Giving Iguanas a Headstart

There’s a reason tourists flock to the Caribbean in the winter: the weather is warm and dry, mosquitoes are almost nonexistent, and there’s enough sunshine and sand to curb any northerners’ case of the winter blues. 

Summer in the Caribbean is another story entirely. The sun that was so inviting in the winter becomes overbearing, temperatures rise from warm to sweltering, the humidity skyrockets, thunderstorms prevail, mosquitoes multiply, and hurricanes force humans to seek storm shelters or evacuate. The Caribbean is downright oppressive in the summer for most humans, and most definitely for me. 

However, for most creatures that are endemic to the Caribbean, summer is not oppressive.  Indeed, summer often provides the conditions they need to reproduce.  Anegada iguanas are among these creatures.  Female Anegada iguanas lay a single clutch of eggs underground in warm moist sand each July.  The eggs incubate for three months and hatch in October.  The hatchling iguanas work together to excavate an escape tunnel to the surface and disperse once above ground.

Reproduction is not an issue for wild Anegada iguanas, which are among the most endangered lizards on earth, but survival of hatchlings is a huge problem due to the presence of introduced nonnative cats. The problem is so severe that Anegada iguanas are in danger of going extinct without help. 

To combat the low recruitment of Anegada iguanas the British Virgin Islands National Park Trust and the San Diego and Fort Worth Zoos established a headstart program for young iguanas on Anegada in 1997.  Headstarting involves collecting juvenile iguanas before the cats can eat them and rearing them in captivity until large enough to fend for themselves in the wild.

Every year since 1997, a team of biologist has descended on Anegada in the heat of summer to locate and protect iguana nest sites. Finding iguana nests is a difficult job though. First, Anegada is a large island with lots of potential nesting habitat (open areas with sandy soil). Second, there are only a couple hundred iguanas remaining on Anegada and not all females reproduce each year. Third, nests are underground and well concealed. 

Eggs are deposited in an underground nest chamber at the end of a long access tunnel excavated in the sand. After laying their eggs female iguanas backfill the nest tunnel and smooth the soil surface to conceal the nest site. 

Finding a nest requires being there during the nesting process or very shortly after, while disturbance to the soil is still visible. Once the soil dries (or it rains) it is almost impossible to identify a nest site. Consequently, finding an Anegada iguana nest is like finding the proverbial needle in a haystack. To give you an idea just how difficult it is, we have never found more than five nests in a given year, despite spending hundreds of man-hours searching over a month-long period each year.

So, every nest is precious and hard won. To protect them, we fence off each site with metal flashing about two feet high. The fences act to contain the hatchling iguanas upon emergence and to exclude cats.  We check each nest several times a day, starting a week before emergence is anticipated, and collect the hatchlings as soon as they emerge and transfer them to the captive facility on island for headstarting.

All of this hard work has paid off. So far, we have returned over 156 headstarted iguanas to the wild on Anegada. Their post-release survival rate has been almost 80% and last year we documented our first nest from a headstarted female. You can bet we’ll be at it again this July!

Glenn Gerber, Ph.D., Scientist, San Diego Zoo Global Institute for Conservation Research.