When you look at the marine iguana (Amblyrynchus cristatus) you can't help feeling you're looking back in time. The dorsal crest, and primitive looking features, inevitably make us think of dinosaurs. But they're nowhere near as ancient as dinosaurs. Geologists believe the Galapagos Islands are no older than ten million years, and even sunken islands that may once have been part of the Galapagos are much more recent than the dinosaurs. Still the marine iguanas do look primitive, and Charles Darwin was unfavorably impressed by their appearance saying they looked "hideous" and "stupid." Marine iguanas might seem slow and even clumsy, but they are highly adapted to their unique niche. The marine iguana is the world's only sea-going lizard. Scientists believe they evolved from a mainland ancestor that arrived in the islands millions of years ago. This individual also gave rise to the land iguana.

They are found throughout the Galapagos Islands, but nowhere else in the world. The largest populations, and the biggest individuals, are found in the western islands of the archipelago. Here the water is coldest, and optimal for the species of seaweed the iguanas feed upon.

A persistent question is whether different populations of the marine iguana are genetically uniform. Some researchers have gone as far to argue that there is more than one species in the islands since there are differences in appearance and size between distantly separated populations. However, the consensus is that the differences do not prevent interbreeding and thus the entire Galapagos population is comprised a single species. The debate continues over whether or not subspecies are valid. (What is the definition of a species?)

They are up to two to three feet in length, with long whip-like tails used for swimming. Their dull black or dark gray color matches the lava rock on which they like to bask. Like all reptiles they are "cold-blooded" (properly called "ectothermic") so they lie in the sun to warm up or move into shade to cool down. After a swim in the ocean to feed on seaweed, they need a few hours to warm up again.

Like all reptiles they are ectothermic, incorrectly called "cold-blooded." Their blood isn't cold, but what this means is that iguanas cannot internally regulate body temperature like mammals and birds. They must rely on the external environment. Consequently, they bask in the sun to warm up, and move into shade to cool down. On a cold morning, they are unable to move fast and wait until the warmth of the sun heats them up enough to swim for feeding. When it's very hot they orient toward the sun to minimize the exposed surface. They'll also cover each other for shade, as shown in the first photo on this page. At night they gather in large numbers to conserve body heat.

Marine iguanas feed exclusively on a few species of red or green algae (seaweed) that grows close to the rock (see photo). These algae grow less than half an inch from the rock surface, so the iguana's mouth is rounded to crop the plant more easily. Only large males venture into the ocean to find food. They can stay down a long time. In an experiment that today might be condemned by animal rights acitivists, Darwin kept one large male submerged in a bucket of seawater for over an hour — it emerged none the worse for the experience.

Females and juveniles feed close to the surf but rarely if ever feed underwater. The cycle of daily feeding depends on the tide and water temperatures. Females and smaller individuals feed at low tide whatever the time of day. The large males wait until the middle of the day by which time they have warmed up enough to enter the water. Sea temperatures around the Galapagos are rather cold for their tropical location and the iguana can lose up to 10ºC of body heat during a dive. Such a loss for a human would probably be fatal. The iguana just swims back to shore and basks in the sun. Underwater predators may include sharks but this is not well-documented. Young sea-lions love to hassle the swimming iguanas and pull their tails, only to be studiousy ignored.

Males during breeding season develop reddish patches, the extent of which varies from island to island. Those on Hood Island may turn almost entirely red. Males fiercely butt heads to determine superiority, sometimes drawing blood. Breeding season varies from island to island. Females dig burrows in soft sand. One to four eggs are laid, and incubate for up to four months. The juveniles are about three or four inches long and vulnerable to predators such as hawks, owls, herons and mockingbirds. Feral animals also take considerable numbers. Once fully grown, their only predator is the Galapagos hawk.

Marine iguana populations fluctuate widely depending on the weather. During El Niño years when warm waters cause their favored algae species to die, huge numbers of iguanas perish of starvation. They'll turn to other seaweeds but these are not nutritious enough to sustain the animals, so that they starve with full stomachs. In 1998 on Fernandina Island, hundreds of iguana corpses littered the ground. Somehow, enough always survive to perpetuate the species. The stringent conservation policies of the Galapagos National Park Service have helped reduce the numbers of wild cats and dogs preying on the young, so barring major environmental disasters, the marine iguana looks set to survive for the indefinite future.

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