Marine Iguana (Amblyrhynchus cristatus)

On my flight to the Galápagos I threw up five times (dry retching after no. 3) which was enough for the passengers on my row to justifiably move away. The next morning, I dragged my frail body across Puerto Ayora to Tortuga Bay, driven by the raw excitement of seeing marine iguanas for the first time – a childhood dream. Although the joy of the experience purged the disease, I still didn’t have an appetite for three days. Perhaps that was a blessing in disguise though as I saved a lot of money on food… 

The marine iguana (Amblyrhynchus cristatus) is endemic to the Galápagos islands. A famously diverse set of islands located 563 miles off the coast of Ecuador. They are the only true marine lizards, specialised in foraging in the sub-tidal and inter-tidal zones. Their diet consists almost exclusively of algae, with two species of red and one green algae making up the majority of the food intake, but there are also reports of iguanas eating sea lion afterbirth.

Being ectothermic (cold blooded), marine iguanas are constrained in their foraging ability by temperature. The optimal thermal range for a marine iguana is 35-39^oC whereas the seas around the Galápagos islands average a chilly 11-23^oC. Only around 5% of the marine iguana population forages out at sea, these tend to be the largest individuals and are mostly males. The smaller animals (females, small males and juveniles) stick around in the inter-tidal zone. The iguanas spend a large amount of time basking before and after foraging to maintain body heat in the preferred range. Higher body temperatures increase foraging ability and the rate of digestion of food. They tend to aggregate in big groups and bask together on the beaches and lava rocks, but secretly they’re just trying to get the perfect cover photo for their new album…

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Marine iguanas have a big flattened tail for propulsion, glands to excrete excess salt, and more densely compacted forelimb than hindlimb bones. This last trait is thought to act like a ship’s ballast, offsetting lung buoyancy and helping maintain control and stability in the water. Females share the same trait but rarely forage at sea, so its likely to be useful for clinging to the inter-tidal lava rocks too – or something else entirely.

Iguanas that have been resting out of view of the ocean will arrive at foraging grounds precisely at low tide, and captive experiments have shown this is not influenced by light or social cues. This internal tidal clock allows them to predict the tide and ensure the best times for foraging can be fully exploited.

It should be a life goal for everyone to reach the level of indifference of a marine iguana. Hitting one with your car lands you a $16,000 fine and 6 months in jail, so what is there to worry about when even to humans you’re untouchable?

Quick facts

• Endemic to the Galápagos islands.
• The world’s only true marine lizard.
• Adaptations such as internal tide clocks and heavy forelimb bones to aid in foraging.

Further reading:

• Hugi, J., & Sánchez-Villagra, M. R. (2012). Life history and skeletal adaptations in the Galapagos marine iguana (Amblyrhynchus cristatus) as reconstructed with bone histological data—a comparative study of iguanines. Journal of Herpetology, 312-324.
  • This paper is quite detailed and compares the bone structure, compactness, and other factors between sexes of marine iguanas and between marine iguanas and other iguana species. There are also some good figures including photos of stained bone pieces showing the microstructure.
• Vitousek, M. N., Rubenstein, D. R., Wikelski, M., Reilly, S., McBrayer, L., & Miles, D. (1997). The evolution of foraging behavior in the Galapagos marine iguana: natural and sexual selection on body size drives ecological, morphological, and behavioral specialization. Lizard Ecology. Cambridge University Press, Cambridge, UK.
  • This is a chapter from a book, so it is set out in an easy to read way. There are subsections on ecology, foraging constraints and preferences, and extra information on the internal clock used by the iguanas to predict the tide. They also talk about odd examples like one population that supplements its diet with a plant that grows on the beach.