Geographic range: Neotropical: Ctenosaura similis is found from Veracruz and Oaxaca in Mexico through Central America (Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, Panama), and islands adjacent to Panama. Certain sources suggest presence of the species in Colombia.

Physical characteristics: Large, bulky lizard with adult males reaching up to 45 cm in length with a tail the size of the body. Body is mostly black in color, but the dorsal surface may show black bands on a greyish background.
The color may also ligthen after basking in the sunlight with yellowish and orange markings becoming evident along the sides. Adult males and females are dimorphic. Adult males have well developed dorsal crests and small dewlaps. The dewlap, the crescent of skin that can be extended under the throat, is not inflated. A small bone bows out to extend the dewlap during times of threat, courtship, or while defending territory. Females lack obvious crests. There is considerable variation with age and sex and therefore identification may be difficult. The lizards have tails ringed with rows of sharp, curved spines, hence the name spiny-tailed iguana. The spines down the back are short. Juveniles tend to be olive-green becoming tan and then finally greyish as they grow.

Food habits: Generally herbivorous, but is also known to have a diverse carnivorous diet that consist of small animals. Ctenosaurs have eaten rodents, bats, frogs, small birds, and a variety of insects. They have even been noted to eat eggs of their own young, and in one case, the tail of a juvenile was found inside an adult male, suggesting cannibalism. Youngsters are primarily insectivorous, switching into herbivorous habits as adults

Reproduction: Sexual maturity is reched at around 3 or 4 years of age. Timing of mating can vary in between populations. Males have a pair hemipenes, which when not in use, lie adjacent to the cloaca within the base of the tail. During mating one hemipenes is everted by the action of muscles and fills with blood. In copulation, which follows courtship behavior, only a single hemipenis is inserted into the female's cloaca, and the sperm travel along a groove in the hemipenis. After mating, convenient nest site is looked for by the females. A borrow is dug out, and up to 30 eggs are deposited. The young iguanas hatch 3 to 4 months later and then take about a week to dig their way out of the nest.

Behavior: More terrestrial than arboreal, C. similis live in colonies. Territories inside the colony are defended by the males. Territorial displays involve color changes, body inflation, jaw-gaping, "push-ups" or rapid nodding of the head, and sometimes, biting and tail thrashing battles. Larger males usually hold bigger and better territories and they mate more often. Combat often occurs when iguanas are attaining or defending a territory or a mate. Young iguanas often remain in groups with one of them temporarily behave as the leader. They engage in mutual tongue-licking and grooming, body and chin rubbing. At night they often sleep together on branches.

Habitat: Can be found around ruins, stone walls, rocky open slopes and branches of large trees along the open borders of the forests. They generally live in dry, arid, open terrain.

Biomes: tropical rainforest, tropical deciduous forest, tropical scrub forest, tropical savanna & grasslands

Conservation: Populations are threatened by habitat destruction and ilegal trade.