Unlike many migratory species, galapagos giant tortoises do not use current environmental conditions to determine the timing of seasonal migrations.
The Galapagos giant tortoise, sometimes referred to as the gardener of the Galapagos Islands, is a creature with migratory habits. During the cool, dry season, the highlands of the volcanic slopes are engulfed by clouds, which allows vegetation to grow, despite the lack of rainwater. On the lower slopes, however, there is no thick layer of fog and there is no vegetation all year round. As a result, adult tortoises spend the dry season in higher areas and then trek back to lower, relatively warmer areas, where there is ample, nutrient-rich vegetation at the beginning of the rainy season.
To find the best food quality and temperature, they often follow the same migration routes over many years. The timing of this migration is critical to maintaining their energy levels, and climate change could undermine giant turtles' ability to migrate at the right time.
The researchers used GPS to track the time and pattern of migration of giant turtles over the years. The study had three main objectives. One of them is to determine whether giant tortoises will adjust the timing of their migration in light of current environmental conditions. Second, if so, what clues do they use to adjust the time, and third, what is the energy effect of the migration time error on the giant turtle?
The researchers hope that this migration will be synchronized with current food and temperature conditions, as many other migratory species do. Many animals, such as ungulates, can track current environmental conditions and migrate accordingly — what researchers sometimes call surfing in the green waves.
Contrary to the researchers' expectations, however, migration has little to do with current conditions such as fog, rain, and temperature. For example, if the climate is unusually dry, giant tortoises do not seem to take this change into account when deciding on the time of migration. It's unclear whether they made immigration decisions based on memories of past situations or simply misassessed the current local situation.
The team was surprised by this mismatch, and they found that the timing of giant turtle migration fluctuated greatly between years, often over two months. This suggests that tortoises may migrate more than just foraging. For example, female tortoises have to make decisions related to nesting, so there's a lot more to learn about the migration of giant tortoises.
Fortunately, this seemingly not-optimal time to migrate may not yet have had a major impact on the health of giant turtles. Because giant tortoises live up to 100 years and are large, poor migration timing affects them less than smaller animals with shorter lifespans. Giant tortoises can survive a year without eating, while other migratory species must eat more regularly to maintain energy levels.
Giant tortoises are important ecosystem engineers in the Galapagos Islands, responsible for the long-distance propagation of seeds, and their migration is key to the survival of many tree and plant species. How changes in tortoise migration times will affect other parts of the ecosystem remain unclear. Since tortoises do not appear to track annual changes in environmental conditions, it is likely that time errors for future migrations will continue to increase.
But there is a concern that migration at some point in the future may not be the best strategy for giant turtles. The number of individuals who exercise over long distances may decrease. This could have a knock-on effect on the entire ecosystem.
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