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What differences on Española Island produced a distinctive subspecies?  The Galápagos archipelago lies along the equator some 1,000 kilometers west of South America. (Click to enlarge) Climate: The Galápagos archipelago, a part of the South American country of Ecuador, is a scattering of islands about 1,000 km west of the mainland. It consists of 13 major islands and about 65 smaller islets or rocks sprinkled along a northwesterly axis across the equator. Of its 7,800 square kilometers, more than half belongs to the largest island, Isabela. The rest of the archipelago is composed of smaller islands, only three of which exceed 500 square kilometers. Although Ecuador is famous for its rain forests, the Galápagos' climate is far from tropical; the lack of high altitudes for pulling moisture from the sky gives the islands more of a dry, desert-like climate, especially on the smaller islands with low altitudes. The climate on arid Española is virtually identical to the coastal climate on the other Galápagos islands where marine iguanas live, so it is easy to exclude this factor as an important cause of differentiation. Food: Beside the occasional feast on sea lion afterbirth, the diet of the marine iguana consists entirely of various species of intertidal seaweed and algae growing on the porous tidal-zone lava rocks. These algae grow most abundantly on the southern shores of the islands, the side exposed to the wind and wave action (Eibl-Eibesfeldt, 1984). The marine iguanas eat by grazing off the rocks, so their heads have evolved for more efficient eating. Their generic name, Amblyrhynchus, means "short snout." Their snouts are flattened in front, allowing them to graze like a horse or a cow; they scrape the food off the rocks with jerky tearing motions, using their uniquely shaped tricuspid teeth. All of the Galápagos' marine iguana subspecies share this diet, snout arrangement, teeth, and eating behavior. Since each of the Galápagos Islands has a similar density of the preferred algae growing on their southern coasts (Eibl-Eibesfeldt, 1984), it is clear that the availability of food probably did not have a major effect on the divergence of venustissimus.  One of the only predators of marine iguanas when they are ashore is the Galápagos hawk, here perching on a rock (middle of photo) on arid Española Island. Predators: Marine iguanas have few natural predators in the Galápagos, and most land predation targets the young or the weakened. Their main marine predator is the shark (Heller, 1903). They can also fall prey to the Galápagos hawk, snakes, and herons when they are hatchlings. Most of the time the adults are too heavy and strong to be attacked by land predators or birds, but the exception is when the females are exhausted after laying their eggs and become easy prey for the Galápagos hawk. Predation does not seem to have had a significant effect on the size or behavior of each island's population for any of the subspecies. Española's predators are the same as the ones on the other islands and do not appear to be a major factor in explaining the divergence of venustissimus.  A pair of marine iguanas basks on the shore near our landing site, north of Punta Suarez. A panga boat disembarks its passengers in the background. |
Española's geography—the southern cliffs: Old and remote Española has eroded and disintegrated heavily since its volcanic creation. The entire southern coast consists of 12 kilometers of treacherous cliffs where a fault line caused the southern third of the island to fall into the ocean long ago. This prevented us from landing our panga on the southern shore, and we were forced to approach the island from the west, where we found a protective cove just north of Punta Suarez. In the still water of the natural harbor, our panga made gentle contact with the rough lava rock that served as our dock, while a nearby basking marine iguana posed as if to welcome us. I eagerly snapped some pictures of the creature before gingerly picking my way inland over the rough black lava bordering the shoreline.  The southern cliffs of Española are evident in this topographic view of the island. A fault line caused the southern third of the island to break off into the ocean long ago. (Click to enlarge) The same cliffs that had prevented our landing on the southern coast of Española also deny venustissimus its preferred habitat—a south-facing shoreline of rocks and sand near the most abundant algae supplies (Eibl-Eibesfeldt, 1984). The presence of cliffs instead of low shoreline appears to be closely related to several adaptations by venustissimus: Brighter breeding colors for males: Each winter the males stake out pieces of lava near the ocean and near beach nest sites. Once a male has firmly established his territory, he mates with the females that pass through his domain. The reduced availability of shoreline near food on Española causes fierce competition for the few suitable breeding sites. Most competition consists of ritualized demonstrations and bluffing, where the appearance of aggressiveness is most important. Brighter coloration is a sign of greater aggressiveness, recognized by other iguanas. The males with the best breeding sites have the largest reproductive success; the cliffs of Española promote breeding success in the brightest-colored males. Brighter breeding colors for females: About five weeks after mating, the females begin competing for the best nesting sites—preferably the soft substrate of beach sand. On most islands the females do not have to compete for nest sites and do not become especially colorful during the breeding season, but this is not the case on Española. Since bright coloration is associated with aggression, during this competitive time the females become nearly as colorful as the males were a month earlier (Eibl-Eisenfeldt, 1984) and, like the males, use these colors to intimidate competitors and achieve breeding success.  Two marine iguanas bask on a rock high on the southern shore of Española, with the cliff line extending to the east beyond them. Cliffside nesting: I was surprised during my visit to Española to see marine iguanas basking on rocks high on the cliffs of the southern coast. I learned that this was a behavior observed only on Española, where venustissimus not only basks on the cliffs, but has adapted the unique practice of actually building nests high in the cliff rocks (Thornton, 1971). Living and nesting on the cliffs of the southern coast keeps venustissimus close to the plentiful algae of the southern coast. The fact that Española's southern coast is devoid of proper nesting beaches appears to have been an important factor in encouraging adaptive behavioral evolution in the venustissimus subspecies. Venustissimus' isolation: Having found a good explanation for how the differences on Española could encourage the emergence of a distinct subspecies, we now must ask how this population stayed genetically isolated enough to begin diverging. Why hasn't the marine iguana, being an able swimmer, swum from island to island, mixing the subspecies? One of the main factors reducing the amount of mixing between Española's marine iguanas and the populations of other islands is Española's small size. With an area of only 61 square kilometers, Española makes up about 0.7 percent of the Galápagos land area. Smaller islands are automatically "smaller targets" for animals swimming or adrift on vegetative rafts (Mackenzie et al., 2002). Although small size is a factor, my observations on Española would reveal other, more significant limitations that restrict immigration to Española from neighboring islands. Continuing our hike toward the southwest corner of Española, we came upon a large venustissimus colony on a rare stretch of beach. En route, I had seen marine iguanas on every sunny lava rock, as well as in groups along the trail. On a small ridge above the white beach, I paused to watch a large male approach from the ocean. A rhythmic undulating motion, starting at his head, flowed through his entire body to the tip of his tail. When breaking waves crashed over him, he casually gripped the porous rock with his long sharp claws, waiting for the danger to pass before continuing on his way to the shore. Relative to its size, this marine iguana appeared to be a quick and powerful swimmer. As I watched the marine iguana swim towards shore, I estimated its swimming speed to be about three kilometers per hour, about the same speed as a good human swimmer. |
