Galápagos Marine Iguanas: A day in the life of World’s only marine lizard
As its name anticipates, the Galápagos marine iguana (Amblyrhynchus cristatus) is a species of marine reptile found only on rocky shores of the Galápagos Islands, Ecuador. This species of lizard can be from 29 to 133 cm in length (head to tail) and mainly lives in colonies. Because of its isolated evolution surrounded by the Ocean on the Galápagos islands, the marine iguana has adapted well to life on the coastline. Among all lizards, this species has developed the unique ability to forage at sea. But as every benefit in nature, this ability comes with a cost: every day for a marine iguana is a challenge to balance the costs and benefits of being the World’s only marine lizard.
The day for a marine iguana starts as early as sunrise, spending the first hours of light basking in a flat position in the sunshine, gathering as much heat as possible. As all reptiles, marine iguanas are ectothermic, meaning that they cannot self regulate their body temperature.
Instead, they need to use the environment they are in to maintain their standard internal temperature. For marine iguanas, the preferred body temperature range is very narrow (35-37ºC). In order to maintain their temperature within this range, Galápagos marine iguanas lay in the sun, using the heat from the sunlight and the dark volcanic rock surface (which can reach up to 60ºC) to regulate their body temperature. Adjustments to the posture are also used to prevent overheating by creating areas of shade on the skin.
Once they have gathered enough heat (usually around midday), it is time for the marine iguanas to start feeding. Most marine iguanas in a population feed in the intertidal zone at low tide, scraping algae off the rocks. In this way, they can remain on land and return back to gather heat on the rocks more quickly.
Only the older and bigger iguanas, which account only for 5% of a population, feed underwater. The flattened tail is very efficient for swimming and the flat head and sharp teeth are useful for scraping algae off rocks, while the long claws keep the iguana clasped to the surface of the rocks. Marine iguanas can dive as deep as 20m and remain underwater for up to 1h, but feeding trips are usually much shorter and shallower. Feeding underwater allows the iguana to access a great abundance and diversity of algae and to escape competition in between other individuals (intra-specific competition).
However, the advantage of swimming underwater comes with a cost. The temperature of the water is usually around 10ºC less than the iguana’s preferred body temperature range and heat is dispersed 25 times faster in water than in air. This makes the iguanas very vulnerable to hypothermia (when a body dissipates more heat that it absorbs), which often leads to death. In order to avoid loosing too much heat, Galápagos marine iguanas feed in the subtidal area only during the hottest part of the day- around midday- and remain underwater for a limited period of time (on average 3 minutes) usually at a depth of around 5m, before coming back to bask in the sunlight to warm up again. When out of the water, the iguana “sneezes” out the excess salt accumulated while feeding at sea.
When the sun goes down, the cooling rate caused by the absence of sunlight is counteracted by aggregation. Piles of up to 50 iguanas sleep closely together at night in order to maintain as much body heat as possible. Some individuals may also sleep alone in crevices or between vegetation.
What will happen tomorrow?
Galápagos marine iguanas are listed as vulnerable in the IUCN Red List of Threatened Species. The small geographic range of this species and the growing changes in Ocean temperatures are considered the main reasons for their vulnerability. El Niño events cause the temperature of the sea to change very rapidly, limiting the growth of many species of seaweeds on which the marine iguanas feed on. On the island of Marchena, the population of marine iguanas was documented to be reduced by 50% after an El Niño event in 2004. It is now estimated that with Global Warming, events like El Niño will be stronger and more frequent, posing a growing challenge to the long-term survival of this species.
Living in a relatively predator-poor environment, another growing threat to marine iguanas are human introduced animals like cats, dogs and rats, against which they have no defence. Although unintentionally, humans also introduced another threat to the environment of the marine iguanas: pathogens. Living in isolations in the Galápagos Islands, marine iguanas have not developed immunity to many common viruses and bacteria. Marine iguanas exposed to tourism, have been seen to regularly show suppressions in the immunity system, which enhances the risk of contracting infections and limits the animal’s capacity to carry out immunological responses such as wound healing.
On a more positive note, Galápagos marine iguanas are completely protected under Ecuador’s laws and almost all their geographical range is part of the Galápagos National Park and the Galápagos Marine Reserve. Alongside conservation policies, further scientific research is needed to better understand the growing environmental and anthropogenic threats to Galápagos marine iguanas and find better solutions to protect World’s only marine lizard.