Explaining the Divergence of the Marine Iguana Subspecies on Espa
Mention the Galápagos Islands, and people generally think of Charles Darwin, evolution, and the giant tortoises (galápagos in Spanish). Lesser known than the island's lumbering "mascot" is the marine iguana, described by Charles Darwin in 1835 as a "hideous looking creature, of a dirty black colour, stupid, and sluggish" (Darwin, 1988). Despite its appearance, Darwin observed the marine iguana closely enough to recognize its main evolutionary adaptations from its land-dwelling Galápagos cousins, the land iguanas, Conolphus pallidus and Conolphus subcristatus. He could not, however, know how genetics and plate tectonics would enable the "lowly" marine iguana to teach us much more.
When I learned that I would be able to take a once-in-a-lifetime biology class trip to the Galápagos, I began doing research to learn much more about the marine iguana. I was surprised to learn that although the Galápagos Islands have the world's only species of marine iguana, there are seven separate subspecies or races among the islands, each one distinguishable from the next (Dowling, 1962, in Thornton, 1971; Garman, 1892, in Eibl-Eibesfeldt, 1984; Eibl-Eibesfeldt, 1956, 1962, in Thornton, 1971). The subspecies from Española Island, Amblyrhynchus cristatus venustissimus, is frequently mentioned as the most unique of the seven races because of its exceptional coloration and unusual behavioral adaptations. This variation within a single species intrigued me. What factors had caused Española's venustissimus to become so different from the races on other islands? Was it climate? The availability of food? Predators? Geography? Also, how had Española's marine iguanas stayed isolated enough from those on other islands to begin to diverge? Since the marine iguana is an able swimmer, why hadn't there been enough immigration between islands, and thus genetic mixing, to keep the populations on Española the same as on the other islands?During my trip to the Galápagos and especially during my stop on Española Island, I made some observations that provided important clues toward what might be the answers to these questions.
What differences on Española Island produced a distinctive subspecies?
As the gentle spray of slow waves misted my panga boat, I scanned the low coast of Española Island, attempting to see beyond the wall of the surf. I was looking for the red and turquoise backs of the Española race of marine iguanas. With a curtain of mist hanging over the lava-rock shore, however, the marine iguanas were impossible to spot, making my excitement mount in anticipation.
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.
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.
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 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.
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.
The iguana's quick, continuous motion was interrupted upon his arrival at the beach. He immediately became clumsy and slow, dragging his plump body with relatively puny legs. Landing on a sun-warmed rock, he prostrated himself. While feeding in the ocean, the ectothermal marine iguana's temperature drops an average 18 to 22 degrees Fahrenheit, from its optimal body temperature of 95 to 99 degrees Fahrenheit (Thornton, 1971). Upon returning from the ocean, they must immediately warm themselves to restore their body temperature.
The iguanas typically make one daily visit to the ocean for a meal. Being smaller, the females usually remain within 20 meters of the shoreline while grazing.
Although the larger males swim out farther and dive deeper than the females, they are rarely seen beyond 400 meters from shore. This is probably due to the fact that their main food is intertidal algae, which is not found further from shore. Drinking water would not pose any problem, however. Due to the lack of fresh water on the Galápagos, the marine iguanas evolved a salt gland enabling them to consume salt water (Schmidt-Nieken and Fange, 1958, in Thornton, 1971). This specialized gland concentrates the excess salt into a salty secretion, which is ejected through the nostrils like a sneeze.It is doubtful that this swimming reptile could survive the cold and fatigue required to swim between islands. Marine iguanas typically limit their time in the ocean to less than one hour, and most of that time is spent stationary and feeding. To swim the 40 kilometers to Española from the nearest island, San Cristobal, would require at least 12 hours if the iguana were able to swim continuously.
The time and effort required to complete the journey could become even greater because of the fact that any iguanas swimming toward Española would also be fighting the ocean current, which flows from southeast to northwest. Even though venustissimus could drift on floating vegetation from Española to other islands, using the current and the wind to help it, it would be virtually impossible for marine iguanas elsewhere to fight the current to swim to Española. This is probably the most important factor in ensuring that the marine iguanas on Española remain isolated from those on other islands despite their excellent swimming ability.
Geologic history and the Nazca plate: Española has been dated by potassium-argon decay and was found to be about 3.3 million years old, making it the oldest island in the current Galápagos. The other islands string out to the northwest of Española, gradually growing younger. San Cristobal, the closest island to Española, is 40 kilometers to the north and is about 2.4 million years old. Fernandina Island is the youngest island, at only 700,000 years old, and has a tall, sharp volcano that is active, like several on nearby Isabela.
Recent improvements in the understanding of plate tectonics have revealed that the Galápagos Islands are the result of the Nazca plate moving eastward over a stationary "hot spot," producing a string of volcanic islands. As the plate moves southeastward at seven centimeters per year, the volcanoes become extinct, are carried toward South America, and eventually erode to the point of becoming submerged. To the east of Española, below the surface of the ocean, are remnants of former Galápagos islands, now seamounts estimated to be between 8 million and 10 million years old.
Interestingly, scientists have recently discovered by genetic analysis that the marine iguana diverged from the land iguana about 8 million years ago (Kunstaetter, 2003). This means that the marine iguana evolved on one of the now submerged islands of the Galápagos before any of the current islands even existed, and it was probably on Española before it was on any other island in the present archipelago. Using the information on plate tectonics and the genetic age of the marine iguana species, it can be inferred that the marine iguana has repeatedly emigrated from eroding older islands to younger ones.
Conclusion: Based on my study and observations, I have concluded that there are two main factors causing the divergence of the Española subspecies. First of all, the unusual cliffs on the southern shore of the island influenced some different behavioral patterns and changes in appearance to occur in its population of marine iguanas. Secondly, two complementary conditions allowed venustissimus to remain isolated on Española: the ocean current flow to the northwest, and the Nazca plate's movement to the southeast. If either the currents or the tectonic plate moved in a different direction, marine iguanas on other islands may have migrated toward Española, mixing up its gene pool. The venustissimus subspecies of marine iguana that has emerged on Española is clearly the result of a combination of unusual factors, and it occupies a special place in Earth's complex mixture of species.
The sun was beginning to set as my group began to put on life jackets and climb onto the sturdy white panga that had brought us. My expedition on Española Island was coming to a close, and I had learned much about the marine iguana. But this new knowledge had succeeded in raising many new questions to explore. Will the races of the marine iguana ever become separate species, or will the island "leapfrog" migrations cause too much genetic mixing? How much longer will Española last before it, too, becomes a seamount? And will the marine iguana ever drift out of the archipelago to the mainland and establish itself somewhere else as successfully as it has in the Galápagos?
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