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Tortoise Tales: Seychelles vs. Galapagos
15 October 2024 | Seychelles & Tanzania, Wildlife
A ‘keystone species’ is a term used to refer to organisms so incredibly vital to their ecosystems that their absence could result in the major alteration of their habitat, or even an ecological collapse. The giant tortoise is an example of such a species. Through their everyday behaviors, these ‘eco-engineers’ create pathways and nest sites for other animals, disperse seeds, and control vegetation growth – literally paving the way for other lifeforms to thrive.
We first encountered giant tortoises on our expeditions to the Galapagos Islands on board Aqua Mare. These iconic reptiles played a significant role in helping Charles Darwin ideate his theory of evolution. Now, with our recently announced voyages to the Seychelles and the Tanzanian Islands, you might have noticed their familiar faces pop up once again in our teaser video. However, is it truly the same giant tortoises inhabiting remote islands in both the Pacific and Indian Ocean?
Let’s dive into the similarities and differences between the Galapagos tortoises and their Seychellois cousins.
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A Brief Overview
Despite looking almost identical, the giant tortoises of the Galapagos and the Seychelles aren’t actually the same species. Scientifically speaking, the Galapagos giant tortoise is named Chelonoidis niger, while the tortoises found in the Seychelles are classified as Aldabrachelys gigantea. This denotes that the two species have followed different branches of evolution and their lineages can be traced to different regions. The ancestors of the Seychelles tortoise, for example, were originally from Madagascar, while the giant tortoises of the Galapagos came from nearby Ecuador.
But how come they look so alike then? This is explained through a concept known as convergent evolution. It’s a phenomenon whereby two different species develop the same traits independently, perhaps due to similar living environments. For instance, the most prominent trait shared by both Galapagos and Seychelles giant tortoises is their size. It’s theorized that this is a result of their evolution in predator-free island habitats.
Key Differences
When looking to differentiate between the two tortoises, the most noticeable distinction is the ‘cervical scute’, an extra plate on the front of the tortoise's shell, directly behind their necks. Most giant tortoises from the Seychelles have a cervical scute, while Galapagos giant tortoises do not. Head shape is another point of difference: Galapagos tortoises sport a larger, broader head in comparison to the Seychelles tortoises’ narrower skulls.
Next, the shells – for context, tortoise shells commonly feature either a smooth dome or a saddle-shape. This latter supposedly allows the tortoise to stretch their necks higher, allowing them to assert dominance and fight off other males during mating season. Among Galapagos tortoises, the variation of their shells’ shapes is much more pronounced compared to their Seychellois counterparts.
Seychelles tortoises possess a consistently domed and smoother shell shape. This corresponds with the theory that since ritual combat between male tortoises is absent in the Seychelles, they have developed lesser need for neck mobility compared to their counterparts in the Galapagos islands.
Finally, while the size of the Galapagos tortoises varies between islands, on average they are still slightly bigger than the Seychelles tortoises. Galapagos tortoises can weigh a whopping 600 to 700 pounds and reach an average of 6.1 feet in length, while Seychelle tortoises weigh an average of 550 pounds and only reach up to 4 feet in length.
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Enduring Similarities
Aside from these differences, the Galapagos and Seychelles tortoises share a remarkable amount of similarities, perhaps owing to their also similar journeys. Despite once being widespread all over the world, with fossil remains of giant tortoises being found in the Americas, Africa and Eurasia, these Giant Tortoises are now only found in remote islands.
Now, you might be thinking, how did these seemingly lethargic creatures make its way to colonize islands literally oceans away? In 1923, naturalist William Beebe wondered exactly that, and set out to test their seaworthiness by unceremoniously chucking one out of his yacht. To his surprise, the tortoise was incredibly buoyant and lifted its head up as it was carried away by the waves like driftwood. This inherent talent shared by both species of giant turtles, along with the fact that they could survive for weeks without needing food or water, made them an unlikely bunch of seafaring explorers, even if they needed an incredulous amount of luck to make landfall. The theory that suggests this was how these giant tortoises reached sealocked islands was all but confirmed when in 2004, an Aldabran giant tortoise washed ashore in Tanzania, over 740 km away, with evidence of it having spent weeks out in the open ocean.
Speaking of resilience, the Seychelles and Galapagos giant tortoises are the longest living terrestrial animals in the world, with a natural lifespan of 100 years. Notably, Adwaita, an Aldabran giant tortoise, was presumed to have lived up to 255 before passing away in 2006, while the Seychelles giant named Jonathan is still alive today at the ripe old age of 191. Representing Galapagos, the giant turtle Harriet is 4th in the list and lived up to 175.
Conservation Efforts
While their longevity and ability to survive originally allowed them to disperse worldwide, it also became one of the reasons for their downfall, as defenseless giant tortoises were rounded up as a food source during long voyages. Thus, it is here that the tale of the Galapagos and Seychelles giant tortoises also intersect. Being a keystone species, their preservation has become a matter of great importance to the Seychelles and Galapagos alike.
Both regions have established dedicated programs to protect their tortoise populations. In the Galapagos, the Charles Darwin Foundation is instrumental in the conservation of giant tortoises, focusing on breeding programs, habitat restoration and environmental education. Similarly in the Seychelles, initiatives have been established to monitor tortoise populations and mitigate the threat of invasive species. Thankfully, these have been impactful, as two giant tortoise subspecies thought to have gone extinct have been recently rediscovered in the Seychelles, while populations are slowly recovering over in the Galapagos.
Click here or contact our Expedition Consultants to find out more.