External Body Parts

Skin and Scales:
An iguana's skin are designed to specifically retain moisture. The scales are individual, thickened skin cells in the outer layer of skin and are mostly keratin, a hard waterproof protein. The folds between scales have very little keratin to make it flexible.

Spines:
Spines or spikes as we commonly refer to it are merely keratinized skin cells. The spines are broken into 3 sections, the nuchal spines - neck spines, dorsal spines - spines on the back and the caudal spines - the tail spines. They also have a crest of spines along the lower edge of their dewlaps. The spines comes in all shapes and sizes, it can be curved, straight, long or short, it all depends on what region or country the iguana comes from. Broken spines in a healthy iguana can be regrown if the damage was not too deep.

Head.

Jowls:
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The jowls are located at the iguana's jaw line and feels soft like a pillow. The jowls becomes more prominent on adult males who are sexually mature. Jowls are intended to make the iguana look bigger and they can enlarge them as a defensive or combat display. The iguana with the biggest appearance usually win all their mock fights (fights resolved by body language alone). They dont really want to fight and get injured so they often swagger, puffup and extend body parts to intimidate their opponents.

Cheek Scales:


A single, large cheek scale is located just below the exterior ear on each side of the iguana's head, near the corner of the jaw, directly on the jowls. It is called the subtympanic shield of subtympanic plate, another common name is the helmet scale. It has iridescent shine like the shell of an abalone and is often outlined in black which makes it more distinct. Some biologists speculate that it is used as a 'bluff' eye to ward off predators. When the iguana gets into protective mode and extends its dewlap, the cheek scales becomes more prominent and can possibly look like a large eye to a predator.

Eyes:

The eye colour ranges from gold or orange to brown or grey. The lower eyelid has what looks like a line of mascara on it which help to reduce the sun's glare. Iguana eyes are different from human eyes, we have both cone cells - colour detectors - and rod cells, which are extremely sensitive to light but dont detect colour. Our daytime colour vision isn't as good as if we had only cones in our retinas.

The retinas of iguanas' eyes contain large numbers of cone cells and double cone cells, both of which are used to perceive colour and contrast. Scientists assume that iguanas see colours more vividly than humans do. While they have excellent full-colour daytime vision, they dont see well at night or in dim light. Iguanas can detect ultraviolet light.

Because the eyes are located on the sides of the head they only have a small field of stereoscopic vision - an area where they see an item with both eyes at once. Iguanas' peripheral vision is good but they will turn their head to point one eye directly at movement or an object of interest to get a better look. Iguanas have limited depth perception so they most often watch things with only one eye at a time.

Eye movement:
Apart from an occasional loud exhale iguanas do not make sounds. They rely on body language to communicate and this is one reason why iguanas are so visually oriented, continually scanning their environment for cues. The eye is a stimulus to an iguana.

When iguanas rest their eyes their eyelids meet in the middle, when they are in a deep sleep the bottom eyelid comes up to the top of the eye. They also have an extra eyelid called a nictitating membrane that is on the inner side of the eye and helps keeping the eye clean and moist.
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When an iguana see something that they do not like or are unfamiliar with they will close the eye facing the object. If you pet your iguana on the head he will also close the eye closets to you, however if he closes both eyes then you know that he is truly enjoying it.

On occasion an iguana bulge its eyes for a second or two - it looks as if it is going to pop out, it is a way of relaxing the eyes. When my iguana does it I often hold my hand and then he rubs his eye against me as if he is trying to clear something off the lens.

Parietal (Third) eye:

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Iguanas have a third eye that are located in the centre of the top of their heads between the parietal bones of the skull. It has a working lens a few tenths of a millimetre across and can only detect light and darkness. While basking in the sun the iguana literally keep one eye on the sky, detecting the shadows of possible bird predators.

It is a true functioning eye, it has a lens and a retina beneath it. Light hitting the retina sends signals to a parietal nerve, which leads into the parietal foramens - a hole at the top of the iguana's head bones. The nerves signals then travel to the pineal body on the brain and then to the brain stem.

Scientists did tests on the parietal eye by covering it up and then documenting the reaction. The iguanas moved, eat and acted like normally but they did not seem to know when to get out of the sun. From these tests, researchers confirmed that the third eye acts as a kind of dosimeter-a way of 'counting' the amount of sunlight the iguana has absorbed and when enough is enough. They parietal eye is crucial to let the iguanas know whether to stay in the sun or move to the shade.

The third eye is also believed to affect a number of hormonal systems, including the thyroid and the pituitary glands, which direct a wide range of activities and behaviours in iguanas. You can do more research about the third eye by reading Richard M. Eakin's book called the The Third Eye (University of California).

Nose and Nostrils:

The nose or snout is located at the extreme front of the head. Inside the nostrils is a nasal chamber with two separate functions. It stores accumulated liquid from the iguana's salt glands until the iguana snorts the liquid out, and it also is responsible for smelling.

Many plants contain large amounts of potassium which iguanas dont need and their kidneys are not designed to get rid of it efficiently. Instead, the iguana has a rhinal (nose) salt gland that acts as a back-up kidney. It is specialized to remove excess amounts of sodium, potassium, bicarbonate and chloride - salts. The ducts of the salt gland drain into the nasal chamber as salt-filled fluid and when a pool of this fluid has collected the iguana simply snorts it out.

Ears:
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Unlike humans, Iguanas have external ear drums. The ear drum or tympanum, appears as a thin, hard, plastic-like covering just above the jowls and cheek scale.

Iguanas have excellent hearing and they will turn their attention to even the faintest sounds. Humans are most sensitive to frequencies between 2,000 to 5,000 hertz range. Iguanas hear best between 500 and 3,000 hertz. Their hearing are tied to their body temperature, hearing best when they are at their 'ideal' temperature, below or above their optimal temps the iguana's hearing diminishes, especially in response to high tones.

Fatty deposits - male:
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Mature males have 2 fatty deposits on the head - just before the neck starts. This is also a characteristic that makes the male appear bigger.

Dewlap:
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The dewlap has multiple purposes, ranging from temperature control to communication. Blood circulates through the dewlap enabling the iguana to extend it on hot days to catch a cool breeze and cool down its body. It also helps the iguana to warm up by extending the dewlap and aligning the body at a right angle to the sun, the dewlap then acts as a great solar collector.

The male's dewlap is usually larger than the female's but both use it for communication. It is used for greeting and as part of mating and territorial display.

The dewlap has a hyoid bone made of cartilage which has some branching cartilage that supports the tongue. It is a delicate and important mechanism and therefore some leashes and harnesses are not recommended for iguanas because they could possibly damage this supporting network.

Trunk.

Neck:

An iguana's neck area is protected from attackers by a thickened, fleshy ridge; by spines that are the longest on the body; and by protective scales (tuberculate scales) that look like small rivets. These 'rivets' also add another level of physical texture and beauty to an already spectacular creature.

Arms and Legs:

Iguanas also use their arms and legs for communication, they will often flick an arm back as if to tell you to leave him alone. They also use their arms and legs to scratch themselves. A complete relaxed iguana throw the arms back along the sides, palms facing upward and kick the legs straight back along the tail.


Hands and feet:

The fingers do not work independently like a humans, they are more like scaly grappling hooks.

Claws:
Iguanas claws are sharp and pointed, intended for climbing. The claws are created from keratinized skin cells and the upper side of the claw is stronger than the underside. As the claw grows the tougher upper portion of the claw curves over the weaker underside, giving the iguana's claw its distinctive shape.

Vent:


Inside the vent is the reproductive organs of both the female and the male.

Femoral Pores:

On the inside of the iguana's thighs is a row of about 12 to 20 large pores called the femoral pores - it runs along the femur bone. Males in breeding season often develop spinelike 'spurs' that protrude and exude a soft waxy substance that carries a pheromone, or scent, produced by glands under the skin. They drag their legs - leaving traces of the pheromone to mark their territory or advertise their position in the social structure. This secretion is fluorescent and varies in colour from species to species.

Tail:
On average the tail length is about 60 to 67 percent of the total length of the iguana. It is used for balancing when climbing, as an undulating propellant while swimming, and as a formidable whip-like weapon. Full grown adults are capable of breaking the legs of a dog with one swift strike. Iguanas can drop their tail as a defence mechanism. Young iguanas can regenerate their tails because several of the tail vertebrae have fracture planes where the vertebrae can snap relatively easy. In older iguanas these fracture planes begin to fuse and the iguana loses the ability to easily sacrifice the tail. When an older iguana loses a part of the tail some regrowth takes place, but the new tail is dark, often black and it never reaches its original full length. If the break was not clean then the tail may even grow back at an odd angle.