Mona Island Ground Iguana

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Mona Island Ground Iguana

Cyclura stejnegeri

ListedFebruary 3, 1978
FamilyIguanidae (Lizard)
DescriptionLarge iguana with a series of rings set close together in the tail and a ridge of flexible, spinelike organs running down the back; the head is covered, at least in part, with larger scales or tubercles; the dorsum is patternless and dark gray, grayish-brown, or black.
HabitatMaritime climates where the humidity levels are high and temperatures are mild and fairly stable.
FoodFruit, flower buds, and new green leaves.
ReproductionClutch of five to 19 eggs hatched early in the fall.
ThreatsHunting, habitat loss, predation by exotics, and pet trade.
RangePuerto Rico


Cyclura stejnegeri is a large iguana (snout-to-vent length, or SVL, in males to 18.2 in [46.3 cm], in females to 20 in [51.0 cm]), also known by the name Mona Island ground iguana. Its dorsum is patternless and dark gray, grayish-brown, or black. Juveniles are very dark and virtually patternless but have about nine pale crossbars on their sides, separated by dark gray areas (often series of gray blotches) extending onto the undersurface of the abdomen; hind limbs are not ocellate. Female Mona Island ground iguanas are strikingly similar to males in outward appearance. They have equivalently well-developed cephalic horns, a mid-dorsal crest, and a dewlap (throat fan). Older females also have large bodies and enlarged, sagging jowl musculature.


C. stejnegeri suns primarily in the morning and late afternoon, basking on exposed lookouts. These iguanas are usually very shy, retreating into the den when disturbed, but they can become more tame when exposed daily to human intrusion. They will enter the water as an escape tactic. Males select territories with retreats attractive to females; females select retreats according to location and structure. Preferred retreats have good drainage, ventilation, a nearly horizontal resting place under a low ceiling, and are within 5 ft (1.5 m) of the surface.

Challenges over mating territory are resolved through fights between males, which are highly formalized tests of strength. These face-to-face pushing matches, performed with mouths wide open, usually last less than 15 minutes and end without violence when one or both contenders back away slowly in retreat. At times, however, battles are longer, lasting up to an hour, and involve biting and tail lashing.

The Mona Island ground iguana eats fruits (especially Opuntia [Cactaceae]), Hippomane mancinella (Euphorbiaceae) leaves, flowers, and some animal matter (especially Pseudosphinx tetrio caterpillars and pupae). Feeding is not a daily requirement; about 95% of the normal daily routine is rest, with or without basking. The species prefers plants that are low in cellulose, low in aromatic compounds, and easy to obtain. The diet of juveniles is like that of adults except for size of items eaten. A large proportion of Mona Island's native trees and shrubs bear fruit under 0.6 in (1.5 cm) in diameter, which the iguanas prefer over leaves. When readily available, fruits are invariably taken to the near exclusion of leaves. In spite of their seasonal occurrence, fruits are the principal plant foods contributing to growth, reproduction, and fat reserves. However, these iguanas will consume mice in captivity.

Mating occurs annually during a two-week period. Males lie in wait for the female located in his territory 6-10 ft (1.8-3 m) from her retreat, with his eyes fixed on the burrow entrance. When she appears, he displays a series of toss-roll head movements, dashes to her with his head lowered, pauses briefly by her side for another series of head movements, and then grabs her tail or loose flank skin to restrain her. Unwilling females may turn and bite or, more commonly, struggle into a crevice, dislodging the male. Females store sperm, and eggs are presumably fathered by more than one male since she may copulate with several males. Eggs are deposited about one month after mating and are warmed by direct sunlight in a tunnel about 3 ft (0.9 m) long. Clutch size is five to 19 soft-shelled eggs. Nests may be guarded by the female, as she returns to the nest one to ten days after depositing the eggs to inspect the site and stand guard against intruders (including other females searching for nest sites). Defense against persistent intruders includes head rolling, broadside challenge, huffing, biting, tail lashing, and delivering sand back into the rival's hole and on the rival. The length of the iguana nesting season on Mona Island is short. The island's climatic regime appears to determine how late in the year an iguana can nest successfully. Hatching normally occurs about three months after laying, in October and early November, the two wettest months of the year, providing optimal conditions for juvenile emergence, feeding, and dispersal before the abrupt onset of the annual dry season in January.


The terrain of Mona Island is marked by sharply weathered limestone, intense heat and glare, and aromas of sea, frangipani blossoms, and pungent shubbery. The island is actually a vast, bean-shaped slab of sedimentary rock situated midway between Puerto Rico and the Dominican Republic. Almost 95% of Mona's 13,600-acre (5,503.7-hectare) area is a relatively homogeneous, slightly domed, and undulating tableland, limited around the windward, northern perimeter by sheer cliffs that drop some 150-280 ft (45.7-85.3 m) to the sea. The cliffs of the south side are steep but break away in many places as talus slopes that descend to a narrow coastal terrace 10-12 ft (3-3.5 m) in elevation. Rainfall averages 32 in (81 cm) annually, but because of its highly permeable limestone substrate and general lack of soil, Mona has no watershed and only supports a slow-growing forest of xerophytic shrubs and trees. Surface depressions in the rock, a few of which exceed 3 ft (0.9 m) in diameter, catch and hold rainwater for periods ranging from hours to, occasionally, months. Water also collects seasonally behind rimstone dams and in some of the caves. The scarce accumulations of soil on the plateau are confined to small, shallow potholes and sparse sinkhole depressions. The coastal terrace is mantled with a thin, spotty layer of sandy soil.


The iguana population on Mona Island, approximately 1,500-2,000, is currently smaller than the habitat could support but appears to be reasonably stable. Although predation on some nesting sites by pigs is currently controlled by fencing, suitable nesting areas appear limited.


Threats to this taxon include feral pigs that prey on eggs, and feral cats, significant predators on hatchlings. Goats are keeping favored food plants from reproducing. Although hunters do not appear to be a serious threat, they do lobby for keeping large feral pig and goat populations on Mona for easy sport hunting.

Conservation and Recovery

The Chelonia-Herpetological Society of the Universidad Metropolitana, in cooperation with the Puerto Rico Department of Natural Resources, has begun a small-scale nest site restoration project. At present, captive husbandry should be secondary to protection/restoration of wild habitats. More intensive wildlife management, especially implementation of a cat control program, is needed. Mona Island has been declared critical habitat for the Mona Island ground iguana, the hawksbill turtle, the Mona boa, and the yellow-shouldered blackbird, all of which are on the U.S. Fish and Wildlife Service list of threatened and endangered species.


Boquerón Ecological Services Field Office
Boquerón, Puerto Rico 00622-0491
Telephone: (787) 851-7297
Fax: (787) 851-7440

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
(404) 679-4000


U. S. Fish and Wildlife Service. 1984. "Mona Iguana Recovery Plan." U. S. Fish and Wildlife Service, Atlanta.

Wiewandt, T. A. 1973. "Mona Amphibians, Reptiles, and Mammals." In Mona and Monita Island: An Assessment of Their Natural and Historical Resources. Environmental Quality Board, Office of the Governor of Puerto Rico, San Juan.