San Francisco State University
Department of Geography

Geography 316:  Biogeography

The Biogeography of  Marine Iguana (Amblyrhynchus cristatus)

Source: Martin Wikelski
by Elisa Gill, a student in Geography 316, Fall 1999

Kingdom:    Animalia
    Phylum:    Chordata
        Class:        Reptilia
            Order:        Squamata
                Family:        Iguanidae
                    Genus:        Amblyrhynchus
                        Species:      Amblyrhynchus cristatus


    The marine iguana is the only sea-going lizard.  The marine iguana (Amblyrhynchus cristatus) is an endemic species to the Galapagos Islands, an archipelago of approximately 25,000 square miles located 600 to 700 miles west of Ecuador in equatorial waters (Carpenter 1966).  They are related to the land iguana, which also lives on t he Galapagos Islands (Carpenter 1966).  The geographical range of the marine iguana is in the coastal areas of all of the islands, islets, and sea rocks of the Galapagos Islands.  There are probably more marine iguanas on the Galapagos Islands than any other creature.

Description of Species

    The marine iguana body is elongate and fusiform.  The average length of an adult male is approximately 1.3 meters and an adult female is approximately 0.6 meters in length.  The weight of these iguanas range between 0.5 kilograms and 1.5 kilograms (Berry 1984).  They are either black or dark grey in color except when males are near mating seasons when they turn shades of red and green.  The dark pigment in the skin helps shield the animals from the effects of ultraviolet rays.  It is also useful in warming the body.  The tail is compressed into a propelling structure which efficiently moves this lizard along the surface or beneath the water.  The long, sharp, recurved claws permit the lizard to hold fast to the lava in heavy seas or when submerged.  The snout of the iguana is short and blunt and this enables it to get its jaws into close contact with the substrate.  The tricuspate teeth are flattened laterally and lie in single rows along the sides of the jaws immediately inside of the labial scales (Carpenter 1966).  Marine iguanas can live up to thirty years (Berry 1984).
    They are adept at both swimming and diving.  Diving times have a duration of usually a few minutes, but can be extended beyond half an hour (Eibl-Eibesfeldt 1984).  Large iguanas can dive to depths of 15 meters. (Eibl-Eibesfeldt 1964).  Marine iguanas swim by using their tails to propel them and hold their legs tightly behind.  An interesting adaptation to marine life is the salt glands which allow the excretion of surplus salt taken into the body.  These glands open into the nose and their secretion is ejected with a jet of air through the nostrils (Dunson 1969).
    These iguanas are found in aggregations of 10 to over 1,000 individuals, usually within 30 meters of the sea (Boersma 1983).
    Before feeding the iguanas must raise their body temperatures to approximately 36 degrees Celsius.  These iguanas are ectotherms and can lose up to 10 degrees Celsius when in the ocean.  To regulate their body temperature they must bask in the sun for long periods of time.  Direct solar radiation infiltrates the iguana when it basks as well as obtaining heat from siting on lava rocks (basalt) that have absorbed heat.  This acquired heat is important to the iguana because to obtain food the iguana must go into the water.  The waters off the Galapagos Islands is controlled by the Humboldt Current.  The Humboldt Current is a cold water current and causes the iguanas to lose heat rapidly.


    Marine iguanas live in colonies where shallow reefs occur with an extensive intertidal zone and a rocky coastline approximately 2 to 5 meters above sea level on the Galapagos Islands.  Softer substrate is needed for egg laying.  There is a clear preference for southern shorelines, which are more exposed to wave action and where algae appear to grow more prolifically (Eibl-Eibesfeldt 1984).  They sleep either exposed on the coastline or even up in mangrove trees.  Young iguanas are found among the adults in most basking aggregations.  Unlike adults, they spend a great amount of time in cracks and crevices.  They are never far away from a hiding place.
    This endemic species has sparked a great deal of academic conversation about its’ origin.  There are land iguanas scattered throughout the world and even in the Galapagos Islands.  The marine iguana is the only iguana that feeds exclusively underwater.  As there is food available on the islands this has made zoologists ponder the origins of this adaptation.
Source: National Geographic

    Marine iguanas are herbivorous.  They feed almost exclusively on marine algae.  Nine species of algae have been identified as food sources.  Tylotus ecuadorianus, Bryopsis indica triseriata, Plocamium pacificum, Priontis abbreviata, Glossophore galapagensis, Loposiponia villum, Ptersosiphonia paucicorticata, Blossevillea galapagensis and Gelidium.  Occasionally they will also eat grasshoppers, crustaceans or even sea lion afterbirth (Carpenter 1966).  They are adapted physiologically to living in the coastal areas of the Galapagos Islands.  Male iguanas prefer to feed on submerged algae or at greater distance from the shore, whereas females generally forage intertidally.  Young iguanas forage only at low tide (Boersma 1983).  Most iguanas feed only once a day.
    After feeding a marine iguana must reset its body temperature and heart rate (from 30 to 100 beats per minute), because the digestive system works best at a constant, relatively high temperature (Wikelski 1999).  By flattening their bodies again the lava rocks, the lizards expose as much skin surface as possible to the sun.  The flow of heat is regulated by vessels in the chest which close and open to regulate body temperature.


Source: Martin Wikelski

    Females reach sexual maturity when 3 to 5 years old.  Males when they are approximately 6 to 8 years old (Dellinger, 1990).  Breeding season begins in December and extends through March. During mating season the male iguana will turn very vibrant colors.  In some animals this would be a means of attracting females, however studies can show no correlation between degree of color in males and reproductive success (Rauch 1988).  Females mate once per season.  A courting male will approach a female with his head down.  As he moves forward he will nod his head rapidly.  As he continues to approach he will circle the female or move up sideways into contact with her, moving his head along her tail.  Then he will through his front leg over the female, moving onto her back.  From this position, he will attempt to bit the skin of the neck or shoulder.  It is common for the female to move away at any stage of the courtship.  After obtaining a sufficient hold on the female the male will twist his tail while at the same time force it under the tail of the female to bring the cloacal regions together.  Copulation lasts approximately three to four minutes (Carpenter 1966).
    Approximately four weeks after mating females will leave the colony and roam between 20 meters to 3 kilometers to sandy areas (Rauch 1988).  Females will compete for sites by rhythmic nodding which is not unlike the behavior exhibited by males when courting.  Choosing the site usually takes between 2 to 3 days.  There the female will dig a hole in which to deposit her eggs.  Digging can take approximately half a day. The female marine iguana can lay between one and six eggs.  The eggs can weigh up to a quarter of the weight of the female(Eibl-Eibesfeldt, 1966).  The female will then guard the nest up to sixteen days.  Incubation of the eggs takes between 89 to 120 days.  When the infants hatch the iguana can weigh about 50 to 60 grams.  Upon hatching the infants tend to look around and then immediately run, often bipedally, to the nearest cover (Eibl-Eibesfeldt 1984).


Natural History

     The marine iguana has adapted to feeding under water by developing a pair of salt glands beneath the skin between the eye and the nostril on each side of the head which periodically ejects forcibly a fine spray of highly saline secretion.  The marine iguana can project this spray to over a foot in distance (Pough, 1989).Marine iguanas spend a great deal of their time basking on basalt to help absorb the heat.  Because the marine iguana is ecothermic the ocean is cold enough to immobilize the lizard if it is immersed too long.  This unique species has adapted to very different temperature variations.

   The marine iguana is an endemic species to the Galapagos Islands.  These islands are located between 600 and 700 miles west of Ecuador in equatorial waters.  They are related to the land iguana, which also lives on the Galapagos Islands, as well as the green iguana which inhabits the mainland of South America.  There are probably more marine iguanas on the Galapagos Islands than any other creature.  Estimates for their population are between 200,000 and 300,000 (Thornton 1971).

   Potential predator of the marine iguana include Galapagos hawk (Buteo galapagoensis), Lava Heron (Butorides sandevalli), Striated Heron (Butorides striatus), Great Blue Heron (Ardea herodias), Frigate Birds (Fergata minor and Fergata mgnificens), and Short-eared Owls (Asio flammeus).  Researchers believe that none of the native predators relied greatly on the marine iguana for food.  This hypothesis comes from the fact that marine iguanas are very docile and don't usually leave an area when molested.  There is also speculation that the meat of adult marine iguanas may be too saline (Thornton 1971).  They give every indication that they have no predators.  Marine iguanas are most vulnerable before birth or when juvenile.  However introduced cats, rats, pigs, dogs, and humans also prey upon iguanas of any age.
    The greatest impact on marine iguana populations has been El Nino.  The warm water that arises off the Galapagos Islands from El Nino causes the algae that the iguanas eat to die.  They are left with only brown algae to eat.  Brown algae is toxic to marine iguanas.  In the second to last El Nino event estimates are that 40 to 50 percent of the population died (Sayre 1984).
    Additional threats to the marine iguana include oil spills and other marine pollution.  These could destroy the algae on which the iguana depend as well as their coastal habitat.


    This endemic species has sparked a great deal of academic conversation about its' origin.  There are land iguanas scattered throughout the world and even in the Galapagos Islands.  However the marine iguana is the only iguana that feeds exclusively underwater.  As there is food available on the islands, this has made zoologists ponder the origins of this adaptation.
    Among Galapagos herpetologists there exist some differences of opinion as to the development of the native iguanas.  The Galapagos iguanas, the land iguana (Conolophus subcristatus and Conolophus pallidus) and the marine iguana (Amblyrhynchus cristatus), are theorized to have diverged between fifteen and twenty million years ago (Wyles and Sarich 1983).  This divergence of species predates the age of the Galapagos Islands, indeed these islands were created by hot spots, approximately 5 million years ago, and have never had contact with the South American continent.  Regardless these two iguanas are more similar in lineage than either is to any continental iguana.
    Immunological studies show high similarity of genetic differentiation in both genera (Higgins and Rand 1974).  There are two suggested theories for this evolution. (1) Divergence of the two taxa from a common ancestor with subsequent radiation, or (2) two separate ancestors from different iguana stocks having colonized the Galapagos recently (Rassman 1995).  There is not an abundance of paleontological information on these taxa so the two theories are continuing to be tested.  These two iguanas are largely considered to be sister taxa.  The lack of resolution of the iguanid phylogenetic relationships is surprising given the multitude of morphological and biochemical studies of this group (Avery and Tanner 1971; Higgins and Rand 1975; Wyles and Barich 1983; Etheridge and de Queiroz 1988).


    A complete population survey of the marine iguana has never been accomplished.  It is unlikely to ever be undertaken because of the nature of the coast lines which these iguanas inhabit.  However estimates of the population approach hundreds of thousands (Boersma 1984).  Visual counts on Santa Cruz Island showed between 1,800 to 2,000 iguanas.  Fernandina Island has an estimated 7,000 to 10,000 iguanas.  Fernandina Island estimates approach 385,000 individuals.  San Cristobal Island estimates 80 iguanas.  Published descriptions of marine iguana abundance (Darwin 1845; Beebe 1924; Pinchot 1931; Slevin 1959; Eibl-Eibesfeldt 1960) indicate that the marine iguana has been abundant over the past 150 years and has abundantly inhabited Fernandina Island since at least 1906 (Boersma 1983).

Map of Distribution:


Other interesting issues
   This species is completely protected by national legislation (Ecuador).  They are listed under C.I.T.E.S. Appendix II (nearly threatened with extinction).
    The marine iguana is a very interesting creature to study.  There does not exist a great deal of background information on this organism and further study seems warranted.  The future of the marine iguanas in unclear.  Extinction of this organism seems unlikely unless El Nino years proceed more frequently and cause their food source to die out.  One adaptation the marine iguana has made to combat the recurrent El Nino is to increase mating after El Nino events.  Populations of marine iguanas have regained number shortly after this event.  One cause for concern about the marine iguana is that the human population of the Galapagos Islands has risen from 4,000 people just 15 years ago to approximately 12,000 people today.  Population pressure on the islands may have adverse effects for the iguana.  Additionally, with the introduction of more people on the island that will mean more pets and domestic livestock.  Dogs, cats and pigs have impacted upon the population of marine iguanas.


Berry, R.J. A Natural History of the Galapagos. Academic Press, London, England 1984

Boersma, P. Patterns of Evolution in Galapagos Organisms. Pacific Division, AAAS. San Francisco, CA. 1983.

Carpenter, C. 1996 "The Marine Iguana of the Galapagos Islands, Its Behavior and Ecology"; Proceedings of the California Academy of Sciences. Vol. 34 (6) pgs.

Cordero, L. "Recovery of the Marine Iguana Population after the El Nino Catastrope"; Noticias de Galapagos 1986 Vol 44 (3); pgs. 4 - 5.

Dawson, W. et. al. "A Reappraisal of the Aquatic Specializations of the Galapagos Marine Iguana"; Evolution Vol 31 (1) 1997; pgs. 891 - 897.

Dellinger, T. 1990 "Sex Identification through Cloacal Probing in Juvenile Marine Iguanas"; Journal of Herpetology. 24 (4), pgs. 424-426.

Eibl-Eibesfeldt, I. Galapagos. Pergamon Press, Oxford, 1984.

Pough, F., et. al. Vertebrate Life. Macmillan Publishing, New York, New York 1989.

Quammer, D. Song of the Dodo. Scriber, New York, New York 1996.

Rassman, K. 1996 "Molecular Genetics and Conservation in the Galapagos" Noticias de Galapagos. 56 (3), pgs. 22 - 25

Rauch, N. "Competition of Marine Iguana Females for Egg-Laying Sites"; Behavior Vol 107 (1-2) 1988; pgs. 91 - 104.

Sayre, R. "Creatures"; Audubon 1984 (1) pg. 98.

Thornton, I. A Natural History of the Galapagos. The Natural History Press, Garden City, New York 1971.

Widelski, M. 1999 "Diving Dragons of the Galapagos"; [Online]. 11/10/99

Widelski, M. & Trillmich, F. "Foraging Strategies of the Galapagos Marine Iguana: Adapting Behavioral Rules to Ontogenetic Size Change"; Behavior Vol 128 (3-4) 1994; pgs. 225 - 279.



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