Male iguanas have large jaw muscles that look like jowls and two fatty deposits on their foreheads, making male iguanas’ heads bigger and blockier than females' heads. A male also has a larger, heavier body and a bulge near the base of his tail where his reproductive organs are located.
Male iguanas also have very noticeable femoral pores on their inner thighs. It takes 18 to 24 months before an iguana is mature enough to determine if it is a male or a female. Once the iguana has reached 6 to 10 inches in length from snout to vent, secondary sexual characteristics appear.