Baby iguanas are often thought by their owners to not have dewlaps, as the owners never see them for the first several weeks. Once the hatchlings are more comfortable around humans, they relax enough to let them down. Other hatchlings, on the other hand, are quite healthy and feisty, and their little dewlaps are rigidly extended whenever humans look their way. Dewlaps rigidly extended is a threatening gesture. It is effective in warning off some predators, other iguanas venturing into their territory, and many humans.
Larger iguanas have larger dewlaps.
When at rest and relaxed, the dewlap may hang down, flopping on the branch or surface on which the iguana is lying when he performs an acknowledgment bob. The relaxed dewlap will hang loosely in drapey folds, quite different from the rigidly extended, flared-out dewlap. In the wild they would do this to another iguana acting aggressively. By shutting it out, it signals that it is not interested in returning the aggression. Out of sight, out of mind.