Marine iguanas: the incredible shrinking lizards of the Galápagos

Marine iguanas (Amblyrhynchus cristatus) are the only lizards known to forage for food in the ocean, living off the algae growing there. But their dependency on the water makes them susceptible to changes caused by the El Niño-Southern Oscillation (ENSO), an irregularly occurring climate cycle that influences temperatures and precipitation around the world.

The iguanas have evolved the ability to adapt and increase their chances of survival during warm phases of the cycle when up to 90% of their populations could perish. 

What is a marine iguana?

Marine iguanas are reptiles endemic to the Galápagos Archipelago, located just over 900 kilometres off the coast of Ecuador. The islands are home to subspecies of these lizards, each differing in size, shape and colour. As they mature, the reptiles can take on shades of red, green, black and grey, depending on their subspecies. Their colours become more vivid in the breeding season. 

Marine iguanas take to the water to feed. They mostly eat red and green algae, both in subtidal and deeper, cooler water. Their blunt noses and sharp teeth allow them to easily graze on the algae growing on rocks. They have also been seen eating grasshoppers, crustaceans and, on some islands, plants that grow on the land.

To cope with the amount of salt they consume while grazing in the ocean, marine iguanas have a specially adapted gland that removes salt from their bodies. They then forcefully expel the salt out of their nostrils in a sneeze-like fashion.  

Their tails are flattened, which gives them extra propulsion when swimming. Having seen the species during his travels aboard HMS Beagle, Charles Darwin noted in a letter that they 'swim quickly and with much elegance' with their legs fixed to their sides and using a 'serpentine movement, like an eel' to propel themselves. 

Marine iguanas are exothermic, so they can't survive for too long in the cool water. They need to bask in the Sun on land to raise their body temperature. They also mate and nest on land.

It's thought that the closest relative of Galápagos iguanas are the spiny-tailed iguanas (Ctenosaura) found in central America. Scientists think the iguanas arrived in the archipelago by rafting and then diverged into the land-dwelling and marine iguanas seen on the Galápagos Islands today.

How and why do marine iguanas shrink?

The International Union for the Conservation of Nature (IUCN) lists marine iguanas as Vulnerable, with one reason being the threats they face from their population fluctuations. These are caused by periodic El Niño events, which bring warmer waters to the Galápagos.

Warmer water leads to the disappearance of the red and green algae that marine iguanas prefer. The amount of brown algae increases, but the lizards can't easily digest this and it may even be toxic to them. This reduction in food causes the animals to die by starvation, with the larger iguanas more at risk.

During El Niño events, 10-90% of marine iguanas can die, causing extreme population fluctuations in this species. 

To combat their lack of food, marine iguanas do what might seem impossible: they shrink, not just in weight but in total body length.

Individuals can become as much as 20% shorter. As cartilage and connective tissue make up only 10% of the animals' length, it's believed their reduction in size could be at least in part the result of the iguanas reabsorbing their own bone matter.

Marine iguanas that shrink have also been found to survive longer than those that are larger. This is likely due to the smaller individuals being able to forage for their reduced food resources more efficiently, using less energy. Females have also been seen to shrink more than males, which is possibly linked to the extra energy females need to produce and carry eggs. 

When the water begins to cool and food becomes abundant again, the iguanas' body length increases once more.

The lizards can shrink and grow multiple times throughout their lives depending on the climate. Marine iguanas live for 12 years on average but some as long as 60 years. ENSO is irregular, but the phases generally last several months each, although prolonged episodes can last for years.

Threats to Galápagos marine iguanas

Human-induced climate change is a concern for this species, as it's anticipated that it may increase the frequency of the warmer El Niño events, which could decimate subpopulations of marine iguanas. 

The lizards also face threats that can't be avoided by shrinking. While they don't have many predators in the ocean, on land the iguanas can fall foul of birds such as hawks and herons. The introduction of non-native predators, such as rats, cats and dogs, is also a problem for them.

An increase in tourism to the Galápagos Islands may also have an impact on their ecosystem - in 2015 the islands saw over 200,000 more tourists than in 1979.

But conservation efforts are having a positive impact. Marine iguanas are protected from over-exploitation by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), listed in Appendix II. The species is also fully protected by law in Ecuador and all populations of the lizards are distributed across protected areas.