New research by the Giant Tortoise Movement Ecology Programme, published in the journal Ecology, shows that Galapagos giant tortoises might be unable to adapt their seasonal migrations in the face of environmental change.
Giant tortoise migrations
Giant tortoises often take the same migration routes over many years in order to find the best food and optimum temperatures. In the cool dry season from June to December, the highlands of the volcano slopes across Galapagos are engulfed in cloud, or the garúa, which allows the vegetation to grow despite the lack of rain. Here the tortoises feed on grasses and small herbs, as there is little vegetation in the lowlands.
Adult tortoises then head back down the slopes, often back to near sea level, for the hot rainy season. This is a journey of 10 km or more that may take up to three weeks to complete in order to find more abundant, thick and nutritious vegetation blooms. The tortoises feed in the lowlands between February and June. At the end of the hot rainy season, the males head back up to the highlands, and the females follow shortly once they have laid their eggs.
It is important to understand how animal behaviour responds to changing environmental conditions to gain insight into the evolution of behaviour and predicting the ability of populations to adapt to an uncertain future. Particularly in this case of how migratory species, such as the Galapagos giant tortoise, will respond to global environmental change.
In Galapagos, climate change is expected to increase the frequency of El Niño and La Niña events, which dramatically impact temperature extremes and the amount of rain and, therefore, vegetation productivity and predictability on a yearly basis.
The GTMEP researchers use GPS tags to track the timings and patterns of tortoise migrations across the Archipelago. In this study, the team tracked 34 adult tortoises on both Isabela and Santa Cruz between 2009 and 2014.
“We had three main goals in the study,” says Guillaume Bastille-Rousseau, lead author on the paper. “One was determining if tortoises adjust their timing of migration to current environmental conditions. Two, if so, what clues do they use to adjust the timing, and three, what are the energetic consequences of migration mis-timing for tortoises?”
The researchers found that there is a weak association between migration and the current weather conditions such as fog, rain and temperature and therefore tortoises are not adjusting their timings based on current conditions. It is not presently clear whether they are basing their migration decisions on memories of past conditions or if they are simply incorrectly assessing the conditions. Male tortoise migrations were more strongly influenced by food availability than those of females. This is probably because the females have the added constraint of nesting which involves decisions based on other environmental factors such as temperature.
The results represent a potentially troubling future for the tortoises, like many species around the world, if they are unable to adjust their migrations in the face of changing climate. If tortoises continue to make decisions based on past averages, they may begin to arrive and depart from their feeding grounds at times which will not provide the best temperatures and vegetation. This could undermine the benefits of migration including their ability to remain healthy.
How you can help
You can find out more about the Giant Tortoise Movement Ecology Programme on our project webpage. There is still a lot to learn about the migration of Galapagos giant tortoises so please help us to continue to fund this important work. Why not buy one of our giant tortoise digital adoptions, donate to our Tortoise Appeal or become a GCT member and help fund the conservation of this species for years to come?
This program is a multi-institutional collaboration among the Charles Darwin Foundation, the Max Planck Institute of Ornithology, Galapagos National Park, the Conservation Medicine Institute of the Saint Louis Zoo, the Houston Zoo and Galapagos Conservation Trust.
You can see the original paper here:
Bastille‐Rousseau, G., Yackulic, C. B., Gibbs, J. P., Frair, J. L., Cabrera, F., and Blake, S. 2019. Migration triggers in a large herbivore: Galápagos giant tortoises navigating resource gradients on volcanoes. Ecology 00( 00):e02658. 10.1002/ecy.2658.