The large, dominant male Galapagos giant tortoises start their annual migration at the beginning of the dry season, slow and steady, from the island lowlands up across the volcanic slopes. Scientists used GPS technology and modern 3D acceleration measurements to find out how fast and how far the tortoises traveled.
Although giant tortoises are able to survive for up to one year without nourishment, they nevertheless wander for large distances searching for food. But only the fully grown animals migrate- up to 10 kilometers, the researchers say – while the young tortoises stay year round in the lowlands. The reason for this and the question of why the animals don’t rest during the dry season are not known yet.
In the cool dry season, the highlands of Santa Cruz are engulfed in fog which allows the vegetation to grow despite the lack of rain. But in the lowlands there’s no thick layer of clouds and the tortoises’ vegetation is not available year round.
Adults, which can weigh up to 250 kilograms, spend the dry season in the higher regions at an elevation of 400 meters above sea level. However, since the food is not as nutritious there, they trek back to the lower zones where there is succulent vegetation in abundance as soon as the rainy season begins.
In order to study the migratory pattern more closely, Stephen Blake from the Max Planck Institute for Ornithology and his colleague Washington Tapia from the Galapagos National Park secured GPS loggers with 3D acceleration monitors onto 17 adult tortoises. This allowed the scientists to determine the animals’ exact position and behavior over a period of two years. In order to gather information on the entire population, the researchers noted the size, sex and location of each tortoise they met on their monthly hikes along the volcanic hillsides. They combined the GPS data with the temperature data and information about availability of vegetation.
Adult females remain in the lowlands until they lay their eggs and then they also make their way to the highlands. In contrast, the smaller tortoises stay in the lower elevated areas all year round. The question of why the younger animals don’t migrate hasn’t been answered by the scientists yet. Stephan Blake said:
Either the energy expenditure of this strenuous hike is too high, or there is still enough food available for the smaller animals. Perhaps the younger animals can’t tolerate the wet cold climate of the higher regions.
In other species, the largest and the most dominant individual does not migrate because it can best defend itself against its competitors. It doesn’t have to leave to survive. However, among the Galapagos tortoises, it’s usually the largest and most dominant individual which takes on this arduous journey.
Bottom line: Scientists used GPS technology and modern 3D acceleration measurements to find out how fast and how far male Galapagos giant tortoises travled during their annual migration.