Marine Iguana

Reptiles of Ecuador | Sauria | Iguanidae | Amblyrhynchus cristatus

English common names. Marine Iguana, Galápagos Marine Iguana.

Spanish common name. Iguana marina.

Recognition. ♂♂ 133 cm ♀♀ 78.5 cm. Amblyrhynchus cristatus is one of four species of iguanas occurring on the Galápagos Islands. The other three are the land iguanas of the genus Conolophus. The Marine Iguana differs from all of them by having a blunt snout, a laterally flattened tail adapted to swimming, and a distinct, usually blackish, background color. Males and females of this species differ from each other in size, shape, and coloration. Adult males are larger, more brightly colored, and have an enlarged middorsal crest. Adult females are usually blackish, and have a smaller middorsal crest.

Marine Iguanas from different islands also differ in size and coloration. Iguanas living on large islands are usually bigger in body size, whereas those on islands smaller than 25 km2 are usually dwarf. Throughout most of the archipelago, the iguanas have a blackish color, but in the southern Islands (Española and Floreana) dominant males acquire a stricking "Christmas" coloration (bright red with blue legs) during the breeding season, though the cause for this unique color change remains enigmatic.

Special thanks to Nina Guayasamin, our official protector of the Marine Iguana for symbolically adopting this species and helping bring the Reptiles of Galápagos project to life.

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Natural history. Extremely common. Amblyrhynchus cristatus is a diurnal iguana that lives in large aggregations (up to 8,000 individuals/km coastline)1 along coastal volcanic outcrops, cliffs up to 50 m high, mangroves, and sandy beaches.2 It is a mostly terrestrial species that spends nearly 5% of its time in the sea, grazing seaweed off the surface of rocks.3 No other lizard in the world forages at sea. Marine Iguanas feed almost exclusively on algae, but may as well ingest beach plants,4 crustaceans, grasshoppers, cockroaches, fish carrion, and even sea lion feces and afterbirth.5 Hatchlings feed on the adult’s feces to obtain the gut microorganisms needed for the digestion of algae.6 Larger Marine Iguanas (heavier than 1.8 kg) dive down to 15 m in depth and for up to 20 minutes to feed,1,3 whereas smaller ones (lighter than 1.2–1.4 kg) forage on exposed intertidal rocks.2,7 After a dive, Marine Iguanas commute to their preffered resting sites (rocks with interspersed crevices or bushes)8 where they “sneeze” excess salt.2

Individuals of Amblyrhynchus cristatus spend the first hours after sunrise basking,2 and usually move into the sea between 7:30 and 8:00.2 During hot afternoons, some Marine Iguanas seek the shade of reef crevices, large boulders, mangroves, and shrubs,2 while others remain in the open. Just before sunset, they retreat into these same hideouts, but they can also spend the night exposed on the reef2 or up on the mangroves.9 Marine Iguanas are preyed upon by introduced species (such as pigs, dogs, cats, and rats) as well as by native species (such as hawks, owls, herons, gulls, snakes [Pseudalsophis biserialis, P. dorsalis, and P. occidentalis] hawkfish, and crabs).1,812 When threatened, Marine Iguanas run into crevices.13

The beginning of the Amblyrhynchus cristatus breeding season varies from island to island,1,6,14 but generally takes place in Dec–Jan2 over a period of slightly over three weeks.7 Male Marine Iguanas defend 1.2–38.8 m2 display territories by bobbing their heads,7 or fighting head to head with intruders.1,7 Between head-push duels, the males bite each other and at times wound each other seriously.7 Only a few males, usually the biggest,15 monopolize most of the matings, copulating with as many as 45 females per season.7 Some smaller, non-territorial "sneaky" males, similar in size to females, attempt to copulate inside territories when territorial males are absent.15 They increase their their chances to fertize females by preparing in advance with pre-copulatory ejaculation.16 Some males appear unable to distinguish the sex of female-sized individuals, with which they attempt to copulate.15 Females of A. cristatus mate only once per season,7 preferentially with the largest territorial males.15 Copulations last 2–20 minutes.7 About a month later, females, which compete fiercely over nesting sites,17 dig 30–40 cm holes in sandy soil17 and lay 1–6 eggs10 that take 89–120 days to hatch.7 Mockingbirds prey upon eggs.17 Hybrids between the Marine Iguana and the Galápagos Land-Iguana (Conolophus subcristatus) exist on Plaza Sur Island.18 The estimated maximum lifespan of A. cristatus is 30 years.19


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Conservation. Vulnerable.20 Amblyrhynchus cristatus is listed in this category because, although there are aggregations of thousands of Marine Iguanas,1,21 the species' extent of occurrence is estimated to be less than 500 km2, it experiences periodic population declines of up to 85% mortality due to El Niño events, and is threatened by pollution and invasive predators.20 There are also multiple genetically distinct populations of A. cristatus, some of which are extremely small in size (for example, the population on northeastern San Cristóbal).22,23 For most of these, there is a lack of solid information about population sizes, which makes them vulnerable to rapid enigmatic declines which may go unnoticed.

Distribution. Amblyrhynchus cristatus is endemic to an estimated 275 km2 area in Galápagos, Ecuador. Marine Iguanas occur on coastal areas of all major islands and most of their surrounding islets.

Distribution of Amblyrhynchus cristatus Distribution of Amblyrhynchus cristatus in and around Española Island Distribution of Amblyrhynchus cristatus in and around Floreana Island Distribution of Amblyrhynchus cristatus in Marchena Island Distribution of Amblyrhynchus cristatus in and around San Cristóbal Island Distribution of Amblyrhynchus cristatus in and around Santa Cruz Island Distribution of Amblyrhynchus cristatus in Santa Fe Island Distribution of Amblyrhynchus cristatus in and around Santiago Island Distribution of Amblyrhynchus cristatus in Pinta Island Distribution of Amblyrhynchus cristatus in Pinzón Island Distribution of Amblyrhynchus cristatus in western Galápagos

Etymology. The generic name Amblyrhynchus, which comes from the Greek words amblys (meaning “obtuse”) and rhynchos (meaning “snout”),24 refers to the blunt snout of the Marine Iguana. The specific epithet cristatus, which comes from the Latin word crista (meaning “ridge”) and the suffix atus (meaning “provided with”),24 refers to the crest of enlarged dorsal scales of the Marine Iguana.13

"The black lava rocks on the beach are frequented by large, disgusting clumsy lizards. They are as black as the porous rocks over which they crawl and seek their prey from the sea. Somebody calls them 'imps of darkness'…"

Charles Darwin, 1835.25

See it in the wild. Although the Marine Iguana can be seen year-round with ~100% certainty on almost any rocky beach in Galápagos, the best time to photograph it is during the peak of the breeding season (Dec–Jan), when males are strikingly colored.

Authors. Alejandro Arteaga and Juan M Guayasamin.

Academic reviewers. Dustin Rubenstein and Amy MacLeod.

Photographers. Jose Vieira and Alejandro Arteaga.

How to cite? Arteaga A, Guayasamin JM (2019) Amblyrhynchus cristatus. In: Arteaga A, Bustamante L, Guayasamin JM (Eds) Reptiles of Ecuador. Available from: www.tropicalherping.com

Literature cited.

1. Laurie WA (1989) Effects of the 1982–83 El Niño-Southern Oscillation event on marine iguanas (Amblyrhynchus cristatus, Bell, 1825) populations in the Galápagos Islands. In: Glynn P (Ed) Global ecological consequences of the 1982–83 El Niño-Southern Oscillation. Elsevier, New York, 121–141.

2. Carpenter C (1966) The marine iguana of the Galápagos Islands, its behavior and physiology. Proceedings of the California Academy of Sciences 34: 329–376.

3. Trillmich KGK, Trillmich F (1986) Foraging strategies of the Marine Iguana, Amblyrhynchus cristatus. Behavior, Ecology and Sociobiology 18: 259–266.

4. Vitousek MN, Rubenstein DR, Wikelski M (2007) The evolution of foraging behavior in the Galápagos marine iguana: natural and sexual selection on body size drives ecological, morphological, and behavioral specialization. In: Reilly SM, McBrayer LD, Miles DB (Eds) Lizard ecology: the evolutionary consequences of foraging mode. Cambridge University Press, New York, 491–507.

5. Wikelski M, Wrege PH (2000) Niche expansion, body size, and survival in Galápagos marine iguanas. Oecologia 124: 107–115.

6. Rubenstein DR, Wikelski M (2003) Seasonal changes in food quality: a proximate cue for reproductive timing in marine iguanas. Ecology 84: 3013–3023.

7. Trillmich KGK (1983) The mating system of the marine iguana (Amblyrhynchus cristatus). Zeitschrift Für Tierpsychologie 63: 141–172.

8. Wikelski M, Trillmich F (1994) Foraging strategies of the Galápagos marine iguana (Amblyrhynchus cristatus): adapting behavioral rules to ontogenetic size change. Behaviour 128: 255–279.

9. Eibl-Eibesfeldt I (1984) The large iguanas of the Galápagos Islands. In: Perry R (Ed) Key environments: Galápagos. Pergamon Press, Oxford, 157–173.

10. Laurie WA, Brown D (1990) Population biology of marine iguanas (Amblyrhynchus cristatus). II. Changes in annual survival rates and the effects of size, sex, age and fecundity in a population crash. Journal of Animal Ecology 59: 529–544.

11. Laurie WA (1983) Marine iguanas in Galápagos. Oryx 17: 18–25.

12. Christian EJ (2017) Demography and conservation of the Floreana racer (Pseudalsophis biserialis biserialis) on Gardner-by-Floreana and Champion islets, Galápagos Islands, Ecuador. PhD thesis, Auckland, New Zealand: Massey University.

13. Van Denburgh J, Slevin JR (1913) Expedition of the California Academy of Sciences to the Galápagos Islands, 1905-1906. IX. The Galapagoan lizards of the genus Tropidurus with notes on iguanas of the genera Conolophus and Amblyrhynchus. Proceedings of the California Academy of Sciences 2: 132–202.

14. Partecke J, von Haeseler A, Wikelski M (2002) Territory establishment in lekking marine iguanas, Amblyrhynchus cristatus: support for the hotshot mechanism. Behavioral Ecology and Sociobiology 51: 579–587.

15. Wikelski M, Carbone C, Trillmich F (1996) Lekking in marine iguanas: female grouping and male reproductive strategies. Animal Behavior 52: 581–596.

16. Wikelski M, Bäurle S (1996) Pre-copulatory ejaculation solves time constraints during copulations in marine iguanas. Proceedings of the Royal Society of London. Series B: Biological Sciences 263: 439–444.

17. Eibl-Eibesfeldt I (1966) Das Verteidigen der Eiablageplätze bei der Hood-Meerechse. Zeitschrift für Tierpsychologie 23: 627–631.

18. Rassmann K, Trillmich F, Tautz D (1997) Hybridization between the Galápagos land and marine iguana (Conolophus subcristatus and Amblyrhynchus cristatus) on Plaza Sur. Journal of Zoology 242: 729–739.

19. Berry RJ (1984) Evolution in the Galápagos Islands. Academic Press, London, 270 pp.

20. Nelson K, Snell H, Wikelski M (2004) Amblyrhynchus cristatus. The IUCN red list of threatened species. Available from: http://www.iucnredlist.org

21. Bowman RI, Berson M, Leviton AE (1983) Patterns of evolution in Galápagos organisms. Pacific Division, AAAS, San Francisco, 568 pp.

22. Miralles A, Macleod A, Rodríguez A, Ibáñez A, Jiménez-Uzcategui G, Quezada G, Vences M, Steinfartz S (2017) Shedding light on the Imps of Darkness: an integrative taxonomic revision of the Galápagos marine iguanas (genus Amblyrhynchus). Zoological Journal of the Linnean Society 181: 678–710.

23. MacLeod A, Steinfartz S (2016) The conservation status of the Galápagos marine iguanas, Amblyrhynchus cristatus: a molecular perspective. Amphibia-Reptilia 37: 91–109.

24. Brown RW (1956) Composition of scientific words. Smithsonian Books, Washington, 882 pp.

25. Keynes RD (2001) Charles Darwin's Beagle diary. Cambridge University Press, Cambridge, 500 pp.