Diet

All iguanas except the largest adults climb into the vegetation to feed, including up the smooth boles of thatch palms.^[1] Even adults have been found 2 m (6 ft 7 in) up in a Pseudophoenix palm to eat the flower buds.^[11] They are primarily herbivorous, feeding on fruits, leaves, and flowers of most available plants. Individuals can survive on very tiny islands from the available plants.^[1] On Alligator Cay the population eats mostly Rhachicallis americana and Suriana maritima, the former is more common and is eaten more, but the lizards show a preference for the latter, which is relatively much more uncommon on the island.^[11][13] Two studies in the remains of their faeces found occasional crab claws, insects, molluscs and fledgling birds, and they have been observed to be occasionally carnivorous.^[1] They also have been found to eat their shed skin. On Alligator Cay three plants were not observed to be eaten: Hymenocallis arenicola and Strumpfia maritima, and the mangrove Rhizophora mangle. More than a quarter of the faeces were made up of fruit. The iguana ate the large fruit of Casasia clusiifolia and Manilkara bahamensis, as well as the small fruit of the sea grape Coccoloba uvifera, and the palms Coccotrinax argentata and Pseudophoenix sargentii. Casasia fruit are produced throughout the year and represented 9.6% of the total faecal matter, the second most prevalent item. The iguanas feed on more leaves when they are forced to during the cool and dry season.^[11]

Biologists Kristen Richardson, John Iverson and Carolyn Kurle investigated the diets of the iguanas. Their 2019 study made use of carbon and nitrogen isotope analysis to estimate iguana diets in order to explain why iguanas on Allen's Cay were ~2 times longer and ~6 times heavier than the same subspecies on nearby Leaf Cay and U Cay, the populations being only a generation removed and separated by a 300 m (980 ft) channel. They had hypothesized that the gigantic iguanas were eating shearwater carcasses killed by mice and barn owls, but they found no evidence for that hypothesis. Instead they determined that their gigantism was due to their consumption of plant material containing more nutrients than the plants on the other cays. The added nutrients came from the ocean in the form of seabird guano; the largest colony of Audubon's shearwaters (Puffinis iherminieri) are located on Allen's Cay. None of the other cays have large populations of seabirds and so are missing the extra amounts of nutrients that arrive in the form of guano on Allen's Cay. Their evidence strongly supports that these iguanas are herbivores and the giants on Allen Cay are so large because their plants contain higher levels of nutrients from seabird guano.^[7][10]

Tourists often feed the iguanas human food items on Leaf Cay^[10] and sometimes U Cay.^[1] The iguanas have come to expect to be fed and may congregate in large numbers awaiting the tourists.^[11]

Mating

Mating occurs in May, and eggs are usually laid in mid-June to mid-July, in nests excavated in sand. The females migrate to suitable areas to nest.^[1] On the newly colonised Allen's Cay there is no sand on the jagged honeycomb limestone beaches, and the iguanas have never bred there.^[7][10] The subspecies previously bred on only the two original islands U and Leaf Cay, but on the islands of Alligator Cay and Flat Rock Reef Cay there are suitable breeding sites.^[1]

Only one in three females nests in a given year, although the largest females nest annually. One to ten eggs are laid; larger females lay more eggs than do smaller females. Hatching occurs with a success rate of 79% in late September and early October after an incubation 80–85 days at nest temperatures of 31 °C (88 °F).^[1]

Status

The IUCN assessed the population to be endangered in 1996 and 2000, but using a more restrictive definition of which lizards were allowed to be counted, in 2018 it assessed that the population was actually critically endangered.^[1]

In the early 1900s the population was almost wiped out, but had increased to some 150 iguanas by 1970 on the only two islands were it bred. In 1982, this had increased to a bit over 200 animals. In the early 1980s the population was growing at some 20% annually, and it had recovered by the end of the century to the limit of the carrying capacity of its original islands.^[1] In 2000, it was estimated that the current global population was less than 1,000 and it was said to be declining,^[6] although it was actually increasing. In 2018 the population was assessed to be 482–632 (mature adult) individuals, with 482 being the amount of mature adult iguanas on U and Leaf Cay in 2016 based on recapture counts. This number does not include over 150 adult iguanas found on islands where it believed they cannot breed.^[1] The population on Allen's Cay was at 20–25 for a number of decades according to Iverson, but it was reduced to ten individuals in 2013 indirectly due to bird conservation activities.^[7][10] In the 1990s, a number of iguanas were moved to nearby islands.^[1] Iverson and P. Hall, a warden of Exumas Cays Land and Sea Park, moved only eight sub-adults to Alligator Cay between 1988 and 1990, of which at least seven had survived ten years later, and these had increased to between 75 and 90 individuals in 1999 based on recapture analysis of 28 caught and tagged iguanas during four visits. Mostly larger lizards were caught and it was expected there were many more juveniles, and the island had not reached carrying capacity.^[11][13] According to the IUCN the population declined over the next decade due to hurricane damage of the vegetation and emigration to the nearby island of Narrow Water Cay. Hines reported seeing 28 iguanas on Alligator Cay and an estimated 38 iguanas on Narrow Water Cay in 2013, and found that apparently some iguanas had successfully emigrated to nearby Warderick Wells Cay. The Flat Rock Reef Cay population appeared in the mid-1990s, the IUCN claims it was purposely introduced without authorisation by unknown people. It rapidly increased to around 200 individuals by 2012, although the IUCN claimed in 2018 that the island would not sustain this population due to its low carrying capacity.^[1]

Threats

The primary threat to this species according to IUCN in 2018 is ecotourism and the feeding of the lizards by tourists. A number of tour operators and sometimes private yachts moor at either of the islands, bringing a few hundred tourists a day to see the lizards on both Leaf and U Cay. Tourists may transmit diseases and parasites and sometimes feed the iguanas unnatural food such as foreign fruit, bread, brownies or meat which is believed to contribute to health problems for the lizards such as faecal impaction and high levels of cholesterol. Despite dogs and cats being banned, tourists continue to bring their pets and a single loose dog could eliminate a population. Furthermore the IUCN alleges that tour operators may have moved some of the largest lizards off to other islands where they cannot scare the tourists, because they found a tagged individual on and islet eight kilometres away from where he was tagged, and they state large iguanas appear to be becoming rarer in the population.^[1] The iguanas are the primary tourist attraction to this area of the Bahamas.^[13]

A century ago, in the early 1900s, the Allen Cays rock iguana was almost wiped out due to being hunted for food by locals (see taxonomy section above).^[1] As of 2003 the animals were still said to be hunted for food and captured for sale in the pet trade.^[14] In 2018 this was repeated by the IUCN, although there is no documentation that this has occurred.^[1]

Recovery efforts

Like all Bahamian rock iguanas, this species is protected in the Bahamas under the Wild Animals Protection Act of 1968.^[6] It is listed in Appendix I of the CITES convention.^[1] Allen's Cay was formerly over-run by the common house mouse (Mus musculus), an invasive species, and these were in their turn attracting barn owls (Tyto alba) from neighbouring islands.^[7] In May 2012, Island Conservation and the Bahamas National Trust worked together to remove invasive house mice from Allen's Cay to protect the Audubon's shearwater and hopefully also native species such as the Allen Cay rock iguana and the Bahama yellowthroat.^[15][16] As it was feared the Allen Cay rock iguana might eat the rodenticide used to rid the island of the mice, eighteen were transplanted to nearby Flat Rock Reef Cay, where another small population already existed.^[7] Sixteen of these had starved to death by 2013.^[7][10] It is now thought this was because the plants on Flat Rock Reef Cay lacked the extra nutrients found on Allen's Cay. After this at least eight iguanas survived on Allen's Cay, and including the two from Flat Rock Reef Cay, there are now ten iguanas thought to be left on the island.^[7] In 2012, Iverson had begun a project to fill a small sinkholes on the island with sand to create suitable breeding ground.^[1][7]

The population on Alligator Cay was released there as part of a conservation translocation program.^[1][11] It was seen as quite successful in 2001.^[11] Alligator Cay and surrounding islands are part of the national Exuma Cays Land and Sea Park.^[11][13]

There are no captive breeding programmes which have been undertaken with this subspecies.^[1]