2017 Útila Research Update

Population, hybridization and nesting ecology of the Critically Endangered Útila Spiny-tailed Iguana Ctenosaura bakeri on the island of Útila, Honduras.

Report submitted by Daisy Maryon and David Lee, University of South Wales

Ctenosaura bakeri
Útila Spiny-tailed Iguana in mangrove habitat. Photo by Thijs van den Burg.

This project investigates the population size, habitat preference, and nesting ecology of Ctenosaura bakeri, and its current rate of hybridization with Ctenosaura similis. There is an urgent need to update our previous studies in light of increased habitat degradation, fragmentation and loss, and poaching of iguanas.

Population size and demography data in the western side of Útila. Continuing from fieldwork in 2016, we established two new sites on the island for population monitoring. This was possible with boat transportation funds provided by the IIF, since we were now able to access the western and interior areas of Útila with greater ease. Nine sites were surveyed across the island in 2016 and 2017, totaling 28 transects from 50–500 meters long. To date in 2017, we have 238 Ctenosaura bakeri sightings along transects — this data will enable us to provide a more accurate estimate of population size and demography. Our population estimate will incorporate natural variation based on habitat type (see below) and location access, as a potential surrogate for hunting pressure.

Since 2016, we have collected biometric data on 181 Ctenosaura bakeri, and were able to determine 69 of those were female and 74 were male. The longest iguana found was a male with a 30.3 cm snout-to-vent length, a tail length of 42 cm, and a weight of 800 grams. The biggest mass found was also a male at 867 grams. The largest female found was 357 grams with a snout-to-vent length of 22.5 cm and tail length of 25.8 cm. Of the females caught in 2016–2017, 55% had regrown tails, while tail regrowth occurred in 31% of the males caught during the same period.

Ctenosaura bakeri
Kayaking to survey sites through the mangrove canals of Útila. Photo by Siel Wellens.
Ctenosaura bakeri
Daisy Maryon releasing an iguana back into the mangrove after processing. Photo by George Lonsdale.

Habitat data collection. We set up transect lines at 50 meter intervals to collect data on habitat structure. Our transects comprised almost all habitat types on Útila and will be used to ground-truth satellite imagery in the second half of this project. During our transect surveys, iguanas were found most often in mangrove habitats. A habitat classification analysis has been generated for Útila using the July 2015 LANDSAT 8 satellite, which enabled us to look at the extent of habitat availability on Útila for Ctenosaura bakeri. In 2015, the extent of mangrove habitat (their preferred habitat) was 8.4 sq. km. The extent of mangrove within Útila’s only protected area, Turtle Harbor Wildlife Refuge, appears to be 1.47 sq. km, meaning only ~17.5% of preferred iguana habitat is currently protected. We will combine the satellite data from the last 15 years with ground-truthed habitat data to gather information on how these areas have changed and how much habitat may have been lost.

The Turtle Harbor Wildlife Refuge appears to be a prime area for hunting both iguana and sea turtles; remains of each were found and hunters caught on camera traps. It is our opinion that this nature reserve should be expanded, or a new nature reserve proposed, and that adequate protection is provided for mangroves.

Genetic analysis. Habitat degradation has changed the degree to which the two iguana species are in contact with each other. This year we collected new samples from 35 Ctenosaura bakeri and 25 Ctenosaura similis. We aim to investigate the current rate of hybridization between the two species and determine what effect this may have on the Ctenosaura bakeri population. Data collection is continuing until late September and genetic analysis will be conducted in our United Kingdom lab starting October 2017.

Ctenosaura bakeri
Útila Spiny-tailed Iguana hatchling in the mangrove. Photo by George Lonsdale.

Hatchling and nesting ecology. Hatchlings have been recorded from February through August so far. The earliest sighting was on 21 February 2017 and the first hatchlings caught and processed were on 21 April 2017. February is a much earlier period for hatchlings than previously thought and this may indicate an extension to the breeding season. Hatchlings were spotted along transects in a range of habitats including sandy beaches, piles of trash, and white, black, and red mangrove. We had one nest viewable with a camera trap. Although this nest was shallow, and we filmed raccoons, cows, feral dogs, and cats walking over the nest, in July it had 100% hatching success from 10 eggs.

Community outreach and environmental awareness. This project has supported an education program run at five schools on Útila with the Kanahau Útila Research and Conservation Facility (KURCF), the Bay Island Conservation Association, and the Whale Shark and Oceanic Research Centre. The program features lectures on Ctenosaura bakeri and their habitats. For the adult Útilan, expat, and tourist communities, we conducted small awareness surveys, citizen science monitoring surveys in the mangroves, and hosted beach clean-ups to expand local knowledge about the species and increase awareness about its conservation. In April 2017, Daisy Maryon and KURCF staff members met with the Instituto de Conservacion Forestal (ICF), the Honduran protected areas agency, in Tegucigalpa and gave a presentation about the threats to Ctenosaura bakeri and our research program. This has led to a good working relationship with ICF, and hopefully a good partnership going forward into the future.

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Kanahau Team and beach cleanup volunteers restoring a nesting beach on the western side. Photo by Siel Wellens.
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Field guide Nahun Molina with a young female Ctenosaura bakeri. Photo by Daisy Maryon.

Capacity Building and Tools. Thanks to the IIF grant, this year we were able to train and pay a field guide, Nahun Molina, to accompany us on population and habitat surveys. This is a small, but vital first step in providing an alternative livelihood to hunting iguanas on Útila. Nahun no longer hunts iguanas on Útila but lives in the community were hunting is common.

The camera traps have been useful in multiple ways. Unexpectedly, they have provided interesting insights into the periods that hunters use particular areas for hunting. We have also accumulated spatial data from them by identifying where Ctenosaura bakeri and Ctenosaura similis coexist naturally and the diurnal activity patterns of iguanas in these areas. All over the island, we’ve gathered data on where potential predators occur. Invasive raccoons have been caught on camera traps within the protected Turtle Harbor Wildlife Refuge and coming into contact with iguana nests.

Future. We intend to continue to monitoring the population and collecting genetic samples, together with Kanahau Útila Research and Conservation Facility. We would like to further explore the interior and western sides of Útila and try to identify an area that could be proposed as a new nature reserve. We also plan to investigate the nesting season in further detail to determine whether Ctenosaura bakeri has a longer, or possibly two, nesting seasons and what parameters may be driving this. We also think it is essential to expand community involvement and awareness about these iguanas, and if possible hire a second guide.

Summary Highlights
  • Deployed camera traps at two sites to monitor iguana interactions and nesting behaviors
  • Recorded the presence of nests and hatchlings, and habitats at both sites with camera traps
  • Determined the sex ratio of hatchlings to establish whether our observed adult sex ratio (fewer females) is present at hatch or develops post-hatching
  • Created 28 distance-sampling line transects to provide an updated estimate of population size
  • Collected biometric data for 181 captured and tagged iguanas to assess body condition and health of the population
  • Collected habitat data from 119 plots across the island
Ctenosaura bakeri
Male and female in branches of black mangrove. Photo by Thijs van den Burg.