There aren’t all that many marine reptiles. Turtles, crocodiles, and sea snakes make up the bulk of the 100 species of marine reptiles. The marine iguana is the only lizard that is able to use both land and aquatic environments!
Iguanas are a family of reptilian lizards (Iguanidae) native to South America, Central America, and some parts of the Caribbean islands. Iguanas are part of the Squamata order of reptiles (or reptilia in Latin), which includes all species of lizards, snakes, and worm lizards. All iguanas are fairly strict herbivores. They graze on various plants for sustenance. Iguanas, like many other reptiles, lay eggs. The incubation period for iguana eggs ranges from two to four months. A distinctive feature of iguanas is their dewlap, which is that funny flap of skin below their chin. Fun fact: cows, dogs, rabbits, and many other vertebrates have dewlaps!
Common species, such as the green iguana (scientific name Iguana iguana), or sometimes called American iguana, are invasive species in places like Texas, Hawaii, and Florida. Iguanas can thrive in a variety of climates, from the tropical rainforests of Puerto Rico to the deserts of the U.S.-Mexico borderlands. The desert iguana, for example, is a native to the brutal Sonoran desert of the southwestern U.S. and northwestern Mexico. The spiny-tailed iguana, in contrast, lives in the jungles of Mexico’s Yucatan peninsula.
Iguanas are a semi-common pet for herpetology enthusiasts. While some types of iguanas can be raised in captivity, the illegal pet trade endangers some populations and species of iguanas. Other genera in the iguana family, such as the Cyclura iguanas of the West Indies, are among the most endangered species in the world.
Marine iguanas, like much of the life on the Galapagos Islands, are an evolutionary anomaly. Scientists believe these lizards floated on ocean debris from mainland Ecuador to the Galapagos millions of years ago. In fact, the lizards may have arrived on the islands before any of the current islands existed! The Galapagos are a volcanic island chain. As the tectonic plates move over the volcanic hotspot, new islands are formed. The islands the lizards initially landed on have eroded into the sea.
Marine iguanas, like all other iguanas, live on an herbivorous diet. Instead of eating land-based plants, these strange lizards eat underwater algae. Marine iguanas typically live to be about 12 years old, but some individuals have reached the ancient age of 60.
Because they have such an unusual lifestyle, marine iguanas have some unusual evolutionary adaptations compared to other lizards. Let’s walk through a day in the life of a marine iguana and see how these adaptations help these creatures.
You wake up on the rocky beach with an empty belly; you’re a hungry iguana. All of your food is in the ocean, so you slink off to the waves for some breakfast.
Marine iguanas have special tails that help them swim. These tails are shaped like crocodile tails, which are long and muscular. They use these tails to swim in a snake-like pattern. Without these tails, these lizards would not be able to maneuver in the water. With the tails, however, these lizards are excellent swimmers that can access all sorts of underwater vegetation. These cold-blooded creatures’ body temperature can drop by more than 10C during one foraging session in the cold Pacific Ocean. Incredibly, these lizards appear to behave normally when their body temperatures are as low as 77F and as high was 104F. That broad range of temperatures surely makes us humans look like weaklings in comparison. Hypothermia sets in when human body temperature sinks below a mere 95F and fever sets in above 100F.
The marine iguanas name may be slightly misleading. While these lizards are killer swimmers, they don’t spend all that much time in the water. In fact, these lizards only spend about 5% of their time, or one hour a day, actually submerged and swimming in the water.
In order to avoid dropping their body temperature too much, these lizards also forage on algae in the inter-tidal zone. When the tide goes out, it exposes algae on rocks that were previously underwater. Inter-tidal feeding doesn’t suck as much heat out of the lizards’ bodies.
Once you find a nice pasture of algae, you use your talons to secure yourself to the rock as you graze. You spend a while using your mouth to pick algae off of rocks. This tasty algae is your main source of food.
These special lizards also evolved long, sharp, strong talons to help them grip on to rocks while they forage. Without these claws, waves could easily sweep the lizards away from their tasty algae patches while they feed. With these claws, the lizards can cling on to rocks through turbulent conditions.
You’ve just spent an hour foraging in the frigid Pacific ocean. With a belly fully of algae, it is time to warm up!
All lizards are cold-blooded animals, which mean they need external heat to regulate their body temperatures. Marine iguanas are very dark colored because that color helps them absorb the most amount of heat from the sunshine. By basking in the sunlight, these lizards raise their body temperatures after being in the cold water.
While eating all that algae in the ocean, you also consumed a lot of ocean water. After your body efficiently concentrates the salt in your body, you spend a while sneezing that salt out of the special salt glands in your nose. After sneezing out your salt and warming up, you realize it is low tide so you forage on in the inter-tidal zone. Then, more basking! That’s more or less the life of a marine iguana.
The white scales around a marine iguana’s face might appear as simply scales with different color pigment. Instead, these scales are white because of an interesting trait. Marine iguanas live in very salty environs. They ingest substantial amounts of salt as they forage for algae underwater. In high concentrations, salts can be toxic and dehydrating. In response to high salt levels, these water-loving lizards evolved a special gland near their nose that excretes excess salts in their bodies. These salt glands are incredibly efficient at expelling salt from these lizards’ bodies. The lizard then spread these salts around their nose through daily activities. Instead of white pigmentation, the white is simply salt deposits!
While these is only one species of marine iguana, there is almost a unique subspecies on every Galapagos Island. Much like the evolution of Darwin’s Finches, these lizards have evolved to exploit their hyper-specific environments.
Interestingly, this lizard appears in many sizes. In an area with a high quality algae pasture, these adult male iguanas can grow to be ten times the weight of adult male iguanas in areas with low quality pasture! Additionally, these heavier lizards can be two times longer than their less-nourished brethren.
Its not always good to be bigger, however. During the weather pattern of El Niño, the warmer ocean waters decrease the lizards’s algae pastures. As these algae pastures become less rich, the larger lizards tend to suffer first and hardest while the smaller lizards tend to do okay. A larger marine iguana needs to eat more algae to keep its body functioning. When there is less algae overall that is much harder to do!
Even though they are in a protected island chain in the middle of the ocean, these marine lizards face various threats. As ocean temperatures warm with climate change, the algae pastures will likely decrease in quality. This means that every year could be akin to a El Niño famine year for these special marine lizards. Non-native animals on the Galapagos are also a threat to these lizards. These lizards did not evolve with land predators, so they don’t know how to react to non-native animals like cats and dogs. Cats and dogs can easily kill the unsuspecting lizards. protect their eggs from non-native animals like rats. Tourism is another threat to these lizards. Iguanas show measurable amounts of stress when tourists approach them. The indirect impacts from fishing tourism and pollution also make life harder for these lizards. For these reasons, the marine iguana is listed as a threatened species.
If you are ever lucky enough to visit the Galapagos Islands, remember to keep your distance from these unique creatures while you appreciate their unusual lifestyle.