Sister Isles Rock Iguana (SIRI) (Cyclura nubila caymanensis)

Mohamed bin Zayed Species project number 142510107

Population dynamics and nesting ecology of the critically endangered iguana Cyclura nubila caymanensis in a heterogeneous landscape.

We are a team of conservation biologists, invested local government staff, and dedicated volunteers working together to study and protect the Sister Isles Rock Iguana (Cyclura nubila caymanensis) on Little Cayman and Cayman Brac. The species is critically endangered and can only be found on these two 10-mile islands 60 miles off the coast of Grand Cayman. Since major development began on the islands about 30 years ago, the iguanas have faced mounting pressures of habitat destruction, vehicle collisions, predation by feral rats, cats, and dogs, and competition from invasive green iguanas. Hatchling iguanas—whose size and inexperience make them very vulnerable—are expected to suffer especially high mortalities under these conditions.

Chief among our goals for this project are to identify and monitor critical nesting sites for protection on Little Cayman, which supports the largest SIRI population of the two islands (estimated between 1,200 and 1,500 in 2010). SIRI nesting strategies are still poorly understood, though several coastal sites have been identified as communal nesting grounds thought to support females from across many home ranges. Not all iguanas nest communally, however, and few single nests have been identified within small patches of suitable substrate in the island’s interior. Due to the generally impenetrable nature of intact, xerophytic shrubland, it is difficult to estimate how pervasive this strategy actually is.

This year, intensive monitoring efforts are focused on known nesting sites along Little Cayman’s west end. Seventy-five total nests have been identified, of which about a third are found in Preston Bay (the island’s largest communal nesting site).  While these numbers seem high, they likely represent a significant reduction in nest densities since Matt Goetz’s island-wide surveys were completed five years ago.  Whether this difference can be attributed to incongruity of methods, loss or shifting of suitable nesting habitat, population reductions, or a combination of factors requires further investigation.

In addition to the identification of nests, a total 137 iguanas were also captured on Little Cayman, measured, PIT-tagged, beaded, and bled for genetic analyses. For comparison of populations, an additional 24 iguanas were captured and processed during a short trip to Cayman Brac. The assignment of unique bead color combinations and permanent PIT-tag IDs to every animal captured will be extremely beneficial to individual recognition over the course of this study. Importantly, this will facilitate investigations of growth, behavior, individual movements, and possible site fidelity. The genetic samples collected will serve to supplement ecological data with analyses of population genetic structure and inbreeding. Currently, a return visit is planned for August to assess hatching success at identified nests and to mark and bleed this year’s hatchlings.