Rhinoceros iguana
Cyclura cornuta cornuta

By Jose Ottenwalder

Description

The rhinoceros iguana attains a large adult size, with SVL up to 560mm in males and 510mm in females. Large males weigh between 6 and 10kg. Adults are grayish brown, dark brown, dark gray, or even black, without pattern, and with the venter less heavily pigmented than the dorsum. Bright colors are entirely absent. Juveniles are similar in appearance to adults, but with approximately nine paler crossbars which disappear relatively soon after hatching. Minimum recorded SVL for juveniles is 80mm.

According to Schwartz and Carey (1977), rhinoceros iguanas can be distinguished from the other two recognized subspecies, the Mona Island iguana (C. cornuta stejnegeri) and the extinct Navassa Island iguana (C. cornuta onchiopsis; Schwartz and Henderson 1991), by a combination of scale counts and femoral pore number. In the Dominican Republic, the only West Indian country where two distinct species of rock iguanas are found, the rhinoceros iguana and Ricord's iguana can be easily differentiated. In contrast to the diagonally barred back and sides of Ricord's iguana, rhinoceros iguanas are uniformly dark on their back and sides (Schwartz and Carey 1977).

Distribution

Rhinoceros iguanas are still widely distributed throughout Hispaniola, including most of its offshore islands (Fig. 3). Their current geographic range is fragmented relative to their more continuous historical distribution, and is strongly associated with xeric regions of lower human population density. Islandwide, 20 or more subpopulations may exist, assuming that at least half of the Haitian populations known 20 years ago still survive (Ottenwalder and Meylan, unpublished manuscript). Most iguana concentrations are found along the southern side of Hispaniola, with the highest numbers in south-southwestern Dominican Republic.

In the Dominican Republic, a minimum of ten subpopulations, many of which contain further subdivided populations, are known from the north-northwest (2, Valle del Cibao Oriental, Llanura Costera del Atlantico north of Cordillera Septentrional), the northeast (1, eastern end of Samana Peninsula), the southeast (2, La Altagracia coastal region south of the line between Bayahibe-Boca de Yuma-Macao, including Isla Saona), and the south-southwest (5, from Bani Province west to the Neiba Valley and south to the Peninsula de Barahona, including Isla Beata). In Haiti, ten or fewer increasingly threatened subpopulations may still exist. Surveys conducted by P. Meylan in 1975 and information gathered by J. Ottenwalder and others during the 1980s indicates that rhinoceros iguana populations, while under heavy hunting and habitat pressure, were until recently known from Tortue Island, Massif du Nord, Gonaives region, southeastern tip of Gonave Island, Petit Gonave Island, Cul-de Sac Valley around Lake Etang Saumatre, and several areas of the Tiburon Peninsula, including Belle Anse, Marigot, Jacmel, Aquin, Les Anglais, Jeremie, Ile Grande Cayemite, Ile Petit Cayemite, and other small offshore islands and cays between Coral and Petit Trou de Nippes (Ottenwalder and Meylan, unpublished manuscript).

Status of populations in the wild

Rhinoceros iguanas were common and widespread until the early 1950s, but accurate information concerning current population estimates on Hispaniola is lacking. Unpublished data based on opportunistic surveys are available for a few localities, but are inadequate for extrapolation to other areas facing different levels of disturbance, particularly in Haiti. Observations on population and habitat trends recorded since the 1970s provide a fair but rough approximation of 10,000 to 17,000. The highest densities are found on the Barahona Peninsula, Isla Beata, and the Valle de Neiba-Cul de Sac region. Moderate populations densities may still exist in localized, isolated areas of the Bani-Azua region in the Dominican Republic, and Petit Gonave Island, the Cayemite island-complex, and the Lake Etang Saumatre basin in Haiti.

Rhinoceros iguana densities are low in the majority of the areas where they presently occur and appear to be declining due to increasing human pressure and the impact of feral mammals. Local extirpations are known from both Dominican Republic and Haiti. Populations are seemingly stable only on Isla Beata and the extreme of the Barahona Peninsula inside Parque Nacional Jaragua, although predation by introduced carnivores has been documented.

Ecology and natural history

Rhinoceros iguanas are most abundant in, although not restricted to, dry forests characterized by xeric, rocky habitats of eroded limestone in coastal terraces and lowlands of the mainland and several offshore islands and small cays. In areas supporting iguanas, mean annual rainfall ranges from 470 to 1000mm, and mean annual temperature is 25°C. With some exceptions, the species ranges in elevation from -35m (Isla Cabritos, Lake Enriquillo) up to 400m. It is found in a variety of subtropical life zones and habitat types, including thorn scrub woodland, dry forest, and transitional semideciduous to subtropical moist forests.

In addition to the habitat conditions described for areas of sympatry with Ricord's iguana, the variety of plant communities occupied by the rhinoceros iguana elsewhere in Hispaniola clearly indicates the ecological generalism and adaptability of this species. While most areas now exhibit a xeric mosaic of habitats as a result of human disturbance, variation in dry forest composition and structure of remaining undisturbed areas is generally influenced by edaphic and climatic factors. Habitat profiles of areas supporting rhinoceros iguanas are given below (vegetation data from Garcia and Alba 1989; DVS/SEA 1990; Hager and Zanoni 1993; DNP/AECI 1993; Mejía and García 1993; Ottenwalder and Meylan, unpublished manuscript; J. Ottenwalder, unpublished data).

Disturbed dry forest

Characterized by declining iguana populations, these areas exist in various stages of succession and are impacted by charcoal production, livestock grazing, fires, hardwood extraction, and other activities. Prosopis juliflora forest, a widespread invasive community occupying areas of former natural dry forest, represents one such habitat. This 6-8m forest is dominated by Prosopis juliflora-Acacia macracantha association, with reduced presence of Bursera simaruba, Phyllostylon rhamnoides, Senna atomaria, Lemaiocereus hystrix, Pilosocereus polygonus, Opuntia moniliformes, Neoabbottia paniculata,and Caesalpinia coriaria. A second type of disturbed dry forest is found in iguana localities around Montecristi in the northwest, at the southeastern end of Altagracia Province including areas in Parque Nacional del Este, and from Las Tablas and Galeón de Bani to Llanura de Azua to Cabral, continuing into the Neiba Valley. The forest canopy is 5-6m, and is dominated by cacti. Trees include Prosopis juliflora, Pilocereus polygonus, Lemaireocereus hystrix, Opuntia moniliformes, Neoabbottia paniculata, and Capparis ferruginea. A very few Guaiacum officinalis and G. sanctum are now left as a result of selective logging. Shrubs include Cylindropuntia caribea, Harrisia nashii, Tournefortia stenophylla, Caesalpinia sp., Cordia globosa, Boerhavia scandens, Turnera diffusa, and Pictetia spinifolia. With the exception of C. caribea, most of these species are expected to eventually disappear with continued habitat alteration.

Natural dry forest on slopes

This community occurs on the north lake of Lake Enriquillo on foothills 100-400m in areas of stony soil with little organic material. The vegetation is characterized by trees, 30% of which are deciduous, approximately 30 species of shrubs, half which are thorny, a few herbs, and prolific vines. The 6-11m canopy includes Bursera simaruba, Guaiacum sanctum, Phyllostylon rhamnoides, Colubrina elliptica, Senna atomaria, Guaiacum officinale, Exostema caribeum, Capparis flexuosa, and C. ferruginea.

Natural dry forest on rocky substrate

This low elevation, primarily cactus community occurs in the lowlands bordering the south Lake Enriquillo basin. The site is characterized by sandy soil and rocks on shorelines, becoming more rocky on slopes with little fine soil. Rainfall is erratic (470-600mm), and vegetation consists of a slow growing shrubby, open tree canopy up to 3-4m. Trees include Acacia scleroxylla, Cameraria linearis, Capparis ferruginea, Guapira brevipetiolulata, Bursera simaruba, Plumeria subsessilis, Opuntia moniliformes, Lemaireocereus hystrix, Pilosocereus polygonus, and a few Prosopis juliflora. Shrubs include Caesalpinia sp., Cordia spp., Isidorea leonardii, Bursera brunei, Comocladia dodonaea, Harrisia nashii, Cylindropuntia caribaea, Guaiacum sanctum, G. officinale, and Tournefortia stenophylla.

Natural dry forest of the Barahona Peninsula

This habitat, occurring south of the Oviedo-Pedernales road, and on Isla Beata, Parque Nacional Jaragua, consists of a mixture of coastal lowland and typical dry forest with high endemism. Soils are dominated by dogtooth limestone formations with little accumulation of alluvial deposits except for low lying zones, depressions, and rock cavities. Annual rainfall ranges from 630-800mm. On the western boundary, vegetation is open and low (5-8m) on limestone rocks, while in the more humid eastern areas, the same vegetation grows more densely and reaches 6-12m. Common plants include Metopium brownei, M. toxiferum, Acacia scleroxyla, Guaiacum sanctum, Bursera simaruba, Plumeria obtusa, Senna atomaria, Capparis cynophallophora, Haitiella ekmanii, Thouinidium inaequilaterum, Coccoloba pubescens, Cameraria linearifolia, Catalpa punctata, Opuntia moniliformis, Tabebuia ostenfeldi, Phyllostylon rhamnoides, Comocladia dodonaea, and Lonchocarpus pycnophyllus.

Semideciduous to transitional semihumid broad-leaved forests

These forests occur along the east and west coasts of Parque Nacional del Este, and the western portion of Isla Saona. Low coastal forests typically occur on reef limestone, with annual rainfall up to 1,300mm, and 25-60% vegetation cover. Plant species include Jaquinia arborea, Coccoloba diversifolia, Conocarpus erectus, Guapira brevipetiolata, Coccoloba uvifera, Borrichia arborescens, Bursera simaruba, Plumeria obtusa, Pilocereus polygonus, Erythoxylum arcolatum, Widelia calicina, Capparis flexuosa, C. cynophallophora, Guaiacum sanctum, and Metopium brownei. Tall semihumid broadleaved limestone forest is characterized by a mosaic of forest patches with varying canopy heights, depending on soil conditions and depth of the water table. Vegetation cover ranges from 25% to 60%, with a canopy dominated by Clusia rosea, Bucida buceras, Coccoloba diversifolia, Bursera simaruba, Krugio-dendrum ferreum, Celtis trinervia, Metopium brownei, Sideroxylon foetidissimun, Swietenia mahogany, Ottoschulsia rhodoxylon, Guaiacum sanctum, and Bumelia salicifolia. The forest floor is dominated by Zamia debilis, with Peperomia epiphytes and vines. Leaf litter and a layer of humus covers rocks. Iguana localities with more mesic conditions are known but these are the exception.

Like other rock iguanas, rhinoceros iguanas are diurnal, spending the night in retreats. Rock crevices, caves, burrows dug in soil or sand, and hollow trunks are also used during the day for resting, cooling, or sheltering. Males defend territories containing retreats attractive to females. High trees and exposed rocks are used by males for basking and overseeing defended areas. Mating takes place at the beginning of or just prior to the first rainy season of the year

Females lay from 2 to 34 eggs, with an average clutch size of 17 (J. Ottenwalder, unpublished data). Females guard nests for several days after laying, and incubation lasts approximately 85 days. Hatchlings weigh about 51g, with a mean SVL of 104mm and total length of 288mm. Females probably become sexually mature at 2-3 years of age. Rhinoceros iguanas feed on fruits, leaves, and flowers of a variety of plants, depending on availability. Additional information on their ecology and natural history is summarized by Schwartz and Henderson (1991).

Habitat

In the Dominican Republic, roughly 35% of rhinoceros iguana habitat has been lost, and approximately 75% of what remains is disturbed. Both figures are much higher for Haiti. Only the natural communities of Isla Beata have been spared noticeable impacts, and extensive dry forest stills remain in Parque Nacional Jaragua. Most of the currently occupied habitat is characterized by fragmented forest patches, including Montecristi, Samana Peninsula, Peninsula de la Altagracia, including Parque del Este and Isla Saona, Llanura de Azua-Bani, Valley de Neiba, and Peninsula de Barahona.

In Haiti, conditions are more critical, and extensive areas of previously diverse plant associations are now dominated by disturbed Prosopis juliflora-Acacia macracantha communities. Large areas of original forest have been extirpated and desertification is progressing rapidly. Among the localities containing habitats supporting iguana populations until the late 1970s are Tortue Island, Riviere Saline, Plaisance, Mole St. Nicolas, Anse Rouge, Gonaives to St. Marc, Gonave Island, Ile Petite Gonave, Mirebalais, Miragoanne to Jeremie, Les Anglais, Cap St. George, Jacmel to Marigot, Belle Anse, Marigot to Anse-a Pitres, and Lake Etang Saumatre in the Cul-de-Sac region (Ottenwalder and Meylan, unpublished manuscript).

Threats

Habitat destruction, due to extraction of hardwoods and fuelwood, charcoal production, agriculture, livestock grazing, and limestone mining, represents the major threat to rhinoceros iguanas in both the Dominican Republic and Haiti. In the Dominican Republic, about 13% of the human population (± 1 million) occupy dry forest regions. These areas are also the most economically depressed, and exploitation of forest habitats for charcoal and fuelwood represent important sources of income. About 75-80% of the total national demand for these products originates from dry forest habitats.

Other important threats are predation by feral dogs, cats, mongoose, and pigs on adults, juveniles, and eggs, and illegal hunting of subadults and adults for food and local trade. The use of iguanas for food in Haiti is extreme in rural areas where iguanas are conspicuous enough that local people are familiar with them. International trade of wild animals from Hispaniola, a conservation problem until the mid 1980s, has been controlled in the Dominican Republic under CITES since 1987, but no such control exists in Haiti.

Current conservation programs

In the Dominican Republic, most rhinoceros iguana populations are either fully or partially protected inside existing national parks and reserves. Protected areas supporting iguana populations include Montecristi National Park, Parque Nacional del Este including Isla Saona, Parque Nacional Jaragua including Isla Beata, Las Caobas Strict Natural Reserve, El Acetillar scenic area, Sierra Martín García National Park, and Lago Enriquillo National Park, including Isla Cabritos. However, the foothill regions in the latter two areas remain only partially protected. Management in most protected areas is not intensive, and in some cases is restricted to legislation.

A number of fragmented populations are found outside protected areas, primarily in the Cibao Occidental Valley, Samana Peninsula (part of the proposed Samana Bay Biosphere Reserve), the dry coastal portion of the Altagracia peninsula, Las Tablas of Bani, Peravia Province, west to Llanura de Azua, the eastern half of the Neiba Valley, including the northeastern slopes of Sierra de Bahoruco above Cabral Lagoon, the lowlands of Puerto Alejandro, and the southern slopes of Sierra Martín García near Bahía de Neiba, the drier slopes of the eastern half of Sierra de Bahoruco, and the dry forest region north of the Pedernales-Cabo Rojo-Oviedo road.

Compliance with international trade regulations is effective, aside from occasional smuggling of animals across the border with Haiti. Rhinoceros iguanas are protected nationally by Dominican wildlife regulations. Enforcement has improved during the past few years, but clearing of the natural habitat for development is not being prevented, and illegal hunting and poaching for food and for the local pet market continues. No formal protected areas are known within the present distribution of iguanas in Haiti. The status of protective legislation is also uncertain, although the rhinoceros iguana was included on a list of protected wildlife by the Ministry of Agriculture during the 1980s. Enforcement of any potentially existing wildlife regulations seems unlikely at present.

Rhinoceros iguanas are the most common rock iguana in captivity. A successful breeding program existed at the Parque Zoológico Nacional of the Dominican Republic (ZooDom) from 1974 to 1994, with an average of 100 young hatching annually. These efforts included experimental reintroductions of captive-bred young to several protected areas in the southwest Dominican Republic. Although a captive colony of almost 300 iguanas representing all age classes was maintained at ZooDom until December 1994, the program was adversely affected by administrative changes. In recent years, the captive breeding and conservation program has been gradually and successfully reactivated.

As of November 1995, rhinoceros iguanas elsewhere in captivity included 39.32.36 individuals at about 20 zoological institutions, with an additional 5.3.3 animals of unassigned subspecies, reported by seven American Zoo and Aquarium Association institutions (Christie, 1995). The actual number may be higher considering holdings at some European zoos and many private collections.

Critical conservation initiatives

• Strengthening of current regulations and legislation protecting iguana populations by increasing fines and designating selected areas as critical habitat whether outside or inside existing protected area boundaries.
• Development of educational awareness campaigns to promote iguana conservation, particularly to discourage subsistence hunting of iguanas for food, local trade, and habitat conversion for charcoal production.
• Development of a national conservation and recovery strategy and working group to include government agencies, non-governmental conservation organizations, and iguana researchers.
• Establishment of research, management, and monitoring programs for wild populations and critical habitats.
• Involvement of local organizations and communities in all iguana conservation, education, and research activities.

Priority projects

1. Assess the current status of wild populations and remaining habitats throughout the species' range.
2. Investigate natural history and ecology, habitat use, and factors limiting numbers in order to develop a conservation strategy and recovery plan for the species. Such studies should be concentrated in areas where rhinoceros iguanas are sympatric with Ricord's iguanas.
3. Control or eradicate exotic predators and herbivore competitors on Isla Cabritos in Lago Enriquillo and Isla Beata.

Contact persons

Jose Ottenwalder
UNDP/GEF Dominican Republic Biodiveristy Project
United Nations Development Program
PO Box 1424, Mirador Sur
Santo Domingo, Dominican Republic
Tel: (809) 534-1134
Fax: (809) 530-5094
E-mail: biodiversidad@codetel.net.do

Angelica Espinal
Departamento Zoologia
Parque Zoological Nacional (ZOODOM)
Santo Domingo, Dominican Republic

Sixto Inchaustegui and Ivon Arias
Grupo Jargua, Inc.
El Vergel 33
Santo Domingo, Dominican Republic
Tel: (809) 472-1036
Fax: (809) 412-1667
E-mail: emys@tricom.net

Departamento de Vida Silvestre
Subsecretaria de Rescursos Naturales
Secretaria de Estado de Agricultura
Santo Domingo, Dominican Republic