Galapagos iguanas are thought to have had a common ancestor that floated out to the islands from the South American continent on rafts of vegetation. The divergence between land and marine iguanas has been estimated at 10.5 million years ago. Geneticists estimate that the pink iguana diverged from the other land iguanas approximately 5.7 million years ago — before most if not all of the current islands existed, while the divergence between the two yellowish iguanas is fairly recent.
There are three species of land iguana found in the Galapagos Islands. The well-known yellowish land iguanas include Conolophus subcristatus, native to six islands, and Conolophus pallidus, found only on the island of Santa Fe. A third species of land iguana (Conolophus marthae), the pink or rosada iguana, was first seen in 1986 and remained unstudied until the 2000s. It is found only on Wolf Volcano at the northern end of Isabela Island. It has a pinkish head, and pinkish and black body and legs, often with black stripes. The new species is morphologically, behaviorally, and genetically distinguished from the other two.
A Land Iguana shows off its amazing cactus-chomping mouth. (Photo © Don Audette)
Land iguanas are large — more than 3 feet long — with males weighing up to 30 pounds. They live in the drier areas of the Islands, and in the mornings can be found sprawled beneath the hot equatorial sun. To escape the heat of the midday sun, they seek the shade of cacti, rocks, trees or other vegetation. At night they sleep in burrows dug in the ground, to conserve their body heat. They feed mainly on low-growing plants and shrubs, as well as fallen fruits and cactus pads. These succulent plants provide them with the moisture they require during long, dry periods. Land iguanas show a fascinating symbiotic interaction with Darwin’s finches, as do giant tortoises, raising themselves off the ground and allowing the little birds to remove ticks.
Land iguanas reach maturity between 8 and 15 years of age. Males are territorial and will aggressively defend specific areas that typically include more than one female. Following the mating period, female iguanas find suitable nesting sites, dig their nesting burrow, and lay between 2 and 20 eggs. The female defends the burrow for a short time, to prevent other females from nesting in the same place. The young iguanas hatch 3-4 months later, and take about a week to dig their way out of the nest. If they survive the first difficult years of life, when food is often scarce and predators are a danger, land iguanas can live for more than 50 years.
In 1959, the status of the extant populations of land iguanas was considered good. Then in 1975, two populations on different islands (Cerro Cartago on Isabela and Conway Bay on Santa Cruz) were decimated in less than six months by feral dog packs. Unlike tortoises, adult iguanas are not predator-proof. Saving them meant removing them from their natural habitat until dogs were eliminated.
A breeding and rearing center was quickly established, but it was not large enough for all of the adults. A management technique used only once before in Galapagos, in the 1930s, was implemented. Thirty-eight Santa Cruz iguanas, about half of the original group brought to the center, were released on the small islets of Venecia off the northwest coast of Santa Cruz. This semi-captive population lived under natural conditions, but the islets had no large areas suitable for nesting. Approximately 100 m3 of soil was moved to Venecia from Santa Cruz and an artificial nesting area was built. The population thrived. The iguanas on Venecia continue to breed today and many of the resulting juveniles are repatriated to Santa Cruz, approximately every three years.
Unlike tortoises, the young land iguanas could not be repatriated to their original habitat unless the introduced predator problem was solved. Dogs eat adults as well as young iguanas, while cats eat only young animals. Once feral dogs had been eliminated on both southern Isabela and northwestern Santa Cruz, iguana repatriations were generally successful.
The land iguanas of Baltra have a very different history. Historically, the Baltra iguanas were the largest in the archipelago. However, when the Hancock Expedition visited the island in 1932 and 1933, the iguanas appeared malnourished. Introduced goats had devastated the vegetation. In an attempt to help the iguanas, members of the expedition transferred 70 iguanas to North Seymour, the island to the north of Baltra where there were no land iguanas and no goats. Within 20 years, the iguanas on Baltra disappeared due to a combination of habitat destruction resulting from the construction of the U.S. air base in World War II, predation by dogs and cats, and competition by feral goats. The informal experiment of the Hancock Expedition had saved the Baltra land iguana from extinction.
In the 1980s, iguanas from North Seymour (where the population seemed to be in decline) were brought to the breeding and rearing center, with the idea of eventually repatriating the young to Baltra. Given that Baltra has two military bases, air force and navy, iguana repatriations required the collaboration not only of the CDF and GNPS, but also the Ecuadorian Armed Forces. The first 35 young iguanas were released in June 1991. A total of 420 iguanas have been repatriated to Baltra and their survival rate appears high. Recent surveys have shown that both populations, Baltra and North Seymour, are healthy and increasing.
Today, all of the populations appear to be healthy, although the National Park must continue to do periodic cat control in critical land iguana habitat at Cerro Cartago and Cerro Dragón. The last repatriations were made to Cerro Dragón in 1991 (except for transfers from Venecia), Cerro Cartago in 1993, and Baltra in 2008. The land iguana breeding and rearing program was considered a complete success and ended in 2008.
Just about every rocky shoreline in the Galapagos Islands is home to the marine iguana (Amblyrhynchus cristatus), the only sea-going lizard in the world. The marine iguana is an extraordinary animal that lives on land but feeds in the sea, grazing on a variety of seaweed – on exposed rocks, in subtidal areas, or by diving deeper into the cold seawater. This habit, totally unique in iguanas and in fact all lizard species of the world, provides them with an abundant food source. However, they cannot withstand the cold temperatures of the sea for too long and must pull out on land to warm up.
A juvenile marine iguana on Isabela Island. (Photo © Kimber Wukitsch)
Marine iguanas also mate and nest on land. While they have few predators in the sea, on land, young iguanas fall prey to hawks, herons, and other birds. Predation by introduced cats has had a major impact on many populations. Found throughout the islands, concentrations of up to 4,500 individuals per mile are not uncommon in some areas. The total population has been estimated at between 200,000 and 300,000.
The short, blunt nose is well-adapted to feeding on algae growing on rocks. The flattened tail is perfect for swimming, propelling the iguana through the water while its legs hang useless at its sides. Iguanas rid themselves of excess salt, consumed along with the algae, by a special gland connected to their nostrils. Marine iguanas are an excellent example of a species well-adapted and continuing to adapt to their environment. While marine iguanas feed mainly on algae, they have also been known to consume crustaceans and grasshoppers. On one or two islands, a small percentage of marine iguanas have been observed feeding on terrestrial vegetation, perhaps an adaptation to the near complete absence of nutritional sea algae during strong El Niño events.
When marine iguanas go hungry, they don’t just become thinner, they get shorter too. A scientist recently found that in times of El Niño-induced famine, the marine iguanas will shrink in length and then regrow as food becomes plentiful again. This finding, reported in the scientific journal Nature, is the first of a shrinking adult vertebrate. The adult iguanas can switch between growth and shrinkage repeatedly throughout their lifetime – a perfect adaptation to the boom and bust cycles in Galapagos associated with El Niño. The researchers postulate that bone absorption accounts for much of the reduction, with iguanas literally digesting part of their bones to survive.
Marine iguanas show their color as they mature – the young are black, while adults range from red and black, to black, green, red and grey, depending on the island, with Española marine iguanas being the most colorful of all, and earning them the nickname “Christmas Iguanas.” Marine iguanas become more colorful in the breeding season, at which time males defend territories on land where they mate with the females, who then lay their eggs in burrows. Marine iguanas lay 2 to 3 large eggs, which hatch between 2 ½ and 4 months later. Marine iguanas are known to live up to 60 years.
El Niño events cause the greatest mortality in marine iguanas, with up to 70% dying in some populations in the great 1982-83 El Niño. Recent research has also found high mortality of Galapagos marine iguanas on Isla Santa Fe resulting from the subtle long-term effects of the 2001 oil spill from the grounded tanker Jessica.