Marine Iguana (Amblyrhynchus cristatus): Galápagos Sea Lizard

What does a marine iguana look like?

Marine iguanas are thickset lizards with short, strong limbs, long, sharp claws and a tail that is flattened from side to side for swimming. A row of spines runs from the back of the head along the back to the tail, longest in adult males. The snout is short and blunt, which gave the genus its name: Amblyrhynchus means “blunt snout” in Greek, and cristatus means “crested.”

The head is topped with conical, pointed scales and bony plates, larger in males. The skin is usually black or dark gray, a color that helps the iguana absorb heat. Young iguanas often have a paler stripe down the back.

Colors vary a lot from island to island, especially in breeding males. Males on the southern islands of Española, Floreana and Santa Fé turn bright brick red with turquoise or green markings in the breeding season, which earned the Española iguanas the nickname “Christmas iguanas.” Males on small northern islands such as Genovesa stay almost entirely black.

How big are marine iguanas?

Marine iguanas vary in size more than almost any other reptile species, because body size depends on how much algae grows around each island. Snout-to-vent length ranges from about 12 to 56 cm (5–22 in), with a tail of 17–84 cm (7–33 in), so the biggest males measure well over 1 m (3.3 ft) in total.

Males are clearly larger than females, with broader heads, and on average they weigh about twice as much. Large size helps males win territories and mates, but it becomes a handicap when food runs short.

Where do marine iguanas live?

The marine iguana is endemic to the Galápagos, meaning it lives nowhere else. It is found on the coasts of all the main islands and many small islets, from the large western islands of Isabela and Fernandina to tiny Wolf and Darwin in the far north. The only confirmed record away from the archipelago is a single iguana that turned up on Isla de la Plata, off mainland Ecuador, in 2014.

Marine iguanas live on rocky lava shores, beaches, salt marshes and mangrove edges, almost always within about 100 m (330 ft) of the sea. Where food is plentiful they can be astonishingly crowded, with up to about 8,000 iguanas per kilometer of shoreline. A 2017 study recognized 11 subspecies, each tied to particular islands; one, from San Cristóbal, was named Amblyrhynchus cristatus godzilla.

What do marine iguanas eat?

Marine iguanas are herbivores that eat almost nothing but marine algae, mainly fine red algae and green algae such as sea lettuce (Ulva). They avoid tough or chemically defended seaweeds and cannot digest brown algae well. A 1 kg (2.2 lb) iguana eats roughly 37 g (1.3 oz) of fresh algae a day.

They gather it in two ways. Most females and smaller males graze on algae exposed on the rocks at low tide, dashing back from the incoming waves. Large males, only about 5 percent of the population, swim out and dive to crop algae from the seabed, typically in water less than 5 m (16 ft) deep. Their flattened tails drive them along, long claws grip the rocks against the surge, and short, blunt snouts with three-pointed teeth scrape algae off the stone. The full story is in How do marine iguanas feed?

Like other plant-eating lizards, marine iguanas need gut bacteria to digest their food. Hatchlings acquire them by eating the droppings of adult iguanas.

Body temperature and basking

The waters around the Galápagos are cold for the tropics, often 11–23 °C (52–73 °F) at the surface, while a marine iguana works best at a body temperature of about 35–39 °C (95–102 °F). Every feeding trip chills the lizard, so marine iguanas spend most of the day basking on dark lava rocks, often lying in heaps.

Their black skin soaks up sunlight. In the water their heart rate slows to about 30 beats a minute, which reduces heat loss, and on land it rises to about 100 beats a minute to spread warmth through the body. A chilled iguana is slow and vulnerable, so it relies on bluffing and biting rather than running. In the midday heat the problem reverses: iguanas pant and raise their bodies, facing the sun, to stay cool. At night many sleep pressed together in groups to save heat.

How do marine iguanas get rid of salt?

Feeding in the sea means swallowing a lot of salt. Reptile kidneys cannot make urine saltier than the blood, so marine iguanas use special salt glands in the nose instead. The glands filter salt from the blood, and the iguana blows out the concentrated fluid in a forceful snort that looks like a sneeze. The salt often dries as a white crust on the head. The process is explained in Why do marine iguanas sneeze?

Behavior and breeding

Marine iguanas live in colonies of about 20 to 500 animals, sometimes more than 1,000. They lie in close contact but do not groom or otherwise interact much. Lava lizards scurry over them to catch flies, and Darwin’s finches, mockingbirds and Sally Lightfoot crabs pick ticks and mites off their skin.

Breeding takes place from about December to March. Large males defend small territories on the breeding grounds for up to three months, bobbing their heads, raising their crests and opening their mouths to show the red inside. Rivals that stand their ground butt heads in ritualized pushing contests that can last hours. Females choose mates mainly by size, but smaller “sneaker” males sometimes mate quickly while the big males are distracted.

About a month after mating, the female digs a burrow 30–80 cm (12–31 in) deep in sand or volcanic ash and lays one to six eggs, usually two or three. The eggs hatch after about three to four months. Hatchlings weigh 40–70 g (1.4–2.5 oz) and run for cover at once, because hawks, herons, gulls and Galápagos racer snakes hunt them on the beaches.

How long do marine iguanas live?

Females mature at about 3–5 years and males at about 6–8 years. Many marine iguanas live about 12 years or less, and survival depends heavily on El Niño events. Some individuals probably live considerably longer, but maximum ages in the wild are poorly documented.

El Niño and shrinking iguanas

Every few years, El Niño events bring warm, nutrient-poor water to the Galápagos. The red and green algae that marine iguanas depend on die back and are replaced by brown algae they cannot digest. Starvation can kill up to 90 percent of the iguanas on some islands, with large males suffering most.

Marine iguanas have a remarkable response: adults can actually get shorter. A long-term study published in 2000 found that some iguanas shrank by up to about 20 percent of their body length during El Niño famines and grew back when food returned. The full story is in Do marine iguanas shrink?

Predators, threats and conservation

Native predators include the Galápagos hawk, short-eared owl, lava gull, herons, Galápagos racer snakes and, in the water, sharks. Most take eggs, hatchlings and young iguanas. Marine iguanas evolved with few predators and are remarkably tame.

The greatest threats come from introduced animals. Cats, rats and pigs eat eggs and young, and feral dogs kill adults, so on inhabited islands such as Santa Cruz, Isabela, Floreana and San Cristóbal few hatchlings survive. Oil spills are another danger: after the tanker Jessica ran aground off San Cristóbal in 2001, the iguana population on nearby Santa Fé lost an estimated 60 percent or more of its members within a year, apparently because even light oiling killed the gut bacteria they need to digest algae. Diseases introduced by people, traffic on island roads and the risk of stronger El Niño events add to the pressure.

The IUCN Red List classifies the marine iguana as Vulnerable, and several island populations, including those of San Cristóbal, Santiago and Genovesa, are considered Endangered. The species is fully protected under Ecuadorian law and listed on Appendix II of CITES. Almost all of its land habitat lies within Galápagos National Park and its feeding grounds within the Galápagos Marine Reserve, and predator control and eradication, such as the removal of rats from Pinzón Island, help young iguanas survive.

Evolution and relatives

The marine iguana’s closest relatives are the Galápagos land iguanas (genus Conolophus). Their common ancestor probably reached the islands by rafting on floating vegetation from Central or South America. Genetic studies suggest the two lineages split around 4.5 million years ago, close to the age of the oldest existing islands. Their nearest mainland relatives are the spiny-tailed iguanas of Mexico and Central America.

Despite millions of years apart, marine and land iguanas can still interbreed. Rare hybrids have been recorded on South Plaza Island, where the two species’ breeding seasons overlap. The mainland green iguana is a more distant cousin.

Marine iguanas and people

Early visitors were not impressed. In 1835 Charles Darwin called them “most disgusting, clumsy lizards” and repeated a nickname, “imps of darkness.” Yet the marine iguana became one of the Galápagos’ most famous animals, alongside the Galápagos giant tortoise, and a living example of how island species adapt to unusual ways of life.

Today marine iguanas are a highlight for visitors, who can watch them basking within a few meters on many trails. Park rules ask everyone to stay at least 2 m (6.5 ft) away, stay on marked paths and never feed or touch wildlife, so the iguanas can go on basking undisturbed.