The Galapagos giant tortoise is one of the most famous animals of the Islands, with the Archipelago itself being named after them (Galapágo is an old Spanish word for tortoise). The giant tortoise arrived in Galapagos from mainland South America 2-3 million years ago, where they underwent diversification into 14 species, differing in their morphology and distribution.
After the death of Lonesome George in 2012, the last Pinta island tortoise, twelve living species are thought to remain in Galapagos across ten islands. Giant tortoises show large variation in size and shape but all species can be classed into two main shell types: domed and saddle-backed. Dome-shelled tortoises lack an upward angle to the front of their carapace (shell), restricting the extent to which they can raise their heads. They tend to live on large, humid islands where there
is lots of vegetation to eat. Saddle-backed tortoises have an upward curve to the front of their carapace, which allows them to stretch up to reach higher growing plants. They tend to live on arid islands in Galapagos, where food is less abundant. The Galapagos giant tortoise spends an average of 16 hours per day resting. The rest of their time is spent eating grasses, fruits and cactus pads. They enjoy bathing in water, but can survive for up to a year without water or food. Small birds,
such as Galapagos finches, can often be seen sitting on the backs of giant tortoises. The birds and tortoises have formed a symbiotic relationship in which the birds peck the ticks out from the folds of the tortoises’ skin. Breeding primarily occurs during the hot season (January to May), although mating may be seen at any time of year. Mating may last for several hours, with the male making loud roaring noises throughout. After mating, the female migrates to a nesting area, where she digs a
hole with her back feet into which she lays 2 to 16 eggs, each the size of a tennis ball. The eggs are incubated by the sun, with the young tortoises hatching after around 130 days. Where to see them: The Santa Cruz highlands and Alcedo Volcano on Isabela have the largest populations of giant tortoises. When to see them: Galapagos giant tortoises are present all year round. They are most
active at midday during the cool season and early morning or late afternoon in the hot season. Threats: Galapagos giant tortoise populations suffered huge declines due to exploitation by whalers, buccaneers and fur sealers. The tortoises provided a source of fresh meat which could be kept alive on a ship for several months without any food or water. This resulted in the loss of between 100,000 and 200,000 tortoises. They were also exploited for their oil which
could be used for burning in lamps. The introduction of several species by humans has had further devastating effects on tortoise populations. Rats, pigs and ants predate tortoise eggs; feral dogs attack adult tortoises; cattle and horses trample nests; and goats compete with tortoises for food. They are also affected by barriers to the migration routes such as farmland fencing and roads, and there are
possible health issues from being in close proximity with farm animals. At least two species of giant tortoise are thought to have gone extinct – including the Pinta tortoise, whose last known survivor (Lonesome George) passed away in June 2012. Conservation actions:
Interactive Galapagos Giant Tortoise Map: You can help us protect Galapagos giant tortoises by donating today or by purchasing one of our giant tortoise adoptions. Learning Outcomes
Darwin and Descent with Modification Figure 1. Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources. In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle, including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape (Figure 1). The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the South American mainland. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that each finch’s varied beaks helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey. Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, or “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change. For example, Darwin observed a population of giant tortoises in the Galápagos Archipelago to have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace’s understanding of this principle came from reading economist Thomas Malthus’ essay that explained this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment. It is the only mechanism known for adaptive evolution. In 1858, Darwin and Wallace (Figure 2) presented papers at the Linnean Society in London that discussed the idea of natural selection. The following year Darwin’s book, On the Origin of Species, was published. His book outlined in considerable detail his arguments for evolution by natural selection. Figure 2. Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858. It is difficult and time-consuming to document and present examples of evolution by natural selection. The Galápagos finches are an excellent example. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important evidence of natural selection. The Grants found changes from one generation to the next in beak shape distribution with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited a variation in their bill shape with some having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, there was a lack of large hard seeds of which the large-billed birds ate; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, the small-billed birds were able to survive and reproduce. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the bill evolved into a much smaller size. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased. Field BiologistMany people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and invigorating. What if your job entailed working in the wilderness? Field biologists by definition work outdoors in the “field.” The term field in this case refers to any location outdoors, even under water. A field biologist typically focuses research on a certain species, group of organisms, or a single habitat (Figure 3). Figure 3. A field biologist tranquilizes a polar bear for study. (credit: Karen Rhode) One objective of many field biologists includes discovering new, unrecorded species. Not only do such findings expand our understanding of the natural world, but they also lead to important innovations in fields such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and nutritive knowledge. Other organisms can play key roles in ecosystems or if rare require protection. When discovered, researchers can use these important species as evidence for environmental regulations and laws. Try ItContribute!Did you have an idea for improving this content? We’d love your input. Improve this pageLearn More What island does the Galápagos tortoise live on?The Galápagos tortoises are iconic animals, often living for over 100 years and weighing as much as 400kg. They're only found on the Galápagos Islands in the Pacific Ocean, nearly 1,000km away from the nearest landmass, Ecuador.
Why are there so few tortoises left on the Galápagos?Tortoise numbers declined from over 250,000 in the 16th century to a low of around 15,000 in the 1970s. This decline was caused by overexploitation of the subspecies for meat and oil, habitat clearance for agriculture, and introduction of non-native animals to the islands, such as rats, goats, and pigs.
How did the Galápagos tortoise adapt?Saddle-backed shells evolved on the arid islands in response to the lack of available food during drought. The front of the carapace angles upward, allowing the tortoise to extend its head higher to reach the higher vegetation, such as cactus pads.
Is there only one Galápagos tortoise?There are 13 living species of Galápagos tortoises, which are also sometimes called giant tortoises. These reptiles are among the longest-lived of all land vertebrates, averaging more than a hundred years. The oldest on record lived to be 175.
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Explain why specific Galapagos tortoise species are able to live only on certain islands
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