Lesser Caymans iguana
Cyclura nubila caymanensis

By Glenn Gerber

Description

Lesser Caymans iguanas are very large iguanas (to 1360mm total length) with pronounced sexual size dimorphism. Maximum recorded SVL and mass of adult males and females are 570mm and 8.5kg, and 472mm and 5.2kg, respectively (G. Gerber, unpublished data). Mean (± SD) SVL and mass of the 10 largest males and females captured on Little Cayman were 544 ± 14mm and 7.6 ± 0.6kg, and 440 ± 18mm and 3.9 ± 0.7kg, respectively. Based on these data, adult females average 81% of the SVL and 51% of the mass of adult males.

Coloration is also variable. Typically, adult males are light gray with tan on the head, tail, limbs, and dorsal mid-line of the body. The dorsal crest scales and head often have a light-blue or reddish-pink hue. Diagonal black bars partially ring the body and tail, but frequently fade with age. The chest and belly are sometimes burnt-orange or rust colored. Females are less brightly colored, lack any blue or red coloration on the head, and frequently have a greenish wash to the entire body. Adults of both sexes have black forefeet. Juveniles are tan or brown with 5-10 pale dorsal chevrons, bordered by black, which break up laterally to form ocelli.

Scale counts distinguishing Lesser Caymans iguanas from other subspecies of C. nubila are given in Schwartz and Carey (1977), and recent molecular studies (S. Davis, personal communication) have revealed genetic differences between the subspecies.

Distribution

The Lesser Caymans iguana is native to two islands: Cayman Brac (38km2) and Little Cayman (28.5km2). The islands are 7.5km apart and are well isolated from other land masses. Grand Cayman is 100km to the WSW, and Cuba and Jamaica are over 200km to the NE and SE, respectively. The Cayman Islands, composed of carbonate rock, are emergent sections of the otherwise submerged Cayman Ridge (Jones 1994). Little Cayman is low-lying with a maximum elevation of 14m, whereas Cayman Brac steadily rises from sea level in the west to 43m in the east. The climate is sub-humid tropical with distinct wet (May-November) and dry (December-April) seasons (Burton 1994a). Vegetation is similar to that which occurs on other limestone formations in the West Indies (Brunt 1994). Both islands have been continually inhabited since the early 1800s. Current human populations for Cayman Brac and Little Cayman are approximately 1,000 and 200, respectively.

Status of populations in the wild

In 1938, iguanas were abundant on Cayman Brac and Little Cayman. At that time, C. Lewis (in Grant 1940) reported that iguanas "are found all over the Brac and Little Cayman" and (in Lewis 1944) that "the colonies of Cyclura on these two islands are reproducing, generally flourishing and are in no danger of extinction." However, in contradiction to the previous statements, Lewis also reported (in Grant 1940) that "the populations are rapidly being reduced by dogs, which these lizards seem unable to escape." Since that time, populations of Lesser Caymans iguanas have declined and are clearly in danger of extinction, particularly on Cayman Brac.

In 1965, Lesser Caymans iguanas were abundant on Cayman Brac only along a relatively small section of the southwest coastline (Carey 1966). Today, the subspecies is nearly extinct on Cayman Brac. Over the last four years, F. Burton (personal communication) has seen only two iguanas on Cayman Brac (both juveniles) and received only two reports of sightings (both adults; Fig. 6). Based on these results, the population probably numbers fewer than 50 individuals.

Presently, Little Cayman still supports a widely, although patchily, distributed iguana population (G. Gerber, unpublished data). The population is reproducing and all age classes are represented, although juvenile mortality is very high due to predation by feral cats. During nine months of field work on Little Cayman in 1993, over 200 iguanas were captured and marked (Fig. 6). However, due to the inaccessibility of much of the interior of the island and the patchy distribution of iguanas, good population estimates are not available; mature iguanas possibly number between 750 and 1,500. The population appears not to have declined significantly since the 1970s (Townson 1980, Stoddart 1980) and 1980s (Blair 1983), and may even be larger today. Nonetheless, dense concentrations of iguanas which occupied the mid-northern (B. Ryan, personal communication) and mid-southern (Grant 1940) coasts of Little Cayman 50+ years ago no longer exist, and a growing population of feral cats and increasing human development severely threaten the long-term survival of iguanas on Little Cayman.

Ecology and natural history

Like other rock iguanas, Lesser Caymans iguanas require suitable forage plants, basking areas, retreats, and nesting sites. On Little Cayman, these requirements are met in a variety of coastal and interior habitats, and iguanas are widely dispersed. Maximum densities occur in dry evergreen bushlands and thickets (sensu Brunt 1994) growing on exposed and highly weathered limestone or dolomite. These habitats provide a diverse assemblage of forage plants, a mosaic of sun and shade for thermoregulation, and an abundance of solution holes in the rock substrate which iguanas of all sizes use as retreats. However, suitable nesting sites in these habitats are restricted to shallow patches of soil that accumulate in small depressions. Consequently, many females migrate to coastal areas with relatively deep sandy soils to nest.

The diet of all age classes consists almost entirely of leaves, flowers, and fruits; however, iguanas occasionally scavenge on animal carcasses (e.g., land crabs) or prey on slow-moving insects (e.g., Lepidopteran larvae). Over 40 plant species have been identified in the diet of iguanas on Little Cayman (G. Gerber, unpublished data). Leaves of Bauhinia divaricata, Capparis flexuosa, C. cynophallophora, Stylothanthes hamata, Ipomea pes-caprae, I. violacea, and Canavalia rosea form a significant portion of the diet year-round. When available, flowers of Tabebuia heterophylla, Ipomea sp., and Canavalia rosea, and fruits of Picrodendron baccatum, Hippomane mancinella, Cordia sebestena, Myrcianthes fragrans, Guapira discolor, Coccoloba uvifera, and Citharexylum fruiticosum are consumed in great quantities. However, many of these fruits are too large to be eaten by hatchling iguanas. Adult iguanas (primarily female) sometimes congregate in areas with fruiting Coccoloba uvifera or Hippomane mancinella trees.

Courtship and mating occur in April and early May, coinciding with the end of the dry season when temperatures and photoperiod are increasing. Mating is polygynous and copulation takes place in male territories. Males compete intensely for territories which are occupied in all seasons. The largest males hold territories in the best habitats and acquire the most matings. Male territories are large, on the order of 1ha, and during the breeding season as many as 10 females may temporarily reside within a single territory. The smallest, youngest males do not hold territories and during the breeding season move from one territory to another attempting to court females and avoid detection by resident males. They sometimes move several kilometers in the process and probably acquire few copulations. Females typically occupy smaller home ranges than males, although they frequently leave their home range to mate or nest, and appear to be territorial only when nesting. The adult sex ratio appears to be skewed toward females.

Females lay a single clutch of 7-25 eggs (mean 15) annually, between late-May and mid-June, coinciding with the beginning of the wet season. Mean SVL of all reproductive females is 385mm (range 308-472). The clutch is deposited in a chamber excavated by the female 10-50cm below the surface. Females nest either in small soil patches within the rocky interior or in large sandy tracts along the coast, and may migrate considerable distances to nest sites. After closing their nest, some females guard the site for periods ranging from one day to several weeks. Guarding appears to be correlated with availability of other suitable nesting sites in the area, and thus the probability that another female will attempt to nest in that location. Females that migrate to large coastal tracts of sand to nest rarely guard, whereas females that nest in small soil patches almost always guard.

Hatchling emergence occurs from early August to early September and incubation averages 72 days (range 63-80). The median percentage of fertile eggs per clutch is 97.5 (range 35-100), and the median percentage of hatchlings emerging from each nest is 91.5 (range 0-100). The sex ratio of hatchlings is unity. Hatchlings average 107mm SVL (range 97-121) and 50g (range 33-69). Juveniles disperse from nest sites soon after emerging and may move long distances before settling. Most juveniles marked after emergence from nests were never seen again, but one was later located 5.2km away. Compared with other rock iguanas, juvenile growth is rapid, averaging approximately 100mm SVL per year during the first two years (G. Gerber, unpublished data). The smallest female observed nesting was 308mm SVL and in her second year.

Habitat

Little Cayman contains large areas of undisturbed habitat where Lesser Caymans iguanas are still widely distributed, although much less abundant than in the past. Many areas appearing to support prime iguana habitat have few if any iguanas. In addition, the pace of development on Little Cayman is accelerating and native habitats are being destroyed and fragmented at an unprecedented rate, especially along the coast where many iguanas nest. Cayman Brac, due to a larger human population, has much less undisturbed habitat than Little Cayman and the population there is nearly extinct. On both islands, populations are far below carrying capacity.

Threats

Threats to Lesser Caymans iguanas include habitat destruction from road construction, commercial and residential real estate development, livestock grazing (on Cayman Brac) and farming practices, predation by feral cats and domestic dogs (and possibly introduced rats), disturbance of sensitive nesting areas, and road casualties. Since the construction of a municipal power generating station on Little Cayman in the early 1990s, habitat destruction associated with road construction and real estate development have increased dramatically and the human population, although still small, has increased several fold. Under the present socioeconomic conditions this pattern can be expected to accelerate. Plans to replace the existing, grass airstrip on Little Cayman with a new, paved airstrip are underway. Proposed sites for this project overlap areas of prime iguana habitat on the west end of the island. Aside from the loss of habitat, such an airstrip will significantly increase human visitation to the island and promote further development. The continued destruction and disturbance of coastal nesting areas on Little Cayman is of particular concern as nesting opportunities for iguanas in the interior appear to be limited due to the paucity of suitable soil patches. Also, the increasing number of feral cats on Little Cayman pose an immediate threat to population recruitment.

Current conservation programs

Iguanas are protected within the Cayman Islands by the Animals Law of 1976 (Davies 1994), but protection of native habitats is lacking. The Development and Planning Law of 1971 provides a legal mechanism to prevent the destruction of terrestrial habitats in the Cayman Islands, but has never been implemented (Davies 1994). Currently, the only protected areas on Cayman Brac and Little Cayman are the Cayman Brac Parrot Preserve (a 65ha tract of potentially important iguana habitat) and the Little Cayman Ramsar Site (an 82ha preserve encompassing Booby Pond and surrounding mangroves; Fig. 6).

The National Trust for the Cayman Islands has had an active iguana conservation program since 1990; however, due to limited resources, efforts have largely concentrated on the Grand Cayman iguana. An intensive field study of Lesser Caymans iguanas was conducted on Little Cayman in 1993 by G. Gerber. Plans to continue this research await further funding.

A captive breeding program does not exist for this subspecies, but may be warranted for the Cayman Brac population if it is genetically distinct from the Little Cayman population. This possibility needs to be investigated. No pure Lesser Caymans iguanas are presently held in zoological institutions (B. Christie, personal communication).

Critical conservation initiatives

• Acquisition and protection of terrestrial habitats on Little Cayman and Cayman Brac.
• Development and enforcement of strict regulations to control domestic and feral animals.
• Control of real estate development, road building, and use of motor vehicles.
• Increased commitment to environmental issues among governmental and private sectors.

Priority projects

1. Eradicate or control feral cats.
2. Conduct field surveys to determine the status of iguanas on Cayman Brac, identify more local subpopulations and nest areas in need of protection on Little Cayman, and obtain accurate population estimates for both islands.
3. Carry out molecular studies to determine the amount of intra- and inter-island genetic variation.
4. Asses feasibility for construction of small supplemental nest areas on Little Cayman. This may provide a simple and cost-effective means of bolstering recruitment against increasing habitat losses.
5. Undertake long-term field studies on Little Cayman to quantify reproductive and other life history parameters. Radiotelemetry will help determine where and how far females travel to nest, dispersal patterns of hatchlings and juveniles, and movements of young males prior to establishing territories.

Contact persons

Glenn Gerber
Department of Ecology and Evolutionary Biology
University of Tennessee
Knoxville, TN 37996 USA
Tel:(423) 974-3065
Fax: (423) 974-3067
E-mail: ggerber@ix.netcom.com

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