According to the Green Iguana Society, iguanas typically bob their heads to assert themselves and lay claim to their territory. A slower bob means they simply want their presence to be known, while a faster bob may signify aggression and agitation.
Melissa Kaplan, author of "Iguanas for Dummies," explains certain characteristics of iguanas that may influence them to bob. For example, male iguanas bob their heads more often as there are more situations in which they might feel inclined to do so. They bob when greeting their owner, when they feel neglected or even when taking a bath. There are also several different types of bobs; these include straight up-and-down bobs or side-to-side bobs.