Grand Cayman iguana
Cyclura nubila lewisi

By Fred Burton

Description

The Grand Cayman iguana was originally described by Grant (1940). Although its distinction from the other two subspecies was based at that time only on its striking blue body coloration and caudal spine lengths, its status as a unique form has been upheld by recent genetic studies (S. Davis, unpublished data). Minor differences in scalation, particularly in the subocular region, are evident but of limited diagnostic value. The Grand Cayman iguana is a large form, similar in size range to the Lesser Caymans iguana, although size statistics have not been obtained from large numbers of animals due to their rarity.

Distribution

Iguanas now survive as three or possibly four partially fragmented subpopulations scattered over approximately 5km2 in eastern Grand Cayman. However, reports dating from the early 1980s suggest that at that time a small population still survived in the Lower Valley and Spotts area of Grand Cayman. A local newspaper report detailed the death of an adult in the extreme northwest of the island in the late 1940s, and Morgan (1994) found fossil remains of iguanas throughout Grand Cayman. Taken together, these reports suggest that the Grand Cayman iguana was distributed widely in dry habitats over the entire island before the advent of human settlement. The present contraction in range to the eastern districts is probably a reflection of human activities impacting primarily the western districts, as 78% of the island's human population lives on this half of the island. Unfortunately, the remaining iguana habitat is also some the best of Grand Cayman's generally poor agricultural land, making its protection socially and politically difficult.

Status of population in the wild

Based on observations made in 1938 by B. Lewis, who stated "the species is nearly extinct, and I doubt whether more than a dozen individuals still exist on the island. In past years they are said to have been numerous in the interior of the east and north...East end people say that since 1925 the 'guanas' have become so scarce that is no longer worthwhile to hunt them," the population appears to have persisted in the low hundreds over the last 70 years. The total wild population is currently estimated to be no more than 100 to 175 individuals (0.2 adults/ha in suitable areas), orders of magnitude below what the habitat appears able to support (Burton and Gould, in preparation). Although this taxon is strictly protected under the Animals Law enacted in 1976, it faces increasing threats and appears to be at high risk of extinction over the next 20-30 years.

Ecology and natural history

Similar to other rock iguanas, the Grand Cayman iguana is almost exclusively herbivorous, occupies rock hole retreats, and nests in inland deposits of red soil. This subspecies is known to consume at least 45 different plant species in 24 different families, with an emphasis on the Fabaceae and the Rubiaceae. Particularly important species include leaves of Rhynchosia minima, Chamaecrista nictitans, and Stylosanthes hamata, whole plants of Spermacoce confusa, and seedlings of Waltheria indica (Burton and Gould, in preparation). All are herbaceous weeds which thrive in disturbed, open areas. When available, iguanas have also been observed to gorge themselves on fruits of plants usually inhabiting stands of primary vegetation, including Picrodendron baccatum, Hippomane mancinella, and Myrcianthes fragrans. Iguanas appear to exploit both primary and disturbed habitats to obtain the variety of foods they consume, indicating that their present diet is probably significantly different from what it was prior to human intervention.

Scat analysis conducted by F. Burton and K. Gould revealed that iguanas consume approximately 80% leaf material and 20% fruits. During the dry season when fruits are unavailable, leaves are consumed exclusively. Because iguanas feed mostly on the leaves of weeds and fruits falling from wild trees, it appears that their purported damage to farmer's crops has been exaggerated. Exceptions to strict herbivory include observations of iguanas feeding on fungus, crabs, and cicadas, although the latter two instances were more likely examples of scavenging rather than active predation. Coprophagy and soil ingestion have also occasionally been observed.

Iguanas on Grand Cayman occur at such low density that it is difficult to assess the degree to which individual spacing patterns are attributable to the availability of suitable retreats rather than territorial behavior or other factors. Whereas males show little long-term fidelity to individual retreats, females appear to occupy small, fixed territories centered on one or two favored retreats. The abundance of suitable refugia may be important in controlling female distribution, while male density may be regulated primarily by intraspecific territorial interactions (Burton and Gould, in preparation).

Habitat

Grand Cayman iguanas are usually observed in dry evergreen thickets and bushlands, as well as traditional agricultural and other man-modified habitats derived from these vegetation communities (Burton and Gould, in preparation). Rather than being confined to a single habitat type, individual iguanas appear to utilize a mosaic of natural and semi-disturbed habitats in response to thermoregulatory opportunity, food, nesting substrate, predator pressure, and human interference.

The interaction between traditional agricultural practices and iguanas is complex, having both benefits and disadvantages. Small scale clearings provide good basking spots and abundant edible herbaceous vegetation, and the mosaic of habitats resulting from small scale agricultural activity likely provides a greater habitat diversity than existed previously. It is possible that these factors to some degree balance the impacts of exotic predators and grasses, trapping, and cattle grazing that accompany agricultural land use (Burton and Gould, in preparation).

At present, inland red earth deposits provide the only natural nesting substrate for iguanas on Grand Cayman. These deposits are essentially restricted to dry evergreen thickets, dry evergreen bushlands, and the agricultural lands derived from these formations. Nesting attempts have not been observed in soil areas converted to grasslands.

Threats

All sites where iguanas are known to occur are subject to human-related threats, including predation by wild and domestic cats and dogs, conversion of land to cattle grazing or intensive farming, habitat destruction for real estate development, road casualties, and trapping or shooting by farmers who perceive iguanas as a threat to their crops. In particular, the intensity of large scale deforestation and new road construction has increased enormously in the eastern districts over the last decade, and under present socioeconomic conditions this trend is expected to accelerate.

Current conservation programs

An integrated conservation program for the Grand Cayman iguana is being implemented by the National Trust for the Cayman Islands. This project incorporates research, habitat protection, captive breeding, reintroduction/restocking, and conservation education. Iguanas feature prominently in the annual National Trust Fair, where 2,000 to 3,000 school children have the opportunity to learn about this and other endangered species. The ultimate goal of the Trust's iguana program is to secure and protect a stable, breeding wild population capable of surviving indefinitely without ongoing human intervention. In addition to the direct goal of preventing the extinction of the iguana, the Trust views this program as a flagship species approach to its more fundamental mission of protecting the unique natural environments of the Cayman Islands.

To help insure a future for the Grand Cayman iguana in the wild, a nature reserve plan for eastern Grand Cayman is in the early stages of development. To date, the National Trust has acquired conservation land through grants of Government land, through donations by U.S. citizens owning land in the Cayman Islands, and by direct purchase using funds raised by popular appeals. At present, the most promising possibility for future introductions appears to be the Salina Reserve in the northeastern part of the island, a 253ha area which contains a small but viable amount of iguana habitat (approximately 2ha). Although there is currently no evidence of wild iguanas in this area, trial translocations indicate that it may be suitable for supporting a limited number of iguanas. The area has subsequently been mapped, its vegetation catalogued, and a low-impact trail network established. The Queen Elizabeth II Botanic Park, a 26ha property jointly managed by the National Trust and the government, has also shown considerable potential as iguana habitat, and is a strong candidate for restocking efforts.

Field research designed to elucidate the status and ecology of Grand Cayman iguanas in the wild has been underway since 1991 in collaboration with the Friends of the National Zoo (Washington, D.C.). Work to date includes assessment of the diet of wild and released captive iguanas, acclimatization to the wild and territorial interactions among released captives, population distribution and habitat utilization, and limited studies of nesting behavior.

A captive breeding program was initiated at the National Trust in 1990. The original breeding stock consisted of animals already in captivity on Grand Cayman, animals donated to the National Trust by the Life Fellowship Sanctuary (Seffner, Florida), and a few wild hatchlings which had roamed into developed areas with traffic and feral predators. As a result of genetic studies undertaken by S. Davis, animals bred in 1990 and 1991 from Life Fellowship were subsequently found to have hybrid ancestry involving Lesser Caymans iguanas, C. nubila caymanensis. These animals were surgically sterilized to exclude them permanently from the breeding program. Additional analysis of captive Grand Cayman iguanas in both the Cayman Islands and U.S. captive collections has shown that genetic variation is low by comparison with Jamaican iguanas and Lesser Caymans iguanas from Little Cayman.

The in situ captive program is intended to function as an integral part of conservation efforts in the wild, and includes release of captive-bred iguanas into protected areas. Sterilized hybrids outfitted with radio transmitters have proven to be an important means for evaluating release protocols for iguanas into various candidate habitats. By tracking and studying released hybrids, a small area within the Trust's existing Salina Reserve capable of supporting at least one breeding female has been identified. These studies have also verified the ability of captive born and reared iguanas to adapt naturally, without pre-release conditioning, to life in the wild. On the basis of these studies, a pair of genetically pure adults was released into the area in 1994.

With funding from the Zoological Society of Milwaukee County and the Foundation for Wildlife Conservation, the National Trust recently constructed a new captive facility where iguanas can be bred and reared for eventual release back into their native habitat. The facility will house up to 12 adults and 32 juveniles.

Critical conservation initiatives

• Establishment of sufficiently large protected and managed areas remote from habitation, where exotic predators and adverse human interference can be effectively controlled and a large breeding population restored.
• Implementation of program for farmers to discourage iguana trapping, shooting, and the practice of allowing domestic dogs and cats to roam freely in areas where iguanas are known to occur.

Priority projects

1. Enhance habitat at the Queen Elizabeth II Botanic Park.
2. Institute feral predator control, particularly for cats in remote agricultural areas of eastern Grand Cayman.
3. Conduct field research and monitoring of the wild population.
4. Produce an educational poster for Grand Cayman iguanas and red-footed boobies.
5. Acquire habitat, particularly agricultural land that could support iguanas and thickets with potential natural iguana habitat adjacent to the Salina Reserve and the Queen Elizabeth II Botanic Park.

Contact persons

Fred Burton
National Trust for the Cayman Islands
PO Box 31116 SMB
Grand Cayman, Cayman Islands
Tel: (345) 949-0121
Fax: (345) 949-7494
E-mail: fjburton@candw.ky

Richard Hudson
Department of Herpetology
Fort Worth Zoo
1989 Colonial Parkway
Fort Worth, TX 76110 USA
Tel: (871) 817-7431
Fax: (871) 817-5637
E-mail: iguanhudso@aol.com