Marine iguanas (Amblyrhynchus cristatus) are a herbivorous reptile found exclusively on the Galapagos Islands off the coast of South America. The genus Amblyrhynchus is monotypic; it contains only one species. The flattened snout of the Amblyrhynchus cristatus, as seen in figure 1, distinguishes it from other iguanas which generally have more pointed snouts. The name stems from the Latin for ‘blunt nose’. As well as their abnormal appearance, these animals exhibit many amazing adaptive capabilities which allow them to survive the unusual conditions they are faced with, in particular the recurrence of El Nino events and the high salinity levels present in their habitat and food sources.
The Galapagos Islands (figure 2) are volcanic islands that are geographically isolated and have never been part of a continent. Consequently they are home to many uniquely evolved species and were famously the inspiration for Darwin’s theory of evolution after his 1831 visit during the voyage of HMS Beagle. Marine Iguanas are thought to be descended from land iguanas that floated to the Galapagos from the rivers of the South American rainforests on vegetation rafts, guided by the Humboldt and El Nino currents (Higgins, 1978). They have since evolved the ability, unique among lizards, to dive up to twenty metres and forage underwater for red and green algae, which they feed on almost exclusively. Their tail is slightly flattened to aid swimming and consequentially they use very little energy when foraging and therefore feed very efficiently. In fact only 10% of their daily energy budget is expended on feeding (Gleeson, 1979).
The high salt content of the algae they feed on is combated by the presence of nasal salt glands which expel excess sodium and potassium to prevent internal salt levels becoming toxically high. Many iguanas display a white crystalline patch around their snouts as a result of these excretions. This feature is an impressive example of the ability of animals to adapt to drastically changing conditions and the marine iguana represents a unique evolutionary path that is fascinating to observe.
Marine iguanas face little threat from inter- or intra- specific competition over food and habitats and have very few predators, but are listed as vulnerable on the International Union for Conservation of Nature (IUCN) red list of threatened species (http://www.iucnredlist.org/details/1086/0) . The main problems posed to their survival are diminishing food sources as a result of El Nino events.
El Nino currents develop periodically off the coast of South Africa and replace the cold, nutrient rich waters of the Humboldt Current with warm and relatively nutrient poor water. They are accompanied by a band of low pressure air and the sudden warming of the ocean can cause thunderstorms, sometime leading to flooding in extreme cases. Figure 4 shows the southern oscillation heat ditribution for the El Nino period of 1997. El Nino events usually occur around every seven years, but from 1990-99 El Nino events became more common and in 1997, the temperature of the water around the Galapagos Islands rose from an average of 18⁰C to 32⁰C. This had catastrophic implications for the population of marine iguanas, as it drastically reduced the growth of the red and green algae they rely on for food. Instead, the conditions brought by El Nino favoured the growth of the brown algae Giffordia mitchelliae (Laurie, 1989), which the iguanas find difficult to digest. This resulted in a mortality rate as high as 90% (Nelson and Wikelski, 2004) with adult males being the most severely affected group, their larger size increasing their energy needs and making them more vulnerable to starvation (Laurie and Brown, 1990). Iguanas are likely to face up to three El Nino events in their lifetime, with average lifespan being twenty three years, and so this extreme population depletion poses a real threat.
The marine iguana has, however, developed an extraordinary mechanism for coping with these extreme fluctuations in food availability. It has the ability to shrink its body size by up to 20% (Thom and Wikelski, 2000) during extended El Nino periods, so that the iguana can survive despite diminished food sources. The average change in bodysize during an El nino period is 6.8cm. There is evidence that smaller iguanas are more energetically efficient foragers and expend less energy during movement on land. As tissue shrinkage in reptiles can only account for up to 10% reductions in size (Enlow, 1969) there is an implication that the iguanas can actually shrink their bones. Even more remarkably, once favourable La Nina conditions return the iguana can revert back to its original size. Shrinkage and reversal may occur several times in each iguanas life, whenever energy expenditure becomes too high to be met by remaining food sources. The benefits of reversal come from the fact that larger males are selected for mating more often than smaller ones and are able to forage for food for longer because their small surface area to volume ratio slows heat loss. This means they are able to reach better foraging grounds that are further away and also that they are have a wider choice of areas in which to forage. This reversal in growth patterns is extremely rare among vertebrates and further exhibits the amazing adaptive capabilities displayed by the Galapagos marine iguana.
The Galapagos marine iguana offers an incredible example of the vast variety of life as well as presenting an opportunity to study ecology and evolution in a unique environment and it should be preserved as a reminder of the capabilities of nature to overcome extreme difficulties.
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