The best way to determine the gender of iguanas is by examining their femoral pores; but their head structure, dewlaps and coloration may provide clues as well. While different iguana species exhibit slightly different secondary sexual characteristics, most mature iguanas are rather easy to sex. If you cannot wait two or three years for your lizard to mature, your veterinarian can determine the gender of your lizard rather easily.
The Technicalities of Terminology
When used by herpetologists, the term “iguana” or “iguanid” may refer to any of approximately 35 species that compose the family Iguanidae. This family includes the iconic green iguanas (Iguana iguana), desert-dwelling chuckwallas (Sauromalus spp.) and desert iguanas (Dipsosaurus dorsalis). However, lizard keepers and reptile enthusiasts usually reserve the term for species that attain large sizes, such as rock iguanas (Cyclura spp.), spiny-tailed iguanas (Ctenosaurus spp.) and green iguanas. You can determine the gender of most of these species by observing their femoral pores and head characteristics.
Boy Parts and Girl Parts
All iguanas have a row of small spots on the undersides of their rear legs, called femoral pores. Males have very large, obvious pores; females have small, indistinct pores. The femoral pores of males often bear gray or white keratinous plugs. The exact function of these pores remains unclear, but they likely help the males to scent-mark their territories. Additionally, male iguanas have two copulatory organs, called hemipenes, which are inside their vents when not in use. These structures are usually visible as two small bulges on the ventral side of the tail base. Females lack hemipenes and have smooth tail bases.
Mature male iguanas usually have larger heads than their female counterparts do. Males often develop massive jowls, which make their jaws and throat areas look larger than the females’. When viewed from below the bottom jaw, the jowls of males are very apparent. Most large iguana species possess a flap of skin below their bottom jaw called a dewlap. These structures -- which are often larger in males -- allow the lizards to communicate with conspecifics and to deter predators.
Probing for the Impatient
Unfortunately, it is difficult for hobbyists to determine the gender of immature iguanas until they have reached sexual maturity. For males this occurs when they reach about 6 inches from snout to vent. Females generally become mature when they reach 10 inches from snout to vent. Your veterinarian can determine the gender of an iguana before maturity by inserting a lubricated, stainless steel probe in the animal’s vent. If the animal is male, the probe will slide into the inverted shaft of one of the hemipenes and therefore penetrate much farther than it would penetrate in a female. Probing is harmless when performed by experienced hands, but hobbyists should always have a veterinarian conduct the procedure.
Various species of iguanas -- and populations within species -- exhibit other sex-based distinctions. The males of many green iguana populations exhibit bright gold or orange colors during the breeding season. Many mature male iguanas have bold, aggressive personalities, while females are often more timid. These bold personalities are especially pronounced during the breeding season, when the males may provide another clue to their gender, by shedding U-shaped seminal plugs.