily use their sight to identify and track prey rather than chemoreception or scent, and employ an ambush technique of catching prey instead of active searching.[14]

Simple phylogeny with Reptilia highlighted in green. Iguanians are within Lepidosauria.

Extant species

Image	Scientific name	Common Name	Distribution
	Iguana delicatissima	Lesser Antillean iguana	The Lesser Antilles on Saint Barth, Anguilla, Sint Eustatius, Guadeloupe, Dominica, and Martinique
	Iguana iguana	Green iguana	Southern Mexico to central Brazil, the Dominican Republic, Paraguay, Bolivia and the Caribbean; specifically Grenada, Aruba, Curaçao, Trinidad and Tobago, St. Lucia, St. Vincent, and Útila
	Iguana melanoderma[3]	Saban black iguana	Saba and Montserrat

There are several other putative species and subspecies with controversial statuses. Two other species formerly considered distinctive due to their nasal horns, the horned Mexican iguana (I. rhinolopha or I. iguana rhinolopha) and the southern Antillean horned iguana (I. insularis) are now considered conspecific with I. iguana as the presence of horns does not necessarily indicate a new species or subspecies. The two described subspecies of I. insularis (the Saint Lucia horned iguana (I. i. sanctaluciae) and the Grenadines horned iguana (I. i. insularis)) are now considered subspecies of I. iguana, although they are genetically very similar and may not be separate subspecies from one another.[16] However, recent studies have still recovered I. rhinolopha and I. insularis as distinct species.[17]

Iguanas have historically featured in the culinary traditions of Mexico and Central America. Iguana meat is also consumed in parts of the United States and Puerto Rico.[18] Also, the eggs of iguana are consumed in some parts of Latin America, such as Nicaragua and Colombia.[19]

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  Green iguana from the island of St. Thomas
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  Green iguana on St. Thomas
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  Another green iguana
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  A Lesser Antillean iguana in the wild in Dominica
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  An invasive green iguana on Grand Cayman
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  A green iguana at an environmental reserve at La Manzanilla, Jalisco, Mexico
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  A green iguana at Butterfly World, Stellenbosch, South Africa
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  A green iguana at Bannergatta, Bangalore, India
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  A green iguana in Indira Gandhi Zoological Park, Visakhapatnam, India
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  A green iguana from Key Largo, Florida
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  A small green iguana at Serra da Capivara National Park, Brazil

Wikimedia Commons has media related to Iguana.

• Frost, D. R. (1989). "A phylogenetic analysis and taxonomy of iguanian lizards". Pub Univ Kansas. 81: 1–65.
• Frost, D. R.; Etheridge, R.; Janies, D.; Titus, T. A. (2001). "Total evidence, sequence alignment, evolution of polychrotid lizards, and a reclassification of the Iguania (Squamata: Iguania)". American Museum Novitates. 3343: 1–39. doi:10.1206/0003-0082(2001)343<0001:TESAEO>2.0.CO;2. hdl:2246/2896.