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Miranda Martinez

Miranda Martinez smiling with big hat on

Invasive Species in Galapagos Expert

The Gentle Giants of the Galapagos: The Environment’s Effects on Morphology

The last rays of sunlight blanket the island of Santa Cruz, glinting off the domed backs of several majestic—yet notoriously slow—Galapagos Tortoises. Their shells seem to absorb the colors of the setting sun, reflecting the beautiful violets, magentas, and blues streaked across the sky.

The Galapagos Tortoises move through the low shrubbery in what looks like slow motion, every step as painstakingly slow as the last. They all seem to be congregating around one of the trees, waiting for the perfect moment to pounce—or in their case just move at a relatively unhurried pace.

Colorful field sketches of Galapagos Tortoise in multiple positions in the grass
Illustration by Miranda Martinez

If slow and steady wins the race, then they must be champions. What they lack in speed they make up in precision as they hone in on their defenseless targets: juicy, ripe guavas. Even though these Galapagos tortoises cannot reach their delicious snacks due to the shape of their shells and relatively short necks, they do not have to travel very far to find what they are looking for.

The soft thud of a newly-fallen fruit piques the curiosity of the slow giants. One by one they make their way to the tree, ready to reap the benefits of good old gravity. Being low to the ground has its rewards as one of the tortoises claims its prize by taking a juicy bite out of the guava. 

When thinking about the diverse life on the Galapagos Islands, one of the most iconic images that most commonly arises is of the Galapagos Tortoise. A large circular shell, scaly dark gray skin, and overall slow behavior are a few of the key characteristics of these interesting animals.

Also known as Geochelone nigra, the Galapagos Tortoise is arguably one of the most fascinating inhabitants of the islands. Although these creatures are usually referred to as one species, there is at least one subspecies located on each of the islands with five residing on Isabela Island. “Out of the 15 subspecies that have been documented, four of them have reached extinction”, leading to more “extensive conservation efforts” that attempt to protect the remaining few[1][2].

Compared to tortoises on other continents, the Galapagos Tortoise is known for being by far the largest overall. Some are even large enough to carry people on their backs which Darwin personally discovered during one of his expeditions to the islands.   

Although the size differences between the various species of tortoises are important, the characteristics of the Galapagos Tortoises show how the environment can affect evolutionary changes. Most of the Galapagos Tortoises have similar overall body structure, but the key differences are between the shell shapes and neck lengths. There are “two prominent forms of tortoise shells throughout the Galapagos Islands: domed and saddle backed”[3].The domed shells have a slight curve near the nape of the tortoise, hindering the upward movement of its neck. The saddle back shells have a more extreme curve, allowing for easier extension of the neck. Each form in its own way shows how the environment has contributed to its overall shape. Those with domed shells have shorter necks while those with saddle backed shells have longer necks. These allow for the Galapagos Tortoises to reach certain vegetation on the islands.

On several of the islands throughout the Galapagos, the domed-shell Galapagos Tortoises are right at home. Subspecies such as C. chathamensis thrive in that environment because of their morphology. Due to the lack of water in these areas, tall vegetation can scarcely grow there, allowing for an abundance of low-lying vegetation. These conditions are ideal for the tortoises since all their sustenance can be found close to the ground, which is easier for them to reach due to their “short neck lengths and low curvature of their shells” [3]. If these Galapagos Tortoises were transported to an island with only tall plants, they would struggle to survive unless they could find trees with ripe fruit—such as the guava tree—that they could sustain themselves with.

Located on a few of the islands are the Galapagos Tortoises with the saddle backed shell shape. They were named this because of how the front of the shell “curves upward at a sharp angle”, resembling a saddle for a horse[4]. Perhaps this is why so many people rode on the backs of these slow animals. A few subspecies—such as Geochelone elephantopus—exhibit these qualities. Since the islands they reside on have a bit more rainfall, the area can support larger vegetation. The curvature of the saddle backed tortoises and their long necks make it ideal for them to reach the higher branches, allowing for easier access to their food.

Although it may seem like the connection between the morphology of the Galapagos Tortoises and the environment they inhabit is coincidental, it demonstrates how environmental conditions influence natural selection. Consider this: a small island containing tall and short vegetation is home to tortoises with both domed and saddle backed shells. Since both types can easily get the resources they need, both can thrive. If one day a drought occurred that wiped out all the tall vegetation, being a domed tortoise with a short neck length would be advantageous since they are still able to feed off the small plants. Overtime, the allele for the domed shape would be selected for, potentially leading to this morphology in the entire population.  

However, if some incidence caused the short vegetation on the island to be depleted, allowing the saddle backed shell to be more advantageous. In either scenario, the adaptation that is most advantageous depends on the environment they are living in.  

Although there are other examples of this type of natural selection throughout the islands, the Galapagos Tortoise is a fan favorite. The fact that they “do not have a natural predator” means that predation is not limiting the shell shape [3]. Rather than being affected by the danger of predators, the resources are the key reason for the struggle to survive. It may seem like shell shape may not be closely tied to predation risk if the tortoises did have any predators on the islands, but a domed shell would make it easier to hide from predators in brush while the saddle backed shell would make it more difficult to find shelter. But due to the lack of predators on the islands, this risk is virtually nonexistent.   

As seen in the example, having a different morphology than the one deemed advantageous within that particular environment can lead to the animal’s demise. Although the environment can cause a shift in which certain morphologies are more advantageous, it is not set in stone; it can easily change over time and favor the opposite morphology.

The Galapagos Tortoises continue munching away at the guavas, mouths comedically covered in the orange innards of the sticky fruit. Another guava drops to the ground, and at a little less than the speed of light, one of the tortoises makes its way to the fallen fruit. It awkwardly snaps at the guava, trying to bite into it to no avail. But luckily it won’t give up that easily. The tortoise follows the guava as it rolls farther away with each failed attempt. Finally, it grabs the stubborn fruit and squishes it with its powerful jaws.

The poor guava never had a chance. After a few more bites, the tortoise also ends up sporting the comedically orange fruit-beard. Whether it doesn’t mind the mess or doesn’t notice it no one may ever know, but the Galapagos Tortoise happily continues to search for another fruit to devour.

Finishing their feast of mouth-watering guava, the Galapagos Tortoises meander over to a small pond for a refreshing sip of water. Will the environment ever change drastically enough that the tortoises’ domed shells and short necks will hinder their survival? Only time will tell, and in the meantime, they live on without a care in the world. Their own little bubble of paradise on these enchanted islands will continue to supply them with tasty and messy fruits for the near future.  

Perhaps the Galapagos Tortoises may think about their next guava adventure, but natural selection waits for no one. Whether it be a hundred or a thousand years into the future, the environment may change in ways that allow different adaptations to be more advantageous than others. Natural selection will always be there to play a key role in the evolutionary changes of species on the Galapagos Islands.   

[1] Caccone, A. (2002). Phylogeography and History of Giant Galápagos Tortoises. Society for the Study of Evolution 56, 2052-2066.

[2] Chiari,Y et al. (2009) Morphometrics Parallel Genetics in a Newly Discovered and Endangered Taxon of Gala´pagos Tortoise. Journal of Morphology 7, 6272.

[3] Chiari, Y & Claude, J. (2011) Study of the carapace shape and growth in two Galápagos tortoise lineages. Journal of Morphology 272, 379-386.

[4] Schafer, S & Krekorian, C. (1983). Agonistic Behavior of the Galapagos Tortoise, Geochelone elephantopus, with Emphasis on Its Relationship to Saddle-Backed Shell Shape. Herpetologica 39, 448-456.