Several species of lizards, including the iguanas, have a pale scale towards the back of their heads marking the parietal eye.

[16] 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.

[17] To accomplish this biomechanically, herbivorous lizards have taller and wider skulls, shorter snouts, and larger bodies relative to carnivorous and omnivorous reptiles.

[21] A female can store sperm from previous mates for several years to continue to fertilize her eggs in case she finds no male within her territory when she is ready to lay again.

Males compete for the females in an area and mark their won territory with a pheromone secreted from the femoral pores on the dorsal side of their hind limbs.

Male behavior during sexual competition involves head bobbing, extending and retracting their dewlap, nuzzling and biting the necks of females, and on occasion, changing color.

[citation needed] 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).

[25] The phylogeny based on whole mitochondrial genomes, though, as proposed by Rest et al. (2003), places the green iguana as the closest relative of the mole skink (Plestiodon egregius).

[25] A study by Breuil et al. (2020) found the taxonomy of the genus Iguana as follows, with I. delicatissima being the most basal member of the group.

Four subspecies of green iguana are recognized under this treatment: I. i. insularis (Saint Vincent & the Grenadines and Grenada), I. i. sanctaluciae (Saint Lucia), I. i. melanoderma (parts of the northern Lesser Antilles, and potentially coastal Venezuela, the Virgin Islands, and Puerto Rico), and I. i. iguana (mainland South America).

[29] The Curaçao population of green iguanas shows major genetic divergence and may also represent an as-of-yet undescribed species or subspecies.