Colours of Wildlife: The Marine Iguana

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Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours ... but actually many media including 'digital' paintings with the computer!"

The Marine Iguana

Today's painting is of a Marine Iguana, Amblyrhynchus cristatus. This time it is not a South African or even an African species. In fact, this strange reptile occurs almost on the other side of the world from South Africa, in the Galapágos islands off the west coast of South America. I'm interested in everything on this Earth – and these lizards are particularly fascinating to me.

First of all, this iguana is one of the few reptile species that live in the ocean. Apart from them, the main sea-dwelling reptiles of our times are sea snakes and sea turtles. Salt water crocodiles occasionally go into the sea, but don't really live there – they prefer rivers and estuaries.

It is strange that today there's only this one marine lizard species. In the past there have been a huge diversity of reptiles – often very large ones – inhabiting the open oceans: mesosaurs, ichthyosaurs, nothosaurs, placodonts, plesiosaurs, pleurosaurs, pliosaurs, true pelagic crocodiles, thalattosaurs, and mosasaurs – the latter being sea-living monitor lizards, of which many land-living ones still exist today including the komodo dragons. But today, all of those are extinct. We still have this one species of marine lizard, which proves that today it is still a viable lifestyle … but why it should be just this one species that takes advantage of it, I would like to know!

This iguana has lots of land-living relatives. On the Galapágos islands there are several other iguana species, and many more are found on the continent of South America and also North America, especially the southern parts. This region is most probably where iguanas originated, from where many spread to colonise islands such as the West Indies and Galapágos. Today, some are found on islands as far away as Fiji, and most strangely of all, a few species occur on the island of Madagascar! It is not yet clear how iguanas (or other land creatures) reach these distant islands. (Even the marine iguana started out as a land-living lizard that somehow came to these islands and only then adapted to swimming in the ocean.) One theory is that individuals clung to trees or logs washed out to sea by storms, eventually landing on the shores of islands where they then managed to survive and – if both males and females were present – start breeding. (Interestingly this is not an absolute requirement since there are lizards in which the females can breed without being impregnated by males – and there are consequently a few all-female island lizard species, perhaps founded by a single female in each case). Iguanas range from small lizards to quite large, the Green Iguana being known to exceed 2 m/6'6".

Among the iguanas, the Marine Iguana is one of the largest, males reaching 1.7 m/5'6", and a bodyweight of 1.5 kg/3.3 lbs. Females are considerably smaller. On land, this creature is rather clumsy, and not very active. Most of the time they spend clinging to rocks and basking in the sun. They do this to increase their body temperature … like all lizards, they cannot generate internal heat the way mammals do but must absorb it from their environment. And of course they also lose heat if the environment should be colder than themselves … which is the problem they have to contend with. The Galapágos islands, in spite of being almost on the equator, are bathed in the cool Humboldt ocean current that flows from the Antarctic seas northward along the west coast of South America. Amongst other things this makes these islands suitable for the northernmost-occurring penguin of all, the Galapágos Penguin, Spheniscus mendiculus. But for a lizard, the cold is a problem. If they cool down too much, their metabolism slows down to a crawl. So, these iguanas feed by (relatively) short dives interspersed with long periods of sunning themselves on the rocks. Fortunately the equatorial sun is very warm and bright, and the weather is dry with almost permanently cloud-free skies.

The man himself, Charles Darwin, was the one who discovered this species – on his famous visit to these islands aboard the ship the HMS Beagle. He called them 'disgusting, clumsy lizards' and 'imps of darkness'. Actually I think they look nice! Not conventionally beautiful but they certainly have character. What especially repulsed Darwin was the salty encrustations around their faces. But that is functional: they have nasal glands that remove excess salt from their bodies, which allows them to feed in the sea water on marine algae. Onshore, they actually sneeze out thin spurts of highly concentrated brine – you can see this if you watch them as they lounge on the rocks! The knobby bumps on their heads catch some of this salt spray, evaporation leaving them with a facial salt crust. Their dark bodies help them absorb heat from the sun more rapidly. The pigment also protect them against the intense ultraviolet radiation.

Actually some of these iguanas can be quite colourful, males of the southern islands sporting red and teal-green colours during the breeding season. There are seven different subspecies, living on different islands, and they differ subtly in appearance. They also differ in size: those on Fernandina and Isabela islands are the largest, and those on Genovesa island are the smallest.

Marine iguanas stake out territories – and harems! A dominant male lays claim to several females; other males will challenge it, mostly by head-bobbing displays rather than actual fighting. Even if they fight, this is just by pushing each other around with their thickly knob-adorned heads.

Dives can last up to an hour and take the lizards down to a depth of 15 m/50 ft. They eat marine algae growing on rocks, scraping them off with their small, blunt teeth. Their bodies are adapted for swimming, their tails being flat and tall, with a high crest of spines running along their midlines and continuing along the back as far as the back of the head. They swim with side-to-side undulations of their bodies, keeping their front and back legs tucked to their sides. Their sharp, curved claws help them cling onto rocks both when they're submerged and when they're sunning themselves on shore. When the sea is rough, the waves can crash over them without dislodging them. Their optimal body temperature is 36 degrees Celsius (about 97 degrees Fahrenheit) but this drops by up to 10 degrees Celsius (18 degrees Fahrenheit) over the course of their dives. They will typically just take a single feeding dive per day, spending all of the rest of their time re-warming.

One of the most fascinating things about these iguanas is that they can change their size as adults in response to environmental factors. In years when there's not a lot of food – which is typically when there's an El Niño phenomenon, when warm waters push away the fertile southern ocean current – the iguanas can shrink by up to 20%! They actually secrete hormones that metabolize some of their bones and tissues. When food is plentiful they regain their lost size.

Marine iguanas reach sexual maturity at the age of about three to five (females) and six to eight (males) years – meaning they are rather slow growers. The breeding season lasts from December through March. The male goes through a little display ritual prior to mating. Then the female heads inland to lay her eggs in a safe, sandy place. Here, the females will compete with each other – also through displays – to get the best patches. Once she has her patch, the female will dig a hole and lay her eggs in it. She will stay there for up to sixteen days, guarding her eggs. But when the little iguanas hatch, they'll run off and be on their own.

Small iguanas can be preyed on by a variety of birds, but adults have few predators to worry about, and can live for up to 30 years. However, a problem is the introduction of predators by humans, most especially cats and dogs. These catch an inordinate number of young iguanas, and some populations seem to be no longer reproducing. There are steps to try and combat this problem … all of the Galapágos islands constitute a nature reserve of global importance. There are some islands that are still predator-free. Overall, there are probably still tens of thousands of these iguanas in the Galapágos, but like everything, we need to watch them carefully and do what we can to conserve them.

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