Marine Iguana Facts

The marine iguana (Amblyrhynchus cristatus) is the only lizard that forages in the ocean. The fierce-looking yet gentle iguana lives in the Galápagos Archipelago. While the lizards are excellent swimmers, they can't cross the distances between islands. So, the islands host several subspecies that differ in terms of size and color.

Description

Marine iguanas have flattened faces, bone-plated heads, thick bodies, relatively short legs, and spines that extend from neck to tail. They have long nails that help them grip slick rocks. Females are mostly black, juveniles are black with lighter dorsal stripes, and males are dark except during the breeding season. At this time, their green, red, yellow, or turquoise colors brighten. The specific colors depend on the subspecies.

Iguana size depends on subspecies and diet, but males are larger than females and have longer spines. Average adult sizes range from 1 to 5 feet in length and 1 to 26 pounds of weight. When food is scarce, marine iguanas lose length as well as weight.

Habitat and Distribution

Marine iguanas are native to the Galápagos Archipelago. While populations on islands tend to be isolated, occasionally a lizard makes it to another island, where it can hybridize with the existing population.

Diet

Marine iguanas forage on red and green algae. Although primarily herbivores, the lizards sometimes supplement their diet with insects, crustaceans, sea lion feces, and sea lion afterbirth. Juvenile marine iguanas eat the feces of adults, presumably to obtain the bacteria needed to digest algae. They start to feed in shallow water when they are a year or two old.

Large male iguanas forage further ashore than females and smaller males. They can spend up to an hour underwater and dive up to 98 feet. Smaller iguanas feed on algae exposed during low tide.

Behavior

Like other lizards, marine iguanas are ectothermic. Exposure to the cold ocean water dramatically lowers body temperature, so iguanas spend time basking along the shore. Their dark coloration helps them absorb heat from the rocks. When the lizards get too hot, they pant and orient their bodies to minimize exposure and increase air circulation.

Marine iguanas ingest a lot of salt from seawater. They have special exocrine glands that extract excess salt, which they expel in a process resembling sneezing.

Reproduction and Offspring

The iguanas live in colonies of 20 to 1,000 lizards. Females become sexually mature between 3 and 5 years of age, while males mature between 6 and 8 years of age. Usually the iguanas breed every other year, but females may breed every year if there is sufficient food. The breeding season occurs at the end of the cold, dry season from December to March. Males start defending territories up to three months before mating. A male threatens a rival by bobbing his head, opening his mouth, and raising his spines. While males may spar with their spines, they don't bite each other and rarely cause injuries. Females select males based on their size, the quality of their territories, and their displays. A female mates with one male, but males may mate with many females.

Females nest about a month after mating. They lay between one and six eggs. The eggs are leathery, white, and about 3.5 by 1.8 inches in size. Females dig nests above the high tide line and up to 1.2 miles inland. If the nest cannot be dug into the soil, the female lays her eggs and guards them. Otherwise, she leaves the nest after the eggs are buried.

Eggs hatch after three or four months. Hatchlings range from 3.7 to 5.1 in body length and weigh between 1.4 and 2.5 ounces. They scurry for cover upon hatching and eventually make their way to the sea.

Conservation Status

The International Union for Conservation of Nature (IUCN) classifies the marine iguana's conservation status as "vulnerable." However, the subspecies found on the Genovesa, Santiago, and San Cristóbal Islands are considered to be endangered. The total population of marine iguanas is estimated to range between 200,000 and 300,000 individuals. The population trend is unknown. Marine iguanas rarely live longer than 12 years, but they can reach an age of 60 years.

Threats

The marine iguana is protected under CITES Appendix II and by Ecuadorian law. While all but 3% of its range lies within the Galápagos National Park and all of its sea range is within the Galápagos Marine Reserve, the lizards still face significant threats. Storms, flooding, and climate change are natural threats. Humans have brought pollution, non-native species, and diseases to the islands, against which the marine iguana has no defenses. Dogs, cats, rats, and pigs feed on the iguanas and their eggs. While motor vehicles pose a threat, speed limits have been lowered to protect them. Exposure to tourists stresses the animals and may affect their survival.

Marine Iguanas and Humans

Ecotourism brings in money to help protect wildlife in the Galápagos, but it takes its toll on the natural habitat and creatures that inhabit it. Marine iguanas are not aggressive toward people and do not defend themselves when handled, so they are at increased risk of disease transmission and stress-related injuries compared with other species.

Sources

• Bartholomew, G.A. "A Field Study of Temperature Relations in the Galápagos Marine Iguana." Copeia. 1966 (2): 241–250, 1966. doi:10.2307/1441131
• Jackson, M.H. Galapagos, a Natural History. pp. 121–125, 1993. ISBN 978-1-895176-07-0.
• Nelson, K., Snell, H. & Wikelski, M. Amblyrhynchus cristatus. The IUCN Red List of Threatened Species 2004: e.T1086A3222951. doi:10.2305/IUCN.UK.2004.RLTS.T1086A3222951.en
• Wikelski, M. and K. Nelson. "Conservation of Galápagos Marine Iguanas (Amblyrhynchus cristatus)." Iguana. 11 (4): 189–197, 2004.
• Wikelski, M. and P.H. Wrege. "Niche expansion, body size, and survival in Galápagos marine iguanas." Oecologia. 124 (1): 107–115, 2000. doi:10.1007/s004420050030