Called “imps of darkness” and “disgusting, clumsy lizards” by Charles Darwin when he first visited the Galapagos Islands, marine iguanas are the only marine reptiles in the world. They may not be the most attractive, but they are certainly one of the most unique creatures that inhabit the planet.
What makes these curious creatures stand out from the rest? How did they become the only reptiles to live in a mostly marine habitat? We explore the lives of these interesting animals.
1. Marine iguanas have black skin that helps them absorb and retain heat
Since marine iguanas are reptiles, they are cold blooded and have difficulty maintaining their body temperature when they dive into the ocean for food. Their black skin helps them absorb sunlight better. During a dive, the body temperature of a marine iguana can drop up to 10 degrees Celsius – when they resurface, they have to bask in the sun to regain body heat. When sunbathing, they are vulnerable to attacks from predators because the cold makes it difficult for them to move quickly.
During mating season, male marine iguanas change to bright colors to attract mates. They can turn blue, green and red – both males and females prefer larger mates because it is a sign of abundant food and larger offspring.
2. Marine iguanas look quite different from land iguanas
Marine iguanas eat algae from the sea, so they have physical features that help them forage better underwater. They have blunt heads, long claws and tough skin that allow them to weather the unforgiving currents in the intertidal and subtidal waters where their food is found. They have flattened tails that make them strong swimmers – they may be clumsy and slow on land, but they are graceful and swift in the water.
Marine iguanas have a nasal gland that filters excess salt that is consumed while eating. They sneeze often to expel the salt from their nostrils, which is why their faces are often covered in patches of salt.
3. Marine iguanas come in a large range of sizes
The largest marine iguanas can grow up to 13kg and five feet long, while the smallest are only about 1-2kg and two feet in length. Their size is determined by how much food is available in their habitat and sea surface temperature. Naturally, if there is less food and the water is colder where they live, they will be smaller in size to conserve limited resources.
4. Marine iguanas helped Charles Darwin formulate his theory of evolution
He may have found them to be utterly hideous, but their existence proved that animals were naturally selected for their ecological adaptations. Land iguanas and marine iguanas are said to have evolved from a common ancestor 8 million years ago, where they rafted across the sea to land in what is now known as South America. Both species are mutually fertile and sometimes mate to produce hybrids where their habitats overlap. The males of this hybrid species are infertile.
5. Marine iguanas may look intimidating, but they are one of the few exclusively vegetarian reptiles in the world
Marine iguanas live on the black lava on the Galapagos Islands and are herbivores that eat the underwater algae and seaweed that grow in the water. Only adult marine iguanas have the strength to dive up to 40 feet underwater to graze on seaweed that grows on rocks, staying submerged for an hour at a time. Young marine iguanas eat algae at low tide.
When the El Nino-Southern Oscillation Event (ENSO) occurs, the high rainfall, sea levels and sea surface temperatures affect algae and seaweed growth – the primary food source of marine iguanas decreasing also decreases survival and reproduction rates of marine iguanas. Large male marine iguanas often die, and the surviving iguanas shrink in size to conserve energy and improve survival rates. When the food supply increases, iguana size increases as well.
6. Male marine iguanas nod at each other during mating displays
Every year, marine iguanas have a breeding season that lasts three months. The males change color from black or grey to vibrant shades of blue, green and red, while females lay up to six eggs in burrows dug in sand or volcanic ash. Larger marine iguanas lay more eggs, small iguanas sometimes only lay one egg at a time. When the baby iguanas hatch after about 100 days, they dig themselves out of their nests and travel to the intertidal area where they start eating algae and seaweed on their own.
Male marine iguanas can mate with multiple females, but female marine iguanas only mate with one male. Female marine iguanas become sexually mature at 3 years, while males take about 6 years. During the mating season, male marine iguanas fast to maintain their territory, losing up to a quarter of their body weight at a time. When two male marine iguanas are looking for the same mate, one of them will bob their heads as a threat display. If the other male responds, they will butt their heads together and push each other until one male leaves.
7. Marine iguanas are hunted by the Galapagos hawk and great blue heron, but their main predators are actually human or related to humans
Marine iguanas have a somewhat symbiotic relationship with mockingbirds – the birds let out a unique cry when a Galapagos hawk is in the area, which the marine iguanas recognize as a signal telling them to run for it. Galapagos hawks hunt fully grown marine iguanas while great blue herons eat hatchlings. Feral dogs and cats brought to the Galapagos islands by human settlers also attack iguanas and their nests.
The Charles Darwin Research Station measures the stress levels of marine iguanas to take note of their health – stress increases when there is excess human activity in the environment and when there is not enough food to eat. This stress results in eggs not being fertilized or laid. The threat of predators, pollution and El Niño mean that the marine iguana population is at risk of extinction.
Marine iguanas have a funny walk like most lizard-like reptiles. They lift themselves up and look like they’re marching.