Natural History of the Marine Iguana

Hola from Isla Isabela!

Nugget of knowledge for the day – Marine iguanas are the only known sea-faring lizards on the planet.  

Research Connection

When engaging in any research that utilizes a particular model organism, it is critical to know as much about it as you can. This not only furthers one's own personal wealth of knowledge, but it also allows researchers to conduct better science and make appropriate and informed decisions about the methodology of the study, the interactions with the animals, and how to best educate the public!

The Marine Iguana (Amblyrhyncus cristatus)

Closeup of a male marine iguana eye.

"The rocks on the coast abounded with great black lizards between three and four feet long; ...It is a hideous-looking creature, of a dirty black color, stupid, and sluggish in its movements." (Darwin, 1845).

As you can see, Charles Darwin had very little appreciate for these animals; beauty, however, is all in the eye of the beholder. In my mind, there is no doubt that these animals are absolutely magnificent. Incredibly well adapted to their equatorial, island environment, marine iguanas are found only on the Galapagos Islands and nowhere else on earth. Upon immediate glance, one may see what looks like a modernized dinosaur, or possibly something that reminds them of a smaller, quadrupedal version of Godzilla. With their stout faces, tough scales, sharp spines and claws, long tail, and powerful legs, they really are something fiercely special and undoubtedly unique.

Physical Description

A male marine iguana at
Station Beach, Santa Cruz.

Physically, they seem to be perfect masters of their domain. They have evolved blunt noses for effective foraging of algae, strong limbs and sharp claws for climbing volcanic rock, tough scales to keep them well protected from the elements, and laterally flattened tails to aid in propulsion under the water. Starting at the front (anterior), you immediately notice their blunt noses, sharp teeth, and heavily scaled head. When scraping algal blooms off the ocean floor or feeding on seaweed (which is algae too), these valuable adaptations allow them to be as efficient as possible when foraging. They also have nasal glands that help them excrete ingested salt, in what usually looks like a rather impolite sneeze (no consideration for neighbors…or us…). Moving along the body, you notice that they can balloon themselves up with air to aid in buoyancy. They also have sharp spines along the top (dorsal) side of their body, both behind the neck and along the vertebrae. The scales over much of their bodies are hardened, pointed, and toughened from the demanding lives they lead; truth be told, if one happens to be a successful marine iguana, one also happens to be a tough, hardy marine iguana. They tend to be grey or black in color, which helps them blend well into the volcanic rock on which they live and forage, although adult males may have green, red, cooper, and/or whitish coloration to them; it’s thought that this particular pattern may result from the algae on which they forage. They say you are what you eat! If you move towards to outer ends on the animal (distally), you’ll also notice their incredibly sharp claws (my hands and arms can attest to this...) as well as the powerful limbs and digits on which these sharp claws sit (my hands and arms can also attest to this...). When scaling lava rock for a living, it would make sense that claws and powerful limbs become valuable assets to the daily grind! As you continue down to the back portion of the animal (posterior), you’ll finally notice their long, horizontally (laterally) flattened tails. These make the animal very adept at swimming and navigating the ocean, particularly when they tuck their legs into their body and laterally ungulate though the water, effectively moving like a snake.

A male marine iguana at
Station Beach, Santa Cruz

Ecology

Marine iguanas spend much of their time both on land and in the ocean. They are found most often spending the daytime in colonies on craggy, bouldered, volcanic rock near the ocean; as the sun is setting, they venture inward towards the shrubs, forbs, and trees to sleep for the night. Much of their time is either spent basking in the sun, foraging, sleeping, or defending their territory (if said lizard happens to be an adult male). They are powerful and elegant swimmers, which come in handy when diving (sometimes greater than 10 meters!) for algae in the ocean. Since their body temperature can fall rapidly in the cooler ocean, they must utilize their foraging time wisely and then make it out safely to sun themselves ( I always think of them recharging their internal temperature batteries while sunning, and then draining that charge while foraging). The adults are primarily the individuals who forage in the ocean, while juveniles feed in the intertidal zones during low tide.  

Life History

One of the females we released.
The beads will help identify her for recapture in the future

When looking at the differences in sex, males are generally considerably larger than females (although we have seen extreme and non-extreme variations of this depending on the island). Adult males tend to both weigh more and be longer than females, while male dorsal spines are both taller and more pronounced than those of females. Males have more variable and colorful patterning to them as well. Juveniles generally look very similar to each other, and are dark grey to black. Breeding usually occurs annually over the course of a three-month timeframe. During this time (anywhere from December to April) males defend their territories from other males with head bobbing, arching their back, or physically driving off competing males through biting or pushing. Since these types of defensive actions take a lot of energy, male marine iguanas tend to bluff and rely on less energetically expensive means of telling other males to “keep off their lawn”.

**Interesting Fact** There is also an additional adaptation found in certain males (who are called satellite males) where they look almost identical to the females, and don’t display the same physical features (phenotypes) as do typical adult males. This way, instead of investing time and energy into getting large and defending a territory, they are sneakier and more subtle, mating with a female while the larger males are busy; hopefully they don’t get caught! Females will usually carry anywhere from 1-6 eggs - our current findings agree with these measures since the most we’ve found so far is 6 - and will bury them in sand to incubate for around 100 days. Mom may guard the nest for a few days, but she will leave the nest to fend for itself.

Conservation

Male and female marine iguanas.

The marine iguana is completely protected here on the Galapagos. Tourists and locals are not allowed to touch or come in contact with these animals unless they have the proper scientific permits issued by the park service. Detailed conservation and research programs have been developed (including ours!) to help us better understand the ecology and environment of the Galapagos, including the marine iguana. Although their populations range in the many thousands across all the islands of the Galapagos, there are several threats to marine iguana that pose many significant risk factors. Feral animals including dogs, cats, and rats will readily eat iguana eggs and juveniles, as will certain natural predators like the Galapagos hawk and potentially sharks. Environmental conditions, primarily from El Nino, can negatively impact marine iguana populations, as may continued increases in global temperatures. Finally, humans are additional primary drivers of marine iguana struggles. From oil spills, pollution, harassment, tourism, boating, and other negative interactions, we have unfortunately done our part in harming marine iguana populations over the years. However, with the park service, the Ecuadorian government, environmentalists, and scientists all pooling resources together in favor of marine iguanas, their future is looking bright.

Just like the habitat in which they live, marine iguanas are uniquely amazing in many ways. If you ever have the chance, I would highly recommend visiting these islands to get a chance at watching them up close. It’s very challenging to write a substitute for that, but I hope you now have a better picture of the model organism for our research!