During our Galapagos cruise, we have the opportunity to see both marine and land iguanas. Both iguanas come from a common ancestor thought to be closely related to the Ctenosaura, a genus of lizard native to Central America. Genetic studies indicate the split between the ancestral forms of marine and land iguanas could have occurred around 10.5 million years ago. It is accepted that there was only one arrival and establishment of this ancestral form sometime earlier when the Galápagos Islands had a different configuration than the present. Today the extant islands are all younger than 5 million years old, the first islands having been lost to the forces of erosion over the millennia as they were moved ever-further from the hotspot under the Nazca tectonic plate.
The marine iguana Amblyrhynchus subcristatus is the only species in the genus, and found in the Galapagos Islands with several subspecies or taxa on different islands. There is substantial variation in size and color among populations, with the smallest individuals inhabiting the northernmost islands, the largest inhabiting the westernmost islands and the most colorful found in the southeastern most islands, all consequences of the marine environment on which they depend for a living.
The only iguana in the world to feed exclusively underwater, the marine iguana has many unique physical adaptations for survival in a harsh environment such as a laterally-compressed tail used in underwater locomotion, long claws useful for climbing and gripping rocks in wave-washed tidal areas, jaw and tooth structure adapted to cropping seaweed off rock, an efficient desalination system to remove the considerable quantities of salt from their bodies and the ability to conserve body heat when underwater for considerable lengths of time. During the year, temperatures can fluctuate dramatically according to the season, but most brutally in the sea. Water temperatures in the Galapagos Islands can vary from as low as 66º F (18º C) to as high as 84º F (28º C). For a reptile, maintaining a healthy temperature in this environment is difficult, but they manage to maintain an almost perfect 96ºF (35.5 C) body temperature throughout the year.
Marine iguanas feed almost exclusively on several species of algae (seaweed) found growing on rocks down to several meters. Large individuals can easily dive to over 12 meters depth, and remain for almost an hour. However, by far more common are dives of shorter duration and depth. Small iguanas prefer to forage at low tides where they can completely avoid getting wet. Breeding takes place just as the seasons change for the warmer. Large males display, spar and fight for territories and breeding rights to females who choose to join them, or not. After mating, females depart for sandy areas where they dig burrows a meter in length and half a meter below the surface. Here, they deposit between one to four eggs, cover up the burrow, and in some cases will remain on site for up to 10 days in order to protect the area from other females who might attempt to dig in the same spot. Incubation is around three to four months, and once the hatchlings emerge, they must be quick and fast to avoid the numerous predators who are awaiting their chance and opportunity for a meal. Although no real data exists, it is thought Galapagos marine iguanas can live to around forty or fifty years old.
The Galapagos land iguanas are currently in the spotlight because of the recent discovery of a new taxa of land iguana in the islands. Up to a few years ago there were considered to be only two separate species of land iguana in the Galapagos islands: Conolophus pallidus and Conolophus subcristatus which had diverged from each other within the last million years. The populations of C. subcristatus can be found on several of the Galapagos Islands, with C. pallidus inhabiting exclusively the island of Santa Fe.
The Galapagos pink land iguana, “iguana rosada” (now known asConolophus marthae) was first seen in 1986, but not identified as a distinct species until 2009. It is found only on the island of Isabela, on the most remote volcano, Volcan Wolf. The iguana is distinguished by its pink body with stripes. Very little of its ecology is known at this time, and approximately 100 of the species are currently thought to exist.
With the latest mitochondrial DNA analysis of this most recent member of the Galapagos land iguana group, it is now clear that diversification occurred much earlier than thought, even earlier than the islands now existing. It appears that the origin of this relict lineage goes back to around 5.7 million years, which leaves researchers trying to understand how one of the oldest lineages in the Galapagos can be living on one of the youngest volcanoes (+/- 250,000 years old).
Land iguanas do not wander into the water. They prefer the arid land along the coastlines, or the arid sides of the taller volcanoes; moist, cool environments are avoided. They feed on a variety of vegetation types, enjoying the bright yellow flowers of Portulaca when in bloom, but also find acceptable the pads, fruits and flowers of the Opuntia cactus. Their appearance is quite difference from their marine cousins, being a yellow to beige/brown color overall, with some populations taking on an orange or reddish coloration during breeding season for some of the males. Their snouts are longer than the snub-nosed marine iguanas (“ambly”= blunt, “rhynchus”=snout), and both sport a spiny crest down their dorsal mid-line. The marine iguana crest however, continues the length of the tail whereas the land iguana crest tapers off mid-way down the spine. The tail of the land iguana is shorter and round, compared to the marine iguana, and the claws, though sharp, are not nearly as long.
Whereas marine iguanas can be found in close body contact and in large groups on shore, land iguanas are more solitary by nature. Males will maintain territories which overlap with the territories of several females, and it has been seen for several individuals to share a burrow in the evenings for warmth. Highly aggressive for the most part, blood is drawn frequently during battles between males. Strict displays and head-bobbing rituals are followed and can be used to distinguish between populations during confrontations. After mating, females are known to travel long distances in search of the perfect nesting area. Some even climb into the caldera of volcanoes in order to use the constant warmth of fumaroles. Incubation lasts from three to four months and depending on the population, anywhere from two to twenty-two eggs are laid in a burrow a few feet deep. Once laid, females leave and the hatchlings are on their own with no parental care as in most reptiles. If they survive the first few years, it is thought possible to live over sixty years for the Galapagos land iguana.
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