Mona Ground Iguana

The Mona Ground Iguana (Cyclura cornuta stejnegeri) is a subspecies of the Rhinoceros Iguana (Cyclura cornuta). It is endemic to Mona Island, Puerto Rico and is the largest native terrestrial lizard in Puerto Rico.


The Mona Ground Iguana is a subspecies of Rhinoceros Iguana belonging to the genus Cyclura which was named by Thomas Barbour in 1937. Its generic name (Cyclura) is derived from the Ancient Greek cyclos (κύκλος) meaning "circular" and ourá (οὐρά) meaning "tail", after the thick-ringed tail characteristic of all Cyclura. The Mona Ground Iguana's specific name, cornuta, is from from the Latin word cornutus, meaning "horned" and refers to the horned projections on the snouts of males of the species. Its subspecific name, stejnegeri was to honor Leonhard Hess Stejneger who described the species in 1902.

There is some debate as to whether this is a valid subspecies and not a different species in its own right. It is known in some scientific circles as Cyclura stejnegeri. Still, others consider it a regional variant of the parent species.

Anatomy and morphology

The Mona Ground iguana is a large bodied, heavy headed lizard with strong legs and a vertically flattened tail which is capable of reaching 1.22 metres in length (from snout to tail). A crest of pointed horned scales extends from the nape of their neck to the tip of their tail. Their color is a uniform gray to olive drab with slight brown or blue colorations. Juveniles differ from adults in that they have gray transversal bands through their bodies. These bands last approximately until they are sexually mature at three years of age.

Males possess bony prominent turbicles on their snouts resembling horns, an adipose pad in the form of a helmet on the occipital region of the head, and a large dewlap. Males of this species, like other species within the Genus Cyclura, are larger than females, and have more prominent dorsal crests and "horns" in addition to femoral pores on their thighs, which are used to release pheromones. Females lack these pores and have shorter crests than the males, making the animals sexually dimorphic.


Mona Ground Iguanas are diurnal and spend most of the day basking in the Sun conserving energy. Mona Ground Iguanas are endemic to Mona Island, Puerto Rico. They are scattered through the entire island, though the Southwest part of the island is only used during the nesting season. They live a considerable portion of their lives underground, and are usually found in talus slopes, caves and sinkhole depressions. The average depth underground that they can be found is 1.5 metres.


Although Mona Ground Iguanas use the whole island as their habitat, only 1% of the territory located on the southwest coast is suitable for nesting because it contains loose sand and receives direct sunlight. The females bury their eggs in the sand and the sunlight incubates the eggs. Males reach sexual maturity at a size of 28-31 cm in length from snout-vent, usually in their third to fourth year while females mature one year later and at a size of 35-40 cm.

Nesting season begins in the second week of June. Usually one female mates with more than one male in the 2 weeks that the mating season lasts. Copulation may last from 15 seconds to 2 minutes and 15 seconds. One month later, nesting begins. Females will dig a long tunnel located 1-2 feet underground where they deposit from 5 to 19 eggs, with 12 being the average. They will guard the nest for several days, but provide no parental care for the hatchlings, which hatch three months later. Hatchlings measure, on average, 32 cm and weigh 73.7 grams and grow at a rate of 5.23 cm per year.


Mona Ground Iguanas, like most Cyclura ssp are primarily herbivorous, consuming leaves, flowers, berries, and fruits from different plant species. A study in 2000 by Dr Allison Alberts of the San Diego Zoo revealed that seeds passing through the digestive tracts of Cycluras germinate more rapidly than those that do not. These seeds in the fruits consumed by cycluras have an adaptive advantage by sprouting before the end of very short rainy seasons. The Mona Ground Iguana is also an important means of distributing these seeds to new areas (particularly since females migrate to nesting sites) and, as the largest native herbivores of their ecosystems, they are essential for maintaining the balance between climate and vegetation. Their diet is very rarely supplemented with insect larvae, crabs, slugs, dead birds, and fungi; individual animals do appear to be opportunistic carnivores. Fewer than a dozen animal species and 71 plant species are found in the Mona ground iguana's diet. Mona Ground Iguanas eat the caterpillar of sphingid moths. These larvae feed on poisonous plants and are aposematically colored and avoided by other predators.

Endangered status

Population numbers are estimated at 1,500 with lower densities than similar iguana-inhabited islands in the West Indies. Immature iguanas are scarce and represent only 5 - 10% of the population, revealing that the population is aging and in decline.

Reasons for decline

Feral pigs pose the most serious threat as they root up iguana nests, and like most Cyclura species, the Mona Ground Iguana nests communally and at high density. Introduced goats and pigs are a major competitor for food and overbrowsing by goats also leads to loss of protective cover from birds of prey such as the osprey and predation of juveniles by feral cats.

Recovery efforts

A headstarting program was put into place by the Puerto Rico Department of Natural and Environmental Resources with cooperation from the IUCN Iguana Specialist Group, the US Fish and Wildlife Service, the Toledo Zoo, and the University of Puerto Rico in 1999 to aid in the recovery of the Mona Ground Iguana. From within the safety of this program, the iguanas are reared until they are large enough to survive in the wild and predators such as the pigs and feral cats are no longer a threat. The Headstart facility also carries out health screening prior to the release of specimens. This health screening has been used to baseline the normal physiologic values of the species, identifying potential future problems due to parasites, diseases, etc which might threatan the population.

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