2018 Taxonomic Studies in Curaçao

Assessing the taxonomic status of the Iguana iguana population on Curaçao and the threat of hybridization with already present non-native I. iguana lineages

Report submitted by Matthijs P. van den Burg (University of Amsterdam) and Catherine Malone (Utah Valley University)

Iguana iguana
Landscape of Curaçao. Photo by Thijs van den Burg.
Iguana iguana
Adult iguana on Curaçao basking on large cactus. Photo by Thijs van den Burg.

Objectives.  Common Green Iguanas were studied on Curaçao to analyze their uncommon appearance. The following objectives outlined our aims.

  1. We will collect genetic data from individuals across Curaçao, which will allow us to 1) identify if non-native iguanas are present within the Iguana iguana population on Curaçao, and 2) if so, whether hybridization between those non-native iguanas and native iguanas has occurred, and then 3) determine the source(s) of the non-natives.
  2. Create and disseminate public education and outreach materials. Depending upon the connections and interest on the island, this may take the form of radio spots, newspaper editorials, and/or public presentations.
  3. Collect morphometric data from specimens in museum collections, as well as from living iguanas on Curaçao and from regional populations. Using morphometric analyzes we will study the presence of locality/regional morphometric characteristics.
  4. We will use our genetic and morphometric data to perform a taxonomic review of the Curaçao population and nearby regional populations.

Background.  The extremely arid climate of the ABC islands makes it a surprising place to find Common Green Iguanas. The mean annual rainfall for Curaçao was 553 mm from 1971–2000. The islands have a distinct dry and wet season that peak between March–June and October–January, respectively. At the onset of the wet season, iguanas are in such a water deficit that they leave the safety of their perches and soak in the puddles. We began our fieldwork on Curaçao on the first days of rain for the season and witnessed the lethal consequences of iguanas soaking in the puddles that form in the road potholes. This was true for iguanas on the main highways, but also within the National Park.

Rainfall data from Curaçao, collected since 1830, shows a cyclic history of 2–4 year periods of extreme droughts every 12–15 years. During, or shortly after these periods, locals report seeing many emaciated and weak iguanas, including iguanas falling out of trees. Additionally, following the last two extreme droughts, local biologists and the public became so alarmed at the drop in iguana numbers that they initiated captive breeding programs and imposed hunting regulations in order to boost population numbers. Interestingly, these measures were undertaken as a response to “overhunting,” but it is equally plausible that the population size on the island naturally fluctuates with extreme drought events. Unfortunately, no systematic data was ever collected to test the impact of drought on iguana populations. However, anecdotal evidence and logic support the idea that these periods would have greatly affected iguana survival as vegetation and fresh water was not, or only scarcely, available for prolonged periods.

Genetic Results.  During seven fieldwork weeks we caught 263 iguanas on Aruba, Bonaire, and Curaçao (ABC islands) combined, and 56 iguanas in Colombia. DNA extraction from a subset of these samples was performed and DNA sequencing and analysis of the mitochondrial (ND4) and nuclear (PAC and MLH3) loci on a large subset of the samples (139, 127, and 6, respectively) has been completed. These data show that all iguanas captured on Curaçao and Bonaire, as well as a subset of iguanas on the Colombian mainland, share mitochondrial haplotypes that belong to the previously identified ‘Curaçao’ clade. At the PAC locus, in addition to ‘Curaçao’ clade haplotypes on Curaçao and Bonaire, we recovered haplotypes from South American clades West and East of the Andes. Given that nuclear DNA coalesces more slowly, this is not unexpected and is likely reflecting older connectivity between populations. Altogether, these data suggests an absence of invasive iguanas on both Curaçao and Bonaire.

In Aruba, mitochondrial haplotypes of both the ‘Curaçao’ and the South American ‘West Andes’ clade were found. A shallow ocean floor and existence of a land bridge between Aruba and the mainland during glacial periods could have facilitated iguana dispersal. This has also been suggested to explain the presence of Crotalus unicolor on Aruba. Lastly, preliminary genetic data from sampling the circumference of Santa Marta mountains (~700 km) in Colombia reveals three mitochondrial groups, one of which corresponds to the ‘Curaçao’ clade. Many of the Colombian individuals in the mtDNA ‘Curaçao’ clade also have nuclear haplotypes restricted to Curaçao and Bonaire. Though very preliminary, this could suggest assortative mating where these Colombian haplogroups come into contact.

Outreach.  Within our outreach and education components, we published five articles in regional newspapers Amigoe and Antilliaans Dagblad, reaching all ABC islands. These articles were used to discuss our activities and the importance of iguanas in the islands’ ecosystems. One additional article will be published to communicate our final findings to the local communities.

Iguana iguana
American Kestrel with a recently caught hatchling iguana. Photo by Thijs van den Burg.

We created educational material for incorporation into the biology curriculum made available to schools through the local NGO, CARMABI. After consultation with their Educational Manager, Cor Hameete, we created documents and exercises for high school and primary school-age students that highlight the biology of iguanas and their importance in nutrient cycling and ecosystem connectivity. For primary school children, we commissioned illustrator Nienke Beets to create a coloring plate that visualizes the tropical dry forest habitat of the ABC islands including multiple parts of the carbon cycle that involve iguanas. Along with the plate, we created accompanying text and exercises for educators to incorporate into their curricula. These files were shared with all ABC island NGOs (CARMABI, Stinapa, Arikok) to be distributed to schools and also to National Park Offices for use with interested local visitors and tourists. Furthermore, we helped build on-island capacity by demonstrating and explaining our fieldwork techniques to park rangers on Curaçao (CARMABI) and on Aruba (Arikok). We spent two days with CARMABI personnel and one day with Arikok personnel and aided their understanding of (iguana) ecology and ecosystem functioning. Lastly, at the Christoffel National Park (Curaçao) we bead-tagged two iguanas so that park rangers can educate visitors about iguanas, ongoing research, and methodology.

It is important to note that, prior to this project, no educational materials regarding island ecology of the ABC islands incorporated Common Green Iguanas.

Iguana iguana
Catherine Malone (left) and Thijs van den Burg (right) collecting morphological data from an iguana on Curaçao. Photo by Wilfredo Falcon.
Iguana iguana
Catherine Malone (right) and Thijs van den Burg (left) collecting morphological data from an iguana on Curaçao. Photo by Wilfredo Falcon.

Morphology Results.  We visited the Smithsonian, D.C., and the Field Museum, Chicago, to collect morphological data from 72 specimens representing the range of I. iguana. These data were also collected during our fieldwork efforts.

Preliminary data analysis show that, in contrary to data published in the 1980s (Bakhuis 1982), Curaçao iguanas do show sexual size dimorphism and attain larger SVL size than previously published. Potential reasons for this prior-to-current discrepancy are 1) higher hunting pressure on iguanas in the past that would have biased data toward a smaller size range, and 2) earlier data were collected during or just following an extensive drought, which could have resulted in higher mortality of larger individuals. Currently, we are still processing our collected data and will complete this in the first half of 2019. However, preliminary morphological data indicates that genetically defined clades cannot be defined by morphology.

Completion of this project component awaits full morphological and genetic data analysis. Our preliminary genetic data indicate that some boundaries of clades recovered from prior genetic studies on Common Green Iguanas have shifted, but the clades themselves remain intact and taxonomic revision seems warranted. In particular, the Curaçao and Bonaire populations maintain genetic integrity for one of the clades whose boundary now extends to the dry region of northeastern Colombia and likely into the Maracaibo Delta (but data are not available from that region yet).

Iguana iguana
Iguana captured on Curaçao. Photo by Thijs van den Burg.
Iguana iguana
Iguana captured in northeastern Colombia. Photo by Thijs van den Burg.

Future.  In the immediate future we will continue perform data collection and analyses to complete the current project, after which we will disseminate its final results to funding organizations/parties, collaborators, local newspapers, and to the scientific community. If taxonomically elevated, the Curaçao/Bonaire group would deserve high conservation and research attention, and a large range of projects would be of interest to gain insight into their ecological adaptations.