Allen's Cay iguana
Cyclura cychlura inornata
By John Iverson
Description
The Allen's Cay iguana is a large (to 1000mm total length) subspecies of C. cychlura characterized by a lack of horn-like frontal or prefrontal scales, rostral scale in contact with the nasal scales, slightly enlarged prefrontal scales separated from frontal scale by four scale rows, usually two portmental scales, and dorsum pigmented gray-black with cream, pink, or orange mottling. Pink or orange pigment is most obvious on the posterior lower labial scales, the preauricular scales, and the enlarged mid-dorsal scale row.
Distribution
Only two breeding populations of this subspecies are known, on Leaf Cay (4ha) and U Cay (also known as Southwest Allen's Cay; 3ha) in the northern Exuma Island chain in the Bahamas. Probably less than seven adults also occur on Allen's Cay (7ha), but no evidence of breeding has been found there during 12 years of study.
Status of populations in the wild
Based on a 17-year mark and recapture study, approximately 130 subadult and adult iguanas (> 8 years old) occur on Leaf Cay and 100 on U Cay. Juvenile population estimates are not precise, but in March are probably near 100 for each island. The entire wild population of this subspecies is less than 500 individuals. The populations have generally been stable over the past 17 years, with recruitment occurring on both islands every year. However, some removals from Leaf Cay by poachers and Bahamian zoo and park personnel are known to have occurred over the past decade. Although the two main populations are generally stable, the two cays are heavily visited by tourists.
Ecology and natural history
Most of the details of the life history of this iguana remain unstudied. What is known has been accumulated during approximately biennial visits by J. Iverson. During these trips, lizards have been marked and recaptured, primarily for growth and survivorship studies. These iguanas can apparently survive on the very smallest rocky islets as long as sufficient vegetation is present for food; however, areas of sand are necessary for nesting. Hatched egg shells have been found on several occasions, but always in sandy areas. The lack of breeding on Allen's Cay may be due to insufficient areas of exposed sand above tidal influence.
Recapture studies have shown that Allen's Cay iguanas average about 113mm SVL (157mm tail length, 56g) in March at approximately six months of age. Average growth rates are over 20mm SVL per year during the first year, declining to about 15mm per year by age 5.5 at about 206mm SVL (Iverson and Mamula 1989; J. Iverson, unpublished). Growth in females then begins to slow, whereas in males growth continues at the same rate until about 300mm SVL. The result is considerable sexual dimorphism in size. The largest known adult male was 476mm SVL and weighed 4.8kg, whereas the largest female was 368mm SVL and weighed only 2.1kg. Large adults of both sexes usually grow less than 10mm per year. Males and females cannot be sexed externally, but can be sexed fairly reliably by hemipenal probing. Based on minimum age for adults first caught in 1980 and still alive in 1992 and/or 1993, some of these iguanas live beyond 25 years of age.
Allen's Cay iguanas are active diurnally, spending the night in burrows they have dug or in natural retreats in or under rocks. They are primarily herbivorous, feeding on fruits, leaves, and flowers of most of the plants present on their tiny islands. They will climb up into the vegetation to feed. They are also opportunistically carnivorous, as evidenced by crab claws in their feces. In addition, humans regularly feed the iguanas (particularly on Leaf Cay) everything from table scraps to fresh produce. The effect of food supplementation on the life history of these lizards remains unknown, but deserves study.
Nothing is known about reproduction in this species, but mating probably occurs in May, with egg-laying commencing in June. Jolly-Seber models of recapture data suggest that survivorship of subadults and adults exceeds 90% per year. During the non-breeding season, these lizards appear to have dominance hierarchies rather than strictly defended territories; however, this may be because tourists frequently feed the iguanas on the main beach areas, perhaps causing a breakdown in the natural social system. Their behavior during the breeding season is unknown.
Habitat
The natural habitat of this iguana on Leaf and U Cays has not been significantly disturbed by human activity, even though a number of introduced ornamental plants occur on these cays (e.g., Casuarina, lilies, palms). All potential habitats on both Leaf and U Cays are occupied by iguanas, including some suboptimal areas of bare, honey-comb limestone. Additional habitat is available on Allen's Cay, but without sandy areas for nesting, the island apparently cannot support a breeding population. Dredging of sand from the harbor between Allen's Cay and Leaf Cay to upland areas on Allen's Cay could double the potential habitat area for this iguana.
Threats
The only significant current threat to these populations is the removal of iguanas by humans. The problem is exacerbated by the fact that these cays offer good anchorage less than a day's sail from Nassau; for example, during March, 15-20 yachts and 1-2 native boats are anchored there each night. Regular reports of actual or attempted poaching are made to the warden of the Exumas Land and Sea Park to the south. In addition, iguanas are occasionally removed for exhibit purposes in Bahamian zoos, parks, and gardens. Illegal exploitation for international trade is undocumented, but probably occurs.
Current conservation programs
The Allen's Cays iguanas are protected nationally under Bahamian law, but enforcement is difficult without a warden present. The warden of the nearby Exumas Land and Sea Park can potentially respond to reports of poaching, but that is not always practical. Fortunately, signs erected on the islands explain the vulnerability of these lizards, and most visitors on yachts radio the authorities if anyone is seen harassing the iguanas. Unfortunately, visitors also enjoy feeding them unnatural foods. Long-term investigations of growth, survivorship, and population status of these iguanas are ongoing by J. Iverson, but a study of their reproductive ecology is urgently needed.
A few captive Allen's Cay iguanas are currently maintained at the Ardastra Zoo and Nature Centre Different on Abaco. Captive breeding is a goal, but long-term plans for any offspring produced need to be developed.
Critical Conservation Initiatives
- Establishment of regular patrols to enforce protection, prevent exploitation, and discourage feeding of iguanas.
- Initiation of a national education program for tourists and residents.
Priority projects
- Collect age-specific reproductive data on the marked population of Allen's Cay iguanas for which long-term growth data already exist.
- Explore the feasibility of modifying sinkholes on Allen's Cay to create nesting habitat for iguanas.
- Continued monitoring of the introduced population on Alligator Cay
Contact person
John Iverson
Department of Biology
Earlham College
Richmond, IN 47374 USA
Tel: (317) 983-1405
Fax: (317) 983-1304
E-mail: johni@earlham.edu
