Although they live in small groups in the wild, iguanas are not considered social animals. They do, however, communicate with each other in a number of ways, most of which would be classified as body language. These include head bobbing, hissing, changes in the dewlap, movement of the tail, and body positioning and movement, among others.
Normally, the dewlap hangs beneath the chin, and may have some folds in it. If it is tucked up under the chin, it is often a sign of submission. If it is rigid and distended, the iguana is demonstrating either fear or a warning signal. When combined with compressing an upright body and raising the crests along the neck and back, the iguana is trying to appear as intimidating as possible. The dewlap is also used during courtship by the male to show he is a prime candidate for mating.
In a normal relaxed state, the body of an iguana appears rather round. If an iguana feels threatened or is aggressive, the iguana will basically pull in his sides to make his body appear taller and larger. This behavior is generally accompanied by an erect posture, with the iguana standing tall on his feet.
Head bobs may be used to provide a number of social cues. A slow head bobbing may be used to greet other iguanas (and people). If the behavior is accompanied by a change in body posture such as raising the body and flaring the dewlap, the bob may become more of an advertisement that the iguana is concerned about his territorial rights. If the bobbing is more rapid, the iguana is trying to indicate that he is becoming more annoyed or is on the offensive.
A combination of hissing and clicking is made by iguanas with their mouths wide open. This behavior signals a warning or defensive posture. It is often accompanied by distention of the dewlap, compressing the body laterally to make it appear taller and provide a more intimidating silhouette, and tail thrashing.
If an iguana is really upset, he will lash his tail. Sometimes, however, he will simply twitch the end of the tail. This is often interpreted as a state of mixed emotions, with the iguana uncertain as to what to do.
Whether the iguana holds his eyes open or shut can also tell you something. Iguanas may shut their eyes if they are very relaxed, or may shut them if they are in sensory overload. Open, glaring eyes can be interpreted as just that - a glare from an iguana who is not happy.