Marine iguanas (Amblyrhynchus cristatus) have probably been living happily on the Galápagos islands for over 10 million years (1). The first human being they saw was the Bishop of Panama in 1535 (2). I don’t suppose they took much notice! They look to me like they are not easy to impress. I guess 10 million years on a bunch of rocks makes you rather laid back!
When I first saw marine iguanas in The Galápagos, I was surprised how small they were. My expectations were I suppose, formed by close-ups taken by BBC camera crews. The seven subspecies do however, vary considerably in size, as do individuals within a given population.
There is nothing more primeval that a pile of iguanas; and they really do lie about in heaps. One of the guides commented that he fantasizes that someone collects them and piles them up on the beach at the end of each day! The casual arm draped by one lizard over the back of another, suggests a certain capacity for affection (below).
This propensity to cuddle up together, may be a result of thermoregulation: “A [more] likely explanation for the formation of beach piles is that iguanas gather at spots that offer a slightly better (i.e., warmer) microclimate during the late afternoon hours, when piles usually develop”. But it may also have something to do with managing ticks, which remarkably enough – given that they take regular dips in the sea – seem to plague these animals (4). It’s a fascinating subject!
Despite their long residence on these islands, and the existence of a number (7) of different races or subspecies, marine iguanas show relatively little evolutionary divergence (1). This is probably because, being a sea-going lizard and good swimmers, they have moved or been dispersed between the different islands. The large males can dive down to 9m for algae in the subtidal zone, but they cool down quickly in the cool waters of the Galápagos archipelago. Many spend their time feeding on exposed algae in the intertidal zone around low tide, but in total only 5% (a little over an hour) of each day is actually spent close to or in the water (3). The rest of the time, I guess, they just spend dozing, at least that’s what it looked like to me. This particular individual – a pinkish variety – was clinging to a rock like Sisyphus! Or was he just thinking? There I go, anthropomorphizing!
Marine iguanas have an endearing habit of sneezing (without the use of a handkerchief!). They blow salt from their noses through a specialised gland which removes it from the salty food they consume. The salt does however, often remain stuck to their heads, which gives the tops of their heads a sort of white, encrusted look (next two slides, below). Amazingly, the nasal salt glands are capable of removing up to 95% of salt ingested as food or with sea water (5); they are veritable desalination units!
Marine iguanas have all sorts of tricks up their sleeves. They can shrink down to a more manageable size when food is in short supply (6); they can change colour to reflect their state of randiness (!); and if all goes well, they can live to a ripe old age of sixty (7). I’m happy to say that there are thought to be a good few hundred thousand Marine iguanas remaining on the Galápagos islands (go count!); but they are subject to predation by invasive cats and dogs, so I hope the next 10 million years will be good for them as the last 10 million.
1) Rassmann, K., et al. “The microevolution of the Galápagos marine iguana Amblyrhynchus cristatus assessed by nuclear and mitochondrial genetic analyses.” Molecular Ecology 6.5 (1997): 437-452.
3) Trillmich, Krisztina GK, and Fritz Trillmich. “Foraging strategies of the marine iguana, Amblyrhynchus cristatus.” Behavioral Ecology and Sociobiology 18.4 (1986): 259-266.
4) Wikelski, M. (1999). Influences of parasites and thermoregulation on grouping tendencies in marine iguanas. Behavioral Ecology 10 (1):22-29. http://beheco.oxfordjournals.org/content/10/1/22.full
5) Shoemaker, Vaughan H., and Kenneth A. Nagy. “Osmoregulation in the Galápagos marine iguana, Amblyrhynchus cristatus.” Physiological zoology(1984): 291-300.
6) Wikelski, Martin, and Corinna Thom. “Marine iguanas shrink to survive El Niño.”Nature 403.6765 (2000): 37-38.