Radiant Reptiles: The Day of the Iguana
From Daniel Defoe’s Robinson Crusoe to Tom Hanks in Cast Away, oceanic islands have provided rich settings for great stories. Isolation and limited resources are ideal conditions for brewing epic drama as protagonists learn to prevail over their circumstances or perish trying. But novelists and movie directors aren’t alone in understanding the allure of remote islands. Scientists have been equally fascinated for hundreds of years—ever since sailors returned to the mainland with tales of strange creatures they’d seen during island encounters.
When Charles Darwin explored the Galapagos and Alfred Russel Wallace ventured into Malaysia during the 19th century, they were immediately drawn into the biological drama that has been unfolding on oceanic islands around the world for millions of years. It’s the gripping saga of adaptive radiation.
In a nutshell, adaptive radiation is the diversification of a single ancestral group of animals into many different species. Darwin’s description of the finches of the Galapagos Islands is a classic account of how these birds, after arriving on the islands, gradually adapted to different niches. The Galapagos Islands are also home to another vivid example—the land iguana and the marine iguana.
Between 200 and 225 million years ago when the original land mass of earth (Pangea) broke up into the subcontinents of Laurasia and Gondwana, the ancient ancestor of the modern iguanid family, which includes roughly 700 lizard species, was distributed in what would become the new world (the Americas). Over time, the animals dispersed into many islands in the Caribbean, islands off the coast of Mexico, as well as the Galapagos, located about 600 miles off the coast of Ecuador. There is also one species on Fiji and two genera on Madagascar. Exactly what this ancient-iguana progenitor looked like remains something of a mystery. Yet despite the tremendous variation between different species of iguanas, certain features (boxy head shape, spiny protrusions along the back, distinctive scale patterns around the mouth and throat) unite them.
The iguanas of the Galapagos descended from animals that made the sea voyage to the tiny island group, possibly atop a raft of vegetation or seaweed or even a land bridge that may have linked the islands to the South American mainland.
The Galapagos Islands themselves are essentially divided into two regions—the arid antral areas and the marine perimeter. The land iguanas and the marine iguanas are each perfectly suited to these two niches. The family resemblance between the two species is clear, and yet the land iguana is much more colorful, with skin that is lighter and bright with patches of yellow-orange and brownish red. This coloration enables it to blend in with the dry soils of the inner islands, where it subsists primarily on prickly pear cactus. Its marine cousin is almost black, like much of the volcanic rock on which it lives.
The marine iguana holds the distinction of being the world’s only aquatic lizard. It can dive to depths of nearly 50 feet and remain submerged for up to 30 minutes—an adaptation that makes possible a diet of seaweed and algae. Another adaptation peculiar to this iguana is a special gland that allows the animals to expel the salt that accumulates in their bodies. The white patches often visible on their heads and necks are salt that they have “sneezed!” The marine iguana’s face is also more blunt than its inland relatives, a feature that makes it easier to crop underwater plants off of rocks.
The West Indies is home to one of the most endangered groups of lizards in the world. The 16 species of rock (or “whorltailed”) iguana make up a vibrant kaleidoscope of adaptations to the many micro-habitats in this island group, which includes the Antilles, the lesser Antilles, and the Bahamas. These iguanas (descendants of ancestors who arrived on the islands during the Miocene, roughly 12 million years ago) are generally bulkier and slightly shorter than the mainland iguana and, depending on the soils and vegetation of the various isles, have developed a dazzling spectrum of colors and patterns to help them hide—from the blue iguana of Grand Cayman Island to the golden San Salvador Island iguana and the sandy-tan Cuban iguana. Rhinoceros iguanas are equipped with horny projections on their snouts and bulbous ornamentation on their heads and necks, while the Bartsch’s iguana (from Booby Cay, Mayaguana) is smooth and streamlined.
The Exuma Island iguana is one of the smallest of the rock iguana species and is found only in seven small cays in the Exuma chain in the Bahamas. The Los Angeles Zoo and Botanical Gardens was once home to the only captive group of these rare lizards—which came to the Zoo in 1998 after being confiscated by the U.S. Fish and Wildlife Service from an individual who tried to smuggle them home from a Caribbean cruise. Like other iguanas they are primarily herbivores though they will eat small lizards and insects on occasion. This species exhibits unusual social behavior that is neither territorial nor hierarchical. Some biologists believe this is an adaptation to the tiny range that these animals occupy—a kind of communal approach to making the most of limited resources.
The South Pacific is home to two subspecies of Fiji Island iguana—the crested and the banded. (The latter can be found in the Zoo’s collection [pictured above], on breeding loan from the government of Fiji via the San Diego Zoo.) Millions of years ago these lizards also survived a sea journey to this island group. Gradually these arboreal lizards became more colorful (to blend with the tropical foliage) and reduced in size, smaller bodies meant greater success living in the treetops. When the last ice age ended, about 18,000 years ago, the sea level slowly rose and their range became even smaller and more isolated.
In addition to the Fiji Island species of iguana, the Zoo’s reptile collection features a mainland species desert iguana and other iguanids including a San Esteban Island chuckwalla, spotted chuckwalla, and Cape Rock lizard, which can be found in the LAIR. Make part of your next Zoo visit a tour of the vibrant adaptations of these ancient survivors.
Originally Published in the Spring 2005 Edition of Zoo View.