Marine Iguanas are certainly among the most unusual creatures in the Galapagos. Charles Darwin made extensive observations on these large, lizard-like reptiles. They certainly well demonstrate the unique evolution and adaption of Galapagos fauna. The males, which are larger than the females, may grow to a length of 4 feet or more (almost half of which is tail). They look fearsome (Darwin called them “hideous”), but are quite harmless. These herbivores feed exclusively on algae growing on rocks near the shore. When feeding, they can remain submerged for up to an hour, though dives of 5 to 10 minutes are more common. The water in the Galapagos is often quite cold and being cold-blooded, iguanas spend much of their time restoring body heat by sunning themselves on the rocks adjacent the shore. Like many of the Galapagos animals, they are usually unperturbed by the presence of humans and may be approached quite closely (this is more true in the populated area around Academy Bay than in more remote areas). The closely related land iguana is lighter colored and usually somewhat larger. It is also less common and often somewhat shy.
Iguanas seem to prefer rocky shore but may also be found in mangrove swamps and beaches. The males are always more brightly colored than the females, with hues of red and green on their backs. As is the case for many other Galapagos fauna, different races or subspecies may be found on different islands. In the case of the marine iguana, differences between race are most apparent in their coloration, particularly of the males. For example, green colors dominate on the Santiago iguanas whereas red dominate on the Espaola race. Females are generally dark gray to black. The faces of both sexes often appear light colored. This is not coloration at all, but is an encrustation that comes from their curious habit of sneezing salt. This is part of the unique adaptions that have evolved that allow these descendents of terrestrial ancestors to live in salt water. A gland connected to the notrils removes salt from the body, which is then expelled by “sneezing”.
Large males assemble “harems” of several females and guard the harem against intrusion by other males. Head bobbing is a threat jesture that warns other males (and sometime humans) to stay clear. Fights occasionally occur between males. These are quite harmless and consist of a contest in which the two males put their heads together and attempt to push their opponent backward. The looser retreats without further fuss.
Iguanas, like the other species unique to the Galapagos, have evolved in an environment in which there are few natural predators. Thus they have few defenses against introduced animals, such as rats that prey on the eggs, cats that prey on young, and dogs that prey on even the adults. These feral animals are steadily reducing the iguana population in many localties. In areas such as Academy Bay and Volcan Ecuador, marine iguanas remain abundant (though notably less so than 20 years ago), but all the individuals are mature adults as few young survive the predation by ferel cats. Thus it appears that population is not being replaced and they may well disappear if nothing is done. Fernandina remains free from feral animals and here, along with many smaller islets, one may observe healthy and thriving populations.