Mona Island iguana
Cyclura cornuta stejnegeri

By Thomas Wiewandt and Miguel Garcia

Description

The Mona Island iguana is a large, robust member of the genus. Adult males and females average 517 and 475mm SVL, and 6.1 and 4.7kg body mass, respectively. Both sexes are ornamented with protruding facial scales and a horn-like, conical scale atop the snout. Also typical of adults are huge, sagging jowl muscles and two prominent pads of fatty connective tissue crowning the head. Body coloration is a uniform gray, olive, or brown in adults. Hatchlings are light gray or tan with dark bands.

At the conclusion of his three-year study, T. Wiewandt (1977, 1982) chose to retain the name C. stejnegeri for this taxon (Barbour and Noble 1916) on the basis of significant differences between the reproductive biologies of the Mona population and its closest living relative on Hispaniola, the rhinoceros iguana, C. cornuta cornuta. In their 1977 revision of the genus, Schwartz and Carey renamed the Mona population C. cornuta stejnegeri based on scale counts of about a dozen individuals of mixed age and sex. Further investigation of all populations in question is clearly needed before meaningful subspecies/species designations can be made. M. Garcia is planning a detailed investigation of systematics in the cornuta complex which should help resolve taxonomy within this group.

Distribution

The Mona Island iguana is endemic to the remote island of Mona, a low-profile limestone plateau situated midway between Puerto Rico and Hispaniola. The 11 by 7km island lies within a deep sea channel known as the Mona Passage, and present submarine banks offer no evidence of former connections with either Puerto Rico or Hispaniola. Mona is part of the Commonwealth of Puerto Rico, administered as a natural reserve by the Division of Natural Reserves and Refuges within the Puerto Rico Department of Natural Resources and the Environment (PR-DNRE). Mona Island has been designated as a National Historical Landmark. The U.S. Fish and Wildlife Service provides some funding for projects related to hunting, fishing, and endangered species. The entire island is occupied by iguanas, and the soil-rich coastal terraces and inland sinkhole depressions are essential for reproduction (Fig. 4).

Status of population in the wild

Based on island-wide surveys conducted from 1972 to 1975, T. Wiewandt calculated a mean density of iguanas on Mona of 0.33/ha and an estimated minimum population size of 2,000. A follow-up census was carried out by J. Moreno in late March and early April, 1995. Moreno's estimate of 1,155 individuals suggested to him that the Mona population had declined over the past 20 years. However, there were significant differences in methodology between the two surveys. Wiewandt utilized a transect width of 12m, whereas Moreno utilized a transect width of 16m. Had Moreno assumed a 12m transect width, his population estimate would rise to 1,540. Conversely, had Wiewandt assumed a 16m transect width, his population estimate would drop to 1,500. It is important to note that because these iguanas have greatly staggered activity patterns, varying between seasons and individuals, the population could conceivably be twice the estimated size. Meaningful comparisons will require established guidelines for future surveys, defined by location, time of day and year, weather conditions, and number of trained observers.

No matter how current census figures are interpreted, it is clear that the Mona iguana population is abnormally small. A survey of the similarly-sized rhinoceros iguana on Petite Gonave Island in Haiti indicated indicated densities 26 times greater than those found on Mona (P. Meylan, personal communication). T. Wiewandt found that immature iguanas were scarce on Mona, representing only 5-10% of the population, and J. Moreno sighted only two juveniles among 118 iguanas seen. This contrasts sharply with M. Garcia's unpublished data for rhinoceros iguanas (C. cornuta cornuta) on Isla Beata in the Dominican Republic, where all age classes are abundant and juveniles comprise approximately one-third of the population (see Grupo Jaragua, 1994). Low iguana densities and the scarcity of juveniles on Mona suggest a senescent and declining population.

Ecology and Natural History

Mona's climate is dry subtropical (800mm rainfall per year), supporting an open canopy forest of short, seasonally deciduous trees, shrubs, cacti, and bromeliads. Rainwater percolates rapidly through the porous limestone substrate, allowing no freshwater streams or ponds. Solution channels and sinkholes that penetrate the island's rock topography offer underground shelters that are utilized by both male and female iguanas, and retreats attractive to females are vigorously defended by males. Some males hold territories year-round, while others defend them only during the brief June mating season.

Because more than 95% of Mona's surface is rock, females must migrate to scarce soil deposits for nesting. The onset of the two-week, mid-summer nesting season appears to be cued by photoperiod and females are especially wary at this time. Most egg laying (74%) occurs in sandy clearings on the island's southern coastal terraces, with the remaining 26% in sinkhole depressions (Haneke 1995). Nesting females fight over favored nest sites and defend completed nests. Mean clutch size is 12 eggs. Surviving eggs hatch approximately 83 days after laying, during the latter half of October. Newly emerged young are large and only the smallest juveniles are susceptible to indigenous predators. Coloration and behavior of hatchlings suggests that aerial predators have long been a threat to this age class (Wiewandt 1977).

Juvenile iguanas are slow-growing, and females require 6 to 7 years to reach sexual maturity. Although longevity records are not available, Mona iguanas, like all large rock iguanas, are probably among the longest lived lizards in the world. Consequently, populations are slow to recover from losses over time.

Mona iguanas are primarily herbivorous, with a strong preference for fruits that fall from native trees. Some animal matter is eagerly taken, especially caterpillars when available. Trees reach their greatest size and diversity in scattered sinkhole depressions, areas that are of particular importance to the welfare of the iguana population.

Habitat

During his surveys, T. Wiewandt sighted iguanas on Mona's plateau at a density of 0.8/km traversed, except on routes along major escarpments and cliffside talus slopes, where iguanas were three to four times more numerous. In contrast, iguanas were rare on much of the island's flat, sandy southwestern coastal terrace, presumably because natural food and shelter are scarce there. Roughly half of the area has been cleared and reforested in mahogany and Casuarina trees, which are of little value to the iguanas because they shade out native understory and bear no edible fruits. Iguana density rises dramatically on this terrace during the summer nesting season when females migrate to coastal lowlands in search of sandy, sunlit clearings (Wiewandt 1977; Haneke 1995).

Historically, Mona's iguanas have been adversely affected by man for centuries, dating back to land use practices of pre-Columbian indians and continuing to the present (Wadsworth 1973). People settling subtropical dry limestone islands have traditionally concentrated their activities near beaches and within soil-rich sinkhole depressions, thereby inadvertently disrupting the life cycle of iguanas during nesting and hatching. Although there are presently no permanent human residents on Mona, a variety of domestic animals inhabit the island, including feral goats, pigs, and cats.

Threats

The most pressing conservation management challenge on Mona today is that of exotic species. Having evolved in the near absence of predators, insular iguanas lack the behavioral and demographic attributes to cope with introduced mammals. Feral pigs regularly plunder iguana nests. Nesting females are unable to protect their eggs from pigs and will cover an empty nest with soil, not recognizing that their eggs are gone. The extent of pig predation in any given year appears to be correlated with nest location and total rainfall during the three months prior to nesting. T. Wiewandt found that along the south coast nest loss ranged from 65-100% in the driest year to less than 5% in an unusually wet year. Haneke's (1995) comparison between coastal and inland nest sites revealed complete failure of all inland nests surveyed. Coastal nesting attempts were more successful, with the highest success in areas protected by pig-proof fences.

Feral cats are also present on Mona, and constitute the most serious threat currently impacting young iguanas. These elusive predators are extremely difficult to study and control. Their dietary preference for small reptiles has been established (Wiewandt 1977) and their interest in young Mona iguanas observed (Moreno 1995). The devastating impact of cats on a population of Turks and Caicos iguanas was clearly documented (Iverson 1978), and there is little doubt that the present scarcity of juveniles on Mona is due primarily to the combined effects of pigs consuming eggs and cats preying on young.

Over 20 years ago, botanist R. Woodbury expressed concern that most of Mona's sinkhole depression forest trees appeared unable to propagate successfully because of intense browsing pressure by feral goats. A follow-up study of forest regeneration within a fenced study plot (Cintrón 1976) confirmed these observations, noting not only a marked increase in the number of native tree seedlings during the first year, but also a 100% increase in accumulated leaf litter within the enclosure. Despite higher than normal rainfall that year, recovery of the plant community was slower than expected. Cintrón and Rogers (1991) further note that successional patterns island-wide are becoming skewed toward toxic, unpalatable plant species. Within depression forest communities, unpalatable shrubs and trees are starting to dominate the understory and also predominate in secondary growth on the plateau in places that were burned or cleared of vegetation during the past 60 years (Wiewandt 1977). Fortunately, Mona's trees are long-lived, allowing them to continue to survive despite heavy browsing pressure. Mona's history of human occupation suggests that goats and pigs were intensely hunted for food by island residents as late as 1942 (Wadsworth 1973). Consequently, much of the damage to vegetation evident today may be relatively recent (Wiewandt 1977).

Although lacking permanent settlements, Mona is a haven for recreational activities, including camping, fishing, swimming, scuba diving, beach combing, exploring, and hunting. Most of these activities are concentrated along the island's sandy coastal terraces and within sinkhole depressions, areas of critical importance for iguana nesting. Haneke (1995) observed that new camping facilities had been recently added in iguana nesting areas at Playa de Pajaros. Mona iguanas are wary and easily disturbed while nesting, and visitors can unintentionally disrupt the egg-laying process. People and feral animals walking through nest site clearings during incubation may cause nest chambers to capsize, denying oxygen to developing eggs. These and other conflicts between iguanas and visitors are bound to intensify as recreational use of the island continues to expand. Goats regularly gather in sinkhole depressions on Mona's plateau, and this may partially explain Heneke's (1995) observation of complete nest failure there. Mona may have already exceeded its carrying capacity for low impact tourist visitation. Better supervision over visitors, particularly by strengthening educational programs, will become increasingly important as the number of people coming to Mona continues to grow.

Another recent concern for conservation of Mona iguanas is the emergence of an undefined disease or parasite that causes blindness. Throughout T. Wiewandt's three-year study, only one animal with cloudy eyes that was obviously blind and emaciated was found. Recently, Haneke (1995) observed 15 blind adults on Mona, all with opaque, bluish eyes and apparently severely undernourished. Ramos (1964; cited in Kuns et al. 1965) lists 16 species of eye flies (family Chloropidae) occurring on Mona, including Hippelates pusio, which has been incriminated in the spread of catarrhal conjunctivitis in the United States. Studies are urgently needed to identify potential pathogens and vectors responsible for blindness in the Mona iguana population.

Current conservation programs

The Mona iguana is listed as threatened by the U.S. Fish and Wildlife Service and the PR-DNRE. In 1984, the U.S. Fish and Wildlife Service approved a recovery plan for the Mona iguana prepared by C. Diaz, PR-DNRE (Diaz 1984). Bringing feral pig and goat populations down to ecologically tolerable levels and maintaining them there would require that 50-70% of each population be removed annually (Baker and Reeser 1972), an extremely costly undertaking that would be a monumental task in Mona's rugged terrain. Realistically, these animals will never be truly brought under control.

During the last 25 years, the PR-DNRE has instituted some important changes. The hunting season on Mona has been moved to a time outside the iguana nesting and incubation seasons. Together with the local herpetological society, Sociedad Chelonia, the government has created several new nesting areas on the southwestern coastal terrace. A number of clearings in the Casuarina forest have been established that are fenced off from goats and pigs but allow iguanas to pass freely. Iguanas have been observed nesting successfully in the new clearings (Chelonia 1993). Fencing of remote nest sites (two 20m x 15m plots) is currently being undertaken by the PR-DNRE, the U.S. Fish and Wildlife Service Caribbean Office, the U.S. Coast Guard, the Sociedad Chelonia, and the Toledo Zoo. Researchers at the PR-DRNE and the Toledo Zoo have additionally begun to assess the nature of the blindness syndrome seen in several adult iguanas. For his graduate research, N. Perez, University of Puerto Rico, will be measuring overall population size, studying recruitment and survivorship of juveniles, quantifying egg loss to pigs and rats, and determining the effect of exotic vegetation on incubation of iguana eggs. A total of 33 blood samples from wild iguanas has been provided to S. Davis for phylogenetic analysis. Over the years, feral cats have occasionally been trapped or shot on Mona. Currently, PR-DNRE is conducting a long-term study to quantify the impact of feral cats on Mona Island wildlife.

Critical conservation initiatives

• Enforcement of existing laws for protection of wildlife and other natural resources.
• Implementation of an active and long-term educational campaign about Mona Island and the conservation of its natural, environmental, and historical resources.

Priority projects

1. Determine the cause of blindness in wild Mona iguanas by pathology examinations of afflicted animals.
2. Monitor and continue to repair existing feral mammal control fences, and expand the fencing program by installing new goat and pig exclosures for sinkhole depressions utilized by nesting iguanas. Care must be taken to locate fences in places that will neither destroy Taino archeological sites nor mar the intrinsic beauty of these areas.
3. Continue to evaluate Mona's plant communities in conjunction with construction of pig and goat exclosures around sinkhole depressions.
4. Expand educational programs for visitors and monitor their activities on Mona.
5. Make follow-up field assessments in order to allow meaningful comparisons between past and present conditions on Mona to be made and to facilitate establishment of a viable population. This work could offer educational opportunities for island personnel.
6. Establish a rigorous procedure for yearly censusing of the iguana population.

Contact persons

Thomas Wiewandt
Wild Horizons, Inc.
PO Box 5118
Tucson, AZ 85703 USA
Tel: (520) 743-4551
Fax: (520) 743-4552
E-mail: wildhorizons@worldnet.att.net

Miguel Garcia
Bureau of Fisheries and Wildlife
PR-DNRE, PO Box 9066600
San Juan, Puerto Rico 00906
Tel: (787) 722-7517
Fax: (787) 724-0365
E-mail: miguelag@umich.edu

Jorge Moreno
Instituto de Educacion Ambiental (INEDA)
Universidad Metropolitana
Apartado 21150
Rio Piedras, Puerto Rico 00928
E-mail: morenjo@ucsu.Colorado.EDU

Robert Matos, Chief
Division of Natural Reserves and Refuges
PR-DNRE
PO Box 9066600
San Juan, Puerto Rico 00906
Tel: (787) 723-6435
Fax: (787) 724-0390

José Luis Chabert
Bureau of Fisheries and Wildlife
PR-DNRE
PO Box 9066600
San Juan, Puerto Rico 00906
Tel: (787) 722-7517
Fax: (787) 724-0365

Nestor Perez
Universidad de Puerto Rico
Departamento de Biologia
PO Box 22360
San Juan, Puerto Rico 00931
Fax: (787) 764-2610
E-mail: nfperez@rrpac.upr.clu.edu