Turks and Caicos iguana
Cyclura carinata carinata

By Glenn Gerber and John Iverson

Description

The Turks and Caicos iguana is a small (up to 770mm total length) rock iguana, characterized by a lack of enlarged scales on the upper surface of the head, rostral scale in contact with the nasal scales, 80 to 110 dorsal crest scales, enlarged, spiny whorls of scales on the tail, and 9 to 10 dark vertical stripes on the dorsolateral wall that fade with age (Schwartz and Carey 1977; Iverson 1979). Body color varies among island populations, from gray or brown to dull green. In some populations, the head and neck have a vermiculated pattern, and the dorsal crest scales and the tail of adult males are pale blue and reddish-brown, respectively. Body size is sexually dimorphic and varies among islands, with the smallest animals occurring on Long Cay on the Turks Bank, where adult males and females average 221mm snout-vent length (SVL) (0.40kg) and 185mm SVL (0.24kg), respectively. Iguanas are largest on Plandon Cay on the east side of the Caicos Bank, where adult males and females average 325mm SVL (1.41kg) and 268mm SVL (0.64kg), respectively (G. Gerber and M. Welch, unpublished data). The largest recorded male and female are from Pine Cay on the west side of the Caicos Bank, measuring 360mm SVL (1.86kg) and 290mm SVL (1.14kg), respectively.

Distribution

This subspecies is native to the Turks and Caicos Islands, located southeast of the Bahamas and approximately 150km north of Hispaniola. The islands lie on two shallow oceanic banks, the Turks Bank and the Caicos Bank, separated by a deep water channel. Although politically separate from the Bahamas, the Turks and Caicos are geologically part of the Bahama Archipelago. There are 10 islands in the Turks Bank and over 100 islands in the Caicos Bank, with a combined surface area of approximately 500km2. Maximum elevation is about 85m and the climate and vegetation are similar to that of the Bahamas. Historically, this subspecies occurred throughout the Turks and Caicos, but has been extirpated from many areas, including most of the larger islands.

Status of populations in the wild

A comprehensive survey of iguana populations in the Turks and Caicos was conducted in 1995 (G. Gerber and M. Welch, unpublished data; Fig. 1). Based on preliminary analyses, about 30,000 adults remain in the wild. Iguanas were found on 56 of 120 cays visited. However, most cays with iguanas are very small (area < 1 ha) and the combined area of all cays with iguanas is only about 28km2. Over half of the area occupied by iguanas consists of five cays (Salt, Joe Grant's, Major Hill, Dellis, and Pine) where iguanas are extremely rare (Fig. 1), probably due to the presence of introduced mammals (Fig. 2; see also Iverson, 1978, and Smith, 1992). The combined area of cays where iguanas are still common (densities can exceed 30 adults per ha; Iverson, 1979) is only 13km2, most of which is accounted for by three cays (Big Ambergris, Little Ambergris, and East Bay), the largest of which (Big Ambergris, 4.3km2) is privately owned and under development. A comparison of the 1995 survey with a less extensive survey conducted in the mid-1970s (Iverson, 1978) suggests that at least 13 iguana populations, most on relatively large islands, have been extirpated over the last 20 years.

Ecology and natural history

Iverson (1979) studied the natural history of this subspecies in the Caicos Islands from 1973 to 1976. This iguana is most abundant in rocky coppice and sandy strand vegetation habitats, and sandy habitat is required for nesting. The Turks and Caicos iguana is diurnal and spends the night in burrows it has dug or in natural retreats in or under rocks. It is primarily herbivorous throughout its life, feeding arboreally or terrestrially on the fruits, flowers, and leaves of at least 58 plant species, as well as occasional insects, mollusks, crustaceans, arachnids, lizards, and carrion (see also Auffenberg, 1982). In captivity, it readily takes both animal and plant food.

Adult males are territorial throughout the year, apparently to guarantee access to food and females. Courtship and mating occur in May, with a single annual clutch of two to nine eggs laid in June. Females defend the nest burrow for several days to several weeks after nesting, but are not territorial during the rest of the year. Hatching occurs in September, after about 90 days incubation, and hatchlings average 80mm SVL and 14.6g.

Growth rates in juveniles average slightly less than 20mm per year until maturity, which in males occurs at about 7 years (220mm SVL, 0.33-0.48kg), and in females at 6 to 7 years (185-200mm SVL, 0.20-0.30kg). Adults grow much more slowly (0.2 to 1.7cm/yr) and show strong sexual dimorphism.

Annual survivorship ranges from about 55% for the first three years of life, to about 67% during years four through six, to 90-95% in adults. Life table analysis suggests that mean cohort generation time is 14 years. Preliminary data suggest that some individuals live at least 20 years.

Habitat

The current human population of the Turks and Caicos is about 8,000, inhabiting eight of the larger islands (Fig. 1). On these islands, considerable habitat has been lost to human activities, particularly tourist-related, and the rate of development is increasing. An even more significant cause of habitat destruction has been the mammalian predators and livestock accompanying modern settlement (Fig. 2). Iguanas are still common only on uninhabited islands that have no introduced mammals. However, many uninhabited islands exist which could support dense populations of iguanas if rid of feral mammals.

Threats

The primary threat to Turks and Caicos iguanas is introduced mammals, particularly cats and dogs. Iverson (1978, 1979) documented the near-extirpation of a population of over 5,000 adult iguanas from Pine Cay (3.9km2) in just three years as a result of predation by feral cats and dogs. Feral livestock (goats, cows, donkeys, and horses) pose a serious threat also, presumably because they compete for food plants, alter the vegetational composition of habitats, and trample soft substrates where iguanas burrow and nest. In 1995, iguanas were found on only five of 26 islands with cats or livestock (G. Gerber and M. Welch, unpublished data). Furthermore, iguanas on these five islands were rare, whereas iguanas on islands without introduced mammals were common. Iguanas are still occasionally eaten by local fishermen, and although illegal exportation for international trade is undocumented, it probably occurs.

Current conservation programs

The Turks and Caicos has a fairly extensive system of national parks, nature reserves, and sanctuaries, a number of which encompass areas supporting iguanas (Fig. 1). Unfortunately, reserves are not immune to the effects of introduced mammals (Fig. 2), and few governmental resources are presently allocated to maintain or enforce protection of non-marine parks. However, with the establishment of the National Trust for the Turks and Caicos Islands in 1994, a significant increase in conservation of terrestrial wildlife and habitats has begun. Largely due to the urging of the National Trust, legislation to protect iguanas within the Turks and Caicos Islands has recently been drafted. In addition, the government has granted the National Trust stewardship of Little Water Cay, which supports a large population of iguanas but needs management due to its popularity with tourists and the threat of invasion by feral cats over a recently formed isthmus to Water Cay. A similar transfer is currently pending for Little Ambergris and East Bay Cays. The National Trust recently initiated a trapping program to remove feral cats from Pine, Water, and (if needed) Little Water Cays. If successful, this program will remove the threat of cats from Little Water Cay and allow iguanas to repopulate Pine and Water Cays. On Little Water Cay, boardwalks and observation towers have been constructed at two popular landing sites to reduce the negative impacts of tourism, and a visitation fee has been instituted with the proceeds supporting conservation activities. In addition, the National Trust has initiated a public education program that includes distribution of an informative poster about iguanas and a tour of all schools to discuss iguanas and other conservation issues.

A preliminary study of genetic variation in the Turks and Caicos iguana using blood samples collected from 29 island populations in 1995 found significant differences among islands and revealed a pattern of strong regional differentiation (M. Welch, unpublished data). This preliminary study utilized microsatellite markers developed for C. nubila, of which only two were informative; further description of inter-island genetic variation awaits the development of markers specific to C. carinata. No captive programs currently exist for this taxon.

Critical conservation initiatives

• Legislation prohibiting the introduction of mammals to uninhabited islands in the Turks and Caicos.
• Incorporation of all uninhabited islands supporting iguanas into the current reserve system.
• Increased governmental commitment to terrestrial conservation, including the provision of resources necessary to enforce compliance with environmental regulations.

Priority projects

1. Eradicate or control introduced mammals on islands uninhabited by humans. Free-ranging domestic livestock should also be captured and relocated to inhabited islands.
2. Complete study of genetic differentiation among island populations.
3. Establish a long-term monitoring program and Geographical Information Systems database of iguana populations.
4. Conduct field studies to determine the conditions necessary to re-establish healthy, self- sustaining populations of the Turks and Caicos iguana on islands uninhabited by humans, supporting suitable habitat, and lacking feral mammals. Results could serve as a valuable model for other West Indian rock iguanas, some which may depend on reintroduction programs for their survival.

Fig. 1. Map of the Turks and Caicos Islands showing ocean banks (gray surrounded by dotted lines), terrestrial reserves (lighter shading around islands), and abundance of iguanas on islands in 1995 (white = not found, hatched = rare, black = common to abundant). Iguanas were found on 24 of 31 cays visited in Chalk Sound. Names of islands inhabited by humans are underlined.

Fig. 2. Map of the Turks and Caicos Islands showing where feral or free-ranging domestic mammals were (black) and were not (white) found in 1995. Labels indicate the types of mammals identified by sight, tracks, or droppings. The label "visited by dogs" indicates islands frequented by dogs residing on nearby islands. Other features labelled as in Fig. 1.

Contact persons

Glenn Gerber and Mark Welch
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
E-mail: mwelch@utk.edu

John Iverson
Department of Biology
Earlham College
Richmond, IN 47374 USA
Tel: (317) 983-1405
Fax: (317) 983-1497
E-mail: johni@earlham.edu

Ethlyn Gibbs-Williams
Turks and Caicos National Trust
Butterfield Square
Providenciales
Turks and Caicos Islands
Tel/Fax: (649) 941-5710
E-mail: tc.nattrust@tciway.tc