Large Billed Ground Finch Expert
How Big Can the Galapagos’ Godzilla Get?
We got off of the inflatable motor boats (zodiacs) onto Tower Island and climbed the same steps that Prince Philip ascended in 1964. When we got to the top, it seemed as if we had travelled through time before man’s existence; Nazca and red footed boobies were all around us and acted as if we were not there. You could hear raw nature and see the wildlife. One thousand kilometers off the coast of Ecuador lies an archipelago that is one of the richest in biodiversity in the world. The Galápagos houses many endemic or rare species, and it was a significant contributor to Darwin’s postulates of the Theory of Evolution. After he returned to England from his 5-year expedition, he began to wonder how species from the islands could vary so greatly, when they are so geographically close.1 Despite their time there being only five weeks the crew of the HMS Beagle discovered a common trend, especially amongst the finches and mockingbirds: the animals had varied traits between the islands, even between islands that are within sight of the others. These differences are due to both dispersal and evolution; the spreading of species, and the genetic changes in a population over time, respectively. The genetic changes often result in morphological alterations in species. Evolution can occur via natural selection, when a certain heritable trait is favored and passed on to future generations. A subcategory of natural selection is sexual selection; when a certain sex prefers a particular heritable trait in the other, which results in more reproductive success. On our trip to the Galápagos Islands in March of 2016, we personally observed this same trend in birds, tortoises, and marine iguanas. The marine iguana, Amblyrhynchus cristatus, is a species endemic to the Galápagos Islands that is present on every large island on the archipelago.
On our first day on the Galápagos we arrived to the Seymour Airport, on Baltra Island, after an approximately two-hour flight from Guayaquil International Airport. The transition, from the cool plane interior to the humid island exterior, was abrupt, but not unwelcomed. We shopped for a few minutes, as typical tourists do, and immediately went to the “Muelle de Pasajeros en Seymour”, the dock of Seymour, to wait for our yacht, the Fragata. While waiting, we all spent time on the sand and observed our natural surroundings. Here, I saw a marine iguana for the first time in my life. These marine iguanas seemed of average size (about one and a half feet long) and a charcoal-like color, to the point where they blended into the dark black lava rocks. Since this was my first encounter with a marine iguana, I didn’t have any expectations of what it would look like in person, and I was simply fascinated by it. In hindsight, I realized that this was really small compared to others that could reach a length of 22 inches, excluding the tail.8
Five days later we woke up just off the coast of Española Island and were preparing for our day. Our guide, Fausto, and our professor, Irby, briefed us and said that we were going to be landing on Española Island, one of the oldest islands on the archipelago, and to expect a major difference in the animals that we saw. We left on the zodiacs at sunset, with the gentle ocean breeze comforting us, and headed towards the island, where we would begin our four-hour hike. We were informed that the waters would be rough and that it would be a dry landing, where the boat goes besides a rock and we climb out. The crew put towels on the rocks to prevent us from slipping, and we got off of the zodiac, which kept a steady acceleration to keep us touching the rocks.
We got onto the rocks and carefully made our way to the beach; aware of every step that we made because with one loose rock, there went our footing and we would fall onto more rocks in the water. Once we made it to the beach, we got off and began to explore; what we saw surprised me and changed my perspective of evolution. On the beach were dozens of Amblyrhynchus cristatus venustissimus, the Christmas Marine Iguana, which are completely different from the marine iguanas on Baltra; they were not only twice the size (three to three and a half feet long), but also, their skin featured a clash of hot pink and turquoise coloration. Their length (excluding the tail) was just as long as the entire body (including the tail) of the marine iguanas on Baltra! This sight got me thinking and I started to ask questions. Why are these iguanas so different if they are located only 120 kilometers apart? Are there even bigger and more elaborate iguanas?
These iguanas spread to all of the islands, via dispersal, and through evolution, they formed their own, new subspecies. Natural selection is the driving force of evolution that favors more fit individuals in a population and makes them more likely to survive. This force acts on individuals in the moment; it does not think about the future, instead it will act accordingly to the current environmental conditions. In other words, natural selection never strives for perfection for the population. Also, the traits that are affected by natural selection must be heritable and facilitate survival. An example of this was seen on a Daphne Major study that showed how varying weather conditions caused finch beak sizes to change in one generation.10
In marine iguanas, body size is the result of a balance between the opposing forces of natural and sexual selection; natural selection favors smaller iguanas, due to El Niño, while sexual selection favors larger males. The El Niño effect is a weather pattern that occurs every couple of winters and results in warm, nutrient-deficient water. This causes food shortages on the marine environment in the Pacific. Natural selection favors the smaller iguanas because larger size prevents efficient energy intake in periods of little food and therefore causes starvation. However, in terms of sexual selection, female marine iguanas prefer larger males with more elaborate coloration. In El Niño years, these two forces would act in opposite directions. It is also seen that since males are larger than females, they have less of a probability to survive El Niño years than females (20-50% compared to 70-80% survival).9 Overall, medium sized males tend to survive and live to the next year.9 There are also other possible effects to the size differences in these marine iguanas. The absence of predators and the availability of food are two factors that could also contribute to the variation in sizes. If the animal’s growth is not restricted to a certain amount of food, then it could grow to much larger sizes.9
One thing that was specifically interesting to me was that I remembered that Baltra Island had an extended history of having feral cats. These were introduced by pirates, towards the end of the 17th century, but recently eradicated in 2004.3 However, there was no history of the invasive species being present on Espanola Island.4 Marine Iguanas here evolved in a place without significant predators!5 In Canada, there was a study that suggested that there was higher variation within animal species living in the absence of predators.6 Since cats are the unnatural predators to the marine iguana, their introduction to the ecosystem could throw the island’s ecology out of wack. Darwin suggested that the marine iguanas came onto land millions of years ago because they had many predators in the water, and none on land.7 What would happen when there are predators, like cats, on land?
Cats – something that is commonly known as a friend and companion to man – are seen to be destructive to the well-being of other animals. On Baltra, the cats brought by man were unnatural predators to the marine iguana population, whereas they were absent on other islands like Espanola.7 A biologically diverse island, like the Galápagos, is one that can be especially sensitive to external factors, such as invasive species. Hopefully man truly learns his mistake, to prevent the need of more eradication efforts, before the consequences lead to the extinction of too many species in such an ecologically rich place.
Travelling nearly 5000 kilometers to visit this ecologically rich and diverse place enabled me to link everything that I learned in my evolutionary biology class to the real world; such as different evolutionary forces, dispersal and the extent to which location could influence the characteristics of a species. After experiencing the “magic” of the Galápagos, I can confidently speak to someone about these topics and give examples with things that I experienced! I guess what a Chinese proverb once said is true, “Tell me and I’ll forget; show me and I may remember; involve me and I’ll understand.”
1Sulloway, F. J. (1982). Darwin's conversion: The Beagle voyage and its aftermath. Journal of the History of Biology, 15(3). 325-396. http://dx.doi.org/10.1007/BF00133143.
2Jardine, N. & McKenzie, D. (1972). Continental drift and the dispersal and evolution of organisms. Nature, 235. 20-24. http://dx.doi.org/10.1038/235020a0.
3Phillips, R. D., Cooke, B., Campbell, K., Carrion, V., Marouez, C., L. & Snell, H. (2005). Eradicating Feral Cats to protect Galapagos Land Iguanas: methods and strategies. Pacific Conservation Biology, 11. 257–267. http://dx.doi.org/10.1071/PC050257.
4Stone, P. A., Snell, H. L., & Snell, H. M. (1994). Behavioral Diversity as Biological Diversity: Introduced Cats and Lava Lizard Wariness. Conservation Biology, 8(2), 569–573. Retrieved from http://www.jstor.org/stable/2386482.
5Roy, K. (2000). Amblyrhynchus cristatus. Animal Diversity Web. Retrieved from http://animaldiversity.org/accounts/Amblyrhynchus_cristatus/.
6Moodie, G. E. (1972). Predation, natural selection and adaptation in an unusual threespine stickleback. Heredity, 28. 155-167. http://dx.doi.org/10.1038/hdy.1972.21.
7Denby, D. (1997). Our far-flung correspondents in Darwin’s wake. The New Yorker, 50(20). 50-62.
8Wikelski, M. & Trillmich, F. (1997). Body size and sexual size dimorphism in marine iguanas fluctuate as a result of opposing natural and sexual selection: an island comparison. Evolution, 51(3). 922-936. http://dx.doi.org/10.2307/2411166.
9Wikelski, M. (2005). Evolution of body size in Galapagos marine iguanas. Proceedings of The Royal Society B, 272(1576). 1985-1993. http://dx.doi.org/10.1098/rspb.2005.3205.
10Searle, J. (2016). Natural Selection: One Mechanism of Evolution [PowerPoint]. Retrieved from Cornell University BIOEE 1780 Evolutionary Biology and Diversity Blackboard: http://blackboard.cornell.edu