The marine iguana (Amblyrhynchus cristatus) is the world’s only lizard that forages for food in the ocean. This extraordinary reptile is found exclusively throughout the Galápagos Archipelago. Its dark, volcanic-rock-like appearance and distinctive spiked crest reflect its specialized lifestyle.
Adult Weight Range
The average adult marine iguana typically weighs between 1 and 3.3 pounds (0.5 and 1.5 kilograms). The species exhibits pronounced sexual dimorphism, with males being significantly larger and heavier than females. Adult females rarely exceed 3.3 pounds (1.5 kg). Males, who defend mating territories and dive for food, show a much wider weight range. The largest recorded males, found on islands like southern Isabela, can reach a maximum body mass of up to 26 pounds (12 kilograms).
Physical Dimensions and Island Diversity
The weight of a marine iguana is directly linked to its physical dimensions. Adult males reach up to 4.3 feet (1.3 meters) in total length, including their long, laterally flattened tail. Females are noticeably shorter, generally measuring around 2 feet (0.6 meters). Body size varies dramatically depending on the specific island population.
The largest iguanas are found on western islands, such as Isabela and Fernandina, where nutrient-rich ocean currents support a higher abundance of preferred algae. Conversely, populations on smaller, more isolated islands like Genovesa are significantly smaller, with the average adult male sometimes weighing as little as 2.2 pounds (1 kg). This variation demonstrates island gigantism and dwarfism within a single species, determined by localized food availability.
The Phenomenon of Size Reduction
The marine iguana has the unique ability to actively and reversibly reduce its size. This phenomenon is a direct response to periods of food scarcity, most notably during El Niño events. When warm ocean currents displace the cold, nutrient-rich water, the iguanas’ preferred red and green algae die off, leaving behind less digestible brown algae.
To survive this famine, adult iguanas can shrink their overall body length by as much as 20% over a two-year period. This reduction is not simply the loss of fat and muscle mass, but involves the active reabsorption of bone and connective tissue. This physiological mechanism allows the lizard to decrease its total body size, thereby reducing its metabolic needs and energy expenditure. Iguanas that shrink the most often have the highest survival rates, and they will regrow to their previous size once favorable food conditions return.
Diet and Its Impact on Body Mass
The diet of the marine iguana consists of marine algae, specifically red and green varieties, which they scrape off submerged rocks using their blunt snout and sharp teeth. This specialized herbivory dictates their entire energetic profile and limits on body mass. Large males often dive offshore to graze on deeper algal beds, while smaller iguanas and females typically restrict their feeding to the intertidal zone during low tide.
Since they are cold-blooded reptiles, their body temperature drops significantly during their forays into the cold Pacific Ocean water. They must spend considerable time basking on dark volcanic rocks to raise their body temperature before they can properly digest the algae. The sheer quantity of algae required to sustain a large body mass, combined with the energy cost of warming up to digest it, establishes a natural limit on how heavy an iguana can become.