Anegada Island iguana
Cyclura pinguis

By Numi Mitchell

Description

The Anegada Island iguana is a relatively large, stout member of its genus. Males have been recorded with SVL reaching 560mm and may grow larger. Juveniles are faintly or boldly patterned with wide gray to moss green bands interspersed with wide gray to black anteriorly directed chevrons. The bands fade and are generally lost as the animals mature. Adults are grayish or brownish-black dorsally with varying amounts of turquoise on the dorsal spines, tail base, fore and hind legs. Occasionally the bluish coloration extends up onto the sides of the individual, particularly in males. Females tend to be relatively dull in color, exhibiting less brilliant blue if any. Ventral coloration of juveniles and adults varies from a solid buffy white to light gray. Sclera of eyes are dull yellow when animals are calm, but flush pink to red with increasing levels of agitation.

Distribution

The common name of the Anegada Island iguana is misleading, as the animal was once distributed over the entire Puerto Rico Bank. Fossils are known from Saint Thomas and Puerto Rico. The iguanas were likely extirpated when localities became densely settled by humans. Vulnerability to predation by humans and their dogs and cats may have resulted in a contraction of its distribution to Anegada. The numerous escape holes coupled with a large expanse of undeveloped land supporting few non-native predators may have allowed the iguana population to persist there. The distribution of iguanas on Anegada today is closely tied to porous limestone habitats.

Status of populations in the wild

Population density in 1968 was estimated at 2.03/ha (Carey 1975). In 1991, this figure had dropped to 0.36/ha in comparable habitat. Extrapolation of density estimates, distribution, and relative habitat quality yields a population estimate for Anegada of 164 individuals (Mitchell, in review). A small restored population also exists on Guana Island with eight founding adults (Goodyear and Lazell 1994), from which three juveniles have been translocated to Necker Island. The total population, including individuals on Anegada, Guana, and Necker, probably consists of fewer than 200 individuals.

Ecology and natural history

Estimates in the late 1960s (Carey, 1975), made before the introduction of domestic livestock, showed small home ranges for both sexes (< 0.1 ha), one principal burrow per animal, a 1:1 sex ratio, and habits that indicated monogamy (apparent pairs inhabited separate but proximate burrows in a joint home range isolated from other pairs). The current population structure is quite different. While previous studies may not have been sensitive to long range movements, it now appears that home ranges are quite large on Anegada: males average 6.6ha (range 2.2-12.3) and females average 4.2ha (range 2.8-5.6). Home ranges broadly overlap and have one or two centers of activity. For males, activity centers may be associated with home ranges of females; one male may have two centers almost a kilometer apart. In 1991, the sex ratio had dropped to 1 female:2 males. Thus male competition for limited females may be responsible for the high degree of home range overlap.

Burrows of both sexes may be located on the old limestone reef-tract or in sandy areas adjacent to it. If available, iguanas will use additional holes or crevices as emergency retreats. Female centers of activity are usually associated with one or several principal burrows. Whereas degraded vegetation may provide for male subsistence, it may not provide females with sufficient energy to allow them both to produce eggs and compete with other animals for forage to support their own growth and metabolism. Reproducing females may have low survivorship, resulting in the present skewed sex ratio. Females usually lay one clutch of about 12-16 eggs per year in late spring or early summer.

Although largely facultative herbivores, all age-groups of these iguanas are opportunistic carnivores. Invertebrates (beetles, caterpillars, centipedes, roaches) form < 1% of the natural diet, although this may be a result of limited availability. The bulk of the diet consists of leaves and fruits.

Habitat

Over the past 20 years, since domestic livestock were released to breed freely island-wide, grazing pressure by goats, sheep, burros, and cattle has radically changed the vegetational composition of Anegada. Not surprisingly, the diet of the iguana is now composed of plant species the feral animals reject. Almost 30% of the iguanas' diet consists of a plant containing secondary compounds (Croton discolor) apparently toxic to ungulates. Over 55% of the diet is composed of fruits (Byrsonima, Coccoloba, Eleaodendron, Eugenia) that the livestock ignore.

Threats

Areas on Anegada that once contained dense populations of iguanas now support few or none. Research indicates that this is due to three major causes, including competitive grazing pressure from free-ranging livestock, predation by feral dogs, and predation of juveniles by feral cats.

Current conservation programs

A major grant has been received from the Environment, Science, and Energy Department of the UK Foreign and Commonwealth Office to facilitate conservation activities on Anegada. Goals of this program are to 1) implement a cat eradication/control feasibility study, 2) expand the current headstart facility, 3) train Senior Terrestrial Warden Rondel Smith in iguana husbandry and facility maintenance, 4) conduct population censusing and mapping at nesting sites and other potential sites where adults may be found, and 5) develop environmental education materials to raise public awareness of the importance and vulnerability of iguanas on Anegada.

In the 1980s, eight iguanas were moved from Anegada to Guana Island, British Virgin Islands, to start a second population in part of the species' former range (Goodyear and Lazell 1994). This is not a limestone island, and does not provide as many natural retreats as Anegada. In the absence of introduced predators, however, the iguanas appear to do well and reproduce in areas that are free of sheep (the only feral grazing competitor present). Currently, approximately 20 adult iguanas are estimated to inhabit Guana. Offspring have been seen each year since 1987, but recruitment is very low over much of the island. Guana Island Wildlife Sanctuary continues to try to rid the island of sheep, which may improve the habitat for iguanas.

Critical conservation initiatives

• Creation of a national park to protect iguanas on Anegada, and maintenance of a reservoir of breeding animals on Guana whose offspring may be used to restock depleted areas on Anegada. Although a national park has been approved in concept by the Anegada Lands Committee, land title issues must be resolved before this project can move forward.
• Livestock purchase and removal from designated protected areas, including Guana Island.

Priority projects

1. Construct livestock exclosure fences around designated protected areas.
2. Assess status and density assessment of iguanas throughout Anegada.
3. Examine the physiology of the declining female population.
4. Conduct vegetation recovery experiments to assess potential for restoration.
5. Expand the headstarting program on Anegada to produce hatchlings for release into restored areas.
6. Carry out genetic studies on both Anegada and Guana Islands.
7. Organize a PHVA workshop to raise local, national, and international awareness.

Contact persons

Numi Mitchell
The Conservation Agency
Branch Office, 67 Howland Avenue
Jamestown, RI 02835 USA
Tel: (401) 423-0866
Fax: (401) 423-0199
E-mail: numi@wsii.com

James Lazell
The Conservation Agency
6 Swinburne Street
Jamestown, RI 02835 USA
Tel: (401) 423-2652
Fax: (401) 423-2396
E-mail: jcinjtown@aol.com

Joseph Smith Abbott
National Parks Trust
Ministry of Natural Resources
Box 860, Road Town, Tortola
British Virgin Islands
Tel: (284) 494-3904
Fax: (284) 494-6383
E-mail: bvinptdirector@caribsurf.com

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

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