Animals try to avoid the hottest and coldest parts of the day and opt to lie low instead. While this seems like a good strategy to avoid heat, it does cost them time when they could be out finding food or a mate. As climate change continues to push environmental temperatures to extreme highs and lows, it's important to understand how much time species will lose sheltering to avoid those extreme temperatures. To quantify how the most drastic prediction of climate change could affect an already vulnerable species, Rafael Lara-Reséndiz from the Universidad Nacional de Córdoba and Centro de Investigaciones Biológicas del Noroeste, Argentina, along with his collaborators, investigated how activity time was affected by the temperatures that Goode's thornscrub tortoises (Gopherus evgoodei) can withstand in the tropical dry forests of Northwest Mexico.
To measure the temperatures individual tortoises were actually experiencing on the forest floor, the researchers built hollow copper models of the tortoises with thermometers inside. They then painted the models leaf-green like the tortoises, so the models would absorb the same amount of heat as the animals. They then placed six of these models throughout the landscape in places where tortoises had previously been seen, to record the temperatures that the tortoises could reach every 30 min for 5 years, providing the scientists with a rich set of temperatures experienced naturally by the animals. By comparing the range of environmental temperatures the model tortoises experienced over the five year period with the temperatures when tortoises are known to be active, the researchers could predict how the amount of time the animals can be out and about could change in the future as temperatures rise.
The researchers found that the time when tortoises were able to be active varied across the months and from year to year, which matches the changes during the seasons in the forest; when the trees lost their leaves during the dry season, the conditions the tortoises experienced also became warmer and dryer. After summer rains, the conditions on the forest floor became more humid and cooler, which in turn gave the tortoises more time at comfortable temperatures. The researchers also found that the tortoises were active during two periods of the day, morning and evenings, and would overheat if active during the hot afternoons. This pattern is likely due to the tree canopy shading the ground beneath where the tortoises live but only during the summer months when the trees still have their leaves.
Additionally, the scientists found that the temperatures predicted by climate change models are likely to increase the amount of daytime when temperatures are too hot, limiting the overall time when tortoises can be active. Tortoises need a minimum of 3.85 h per day to forage sufficiently and mate; if a region is too inhospitable for an animal to be sufficiently active, then it is unlikely that these creatures will be able to make it their home. Given the temperature changes that are predicted to strike this region, the researchers suspect that only a few populations would survive to 2070 under the most pessimistic scenarios.
Although this study predicts tortoises will lose time for foraging and mating under alarming climate change scenarios, the researchers also suggest that tortoises may begin foraging earlier in the morning or later in the evenings to make up for lost time during hot afternoons or move to areas more protected from the heat. To learn more about how rainfall and variation in temperature affect other aspects of tortoise activity times, the researchers are conducting additional studies to inform conservation management.
