Natural history, ecology, and conservation of the West Indian Stout Rock Iguana (Cyclura pinguis)

The stout iguana (Cyclura pinguis) is one of nine species of West Indian rock iguana that collectively are recognized as one of the most endangered groups of organisms in the world. The species was once found throughout the Greater Puerto Rican Bank, but currently survives exclusively in the British Virgin Islands (BVI), with the only remaining natural population occurring on Anegada. Due to grazing competition with feral livestock, predation by non-native mammalian species, and habitat loss, the Anegada population is both critically endangered and conservation dependent, relying on the headstarting of wild-born hatchlings to maintain population abundance. Beginning in 1984, insurance populations of C. pinguis were artificially established on Guana, Necker, and several other small, privately-owned islands within the BVI to serve as a hedge against the extinction of the species. These translocated populations have never been considered as legitimate components of the population in terms of conservation planning or management; however, it appears that that situation may soon be changing.

Our understanding of C. pinguis natural history and ecology is still fairly limited, and there is very little known about the potential differences in life history, behavior, or ecology that may occur between source and recipient populations. This knowledge could have important implications for conservation. Therefore, using data collected from 2003 to 2014, I compared age-specific iguana body sizes and growth rates among islands; assessed hatchling thermal ecophysiology, including the thermal sensitivity of locomotor performance metrics, between the Guana and Anegada populations; and performed population abundance estimation on Guana Island.

My results suggest that wild hatchling, yearling, and sub-adult C. pinguis on both Guana and Necker Islands grew faster and attained larger and heavier age-specific body sizes than animals in the Anegada Headstart Facility. In addition, wild, mature adult C. pinguis were heavier in the Guana population than in either the Necker or the Anegada populations, but were similar in length among islands. There are likely both physical (thermal, moisture, and light regimes) and biological (density, sensory cues, behavioral interactions, and proper diet) factors influencing these trends. Adult body condition differences between Guana and Anegada are widely attributed to differences in food availability and food nutritional quality between islands, while the reduced body condition in the Necker adult population is more likely the result of suppressed fat deposition due to higher iguana density and social stress near artificial resource concentrations.

In the wild, hatchling C. pinguis maintained operative body temperatures between 27.6 and 43 °C that were, on average, over 6 °C higher than the ambient temperature. Laboratory-assessed preferred hatchling temperature was approximately 39.4 °C (80% set-point range = 38.3 to 40.1 °C) and aligned closely to the peak endurance and sprint performance temperature of approximately 38 °C. Endurance capacity between hatchlings from Guana and Anegada differed by both island and thermal treatment group, with Guana hatchlings having significantly greater endurance capacity.

In the 30 years since its founding, the Guana Island C. pinguis population has increased dramatically from its original abundance of eight individuals. The results of my abundance surveys indicated that in 2014, the island-wide post-hatch year population was approximately 417 individuals (densities ranged from approximately 0.96 to 4.6 iguanas per hectare). There were also roughly 151 hatchlings within the core research area at beginning of the 2014 sampling period. During October, which coincides with the hatching period, daily apparent survival for the hatchling age-class was estimated to be between 0.93 and 0.97. Consequently, there are currently more stout iguanas in the unofficial and unmanaged insurance populations than there are in the official, natural population. Conservation insurance populations will provide both a valuable hedge and additional operative potential for the conservation management of C. pinguis. Therefore, in the face of continuing conservation threats, the inclusion of these artificially established populations in recovery planning may well be critical to the long-term persistence of the species.