- Researchers are using a button-shaped device to gather data about desert tortoises (Gopherus agassizii) and their habitats in the Mojave Desert in Southern California.
- Using iButtons, the researchers are trying to understand how hot the tortoises get, and the temperatures that they prefer in the burrows where they spend most of their time.
- Identifying the critically endangered species’ temperature preferences is an urgent task: the tortoise faces threats to its survival from various quarters such as rising temperatures, habitat loss, and attacks by predators.
- With this research, scientists say they hope to find habitats that are safer and where the thermal conditions are suitable for the long-term survival of the tortoises.
A swanky iPhone or a sleek new iPad might sound like a perfect holiday gift for many. Tom Radzio feels the same joy when he retrieves an iButton and downloads data from it.
“It’s like opening a Christmas present,” Radzio, a researcher at the San Diego Zoo Wildlife Alliance, told Mongabay in a video interview.
The device he referred to isn’t a fancy phone or a fitness tracker. Rather, it’s a button-shaped device filled with sensors that have helped Radzio and his team study desert tortoises and their habitats. Since March this year, they’ve attached iButtons to 95 juvenile and 20 adult Mojave Desert tortoises (Gopherus agassizii), a critically endangered species native to the Mojave Desert in Southern California. The goal: find out how hot the animals are getting and understand the temperatures they’ve been living through. Each time he retrieves an iButton from a tortoise, Radzio said, he ends up with a treasure trove of information.
“You get to see what the animals have been doing,” he said. “Did it come out of its burrow on a particular day? What kind of temperatures have they experienced?”
Getting answers to these questions is critical. Identifying the temperature and humidity at which tortoises are moving helps scientists determine the characteristics of their preferred habitats, and aids in their understanding of how global warming is impacting tortoises and the ecosystems they live in.
Mojave tortoises face an onslaught of threats to their survival. Extreme drought, rampant habitat loss spurred by urbanization, increasing attacks by predators, and getting hit by cars are all factors that have played a role in the steady decline of their population. Given their slow reproduction rate, and the long time it takes for the animals to reach maturity, conservationists say it’s essential to gather more data on their habitats that can inform urgently needed conservation measures. The tortoises and their burrows also play a larger role in their ecosystem.
“Their burrows provide refugium for many species, now documented over 100,” Radzio said. “Some of the species actually require tortoise burrows for part of their life cycle because the burrow provides a stable environment that is not as extreme as at the surface.”
However, accessing data on where the tortoises are isn’t always easy. Desert tortoises spend most of their time in burrows or under rocks, in order to regulate their body temperature and minimize water loss. Since they live near the ground, the temperatures they experience are typically higher than the readings recorded by weather stations.
It was to overcome this problem that Radzio started using iButtons — a device that has long been used for medical purposes and in the food-shipping industry. “You can program it to record temperature at preselected intervals, and some iButtons also record humidity,” Radzio said. “We get a long time series of data on temperature and humidity in their environment — data which we cannot collect by hand at the same level of frequency and spatial extent.”
So far, Radzio and his team have used the devices to sample the environment available to tortoises during two important, and vulnerable, life stages. One to understand how juvenile tortoises are choosing habitats that help them conserve energy and water during hot and dry summer months. And second, to figure out if there’s a pattern behind how the animals select their habitats for nesting.
“In the egg stage, temperature has really strong effects on development, sex ratio, size, and other attributes that can affect the animal’s ability to persist,” Radzio said.
Since the research is still ongoing, Radzio said concrete patterns are yet to emerge from the preliminary data collected so far. Nonetheless, he said, there are certain trends that have caught his attention.
For example, Radzio’s team lined 21 burrows with iButtons every 10 centimeters (4 inches). They found that the temperatures varied rapidly from one location in the burrow to another. “During spring, the temperature can be 43° Celsius [109.4° Fahrenheit] at the entrance in the middle of day, but deep inside, it’s somewhere close to 26°C [78.8°F],” Radzio said.
Moreover, the team found that well-hydrated tortoises often pick spots in the burrow where the temperature hovers around 32°C (89.6°F). “The predictability with which the tortoises selected temperatures, it was almost like clockwork. It likely tells us something about their habitat needs,” Radzio said.
One of the big questions, Radzio said, is to find out how juveniles survive summers on limited energy and water reserves. Initial observations from his experiment suggest that they dig deeper, cooler burrows when temperatures increase, and often emerge from the burrows at night to experience cooler surface temperatures. “By simultaneously recording temperature and humidity experienced by tortoises during physiologically challenging summer months, iButtons can provide valuable insights into juvenile tortoise habitat requirements and potential responses to climate change,” he said.
Radzio said these preliminary observations will be further validated as the team continues its research. Additionally, they’re collaborating with researchers at the University of North Carolina at Chapel Hill to study the relationship between the thermal profiles of habitats and the animals’ physiology.
“We will run controlled physiological experiments in the lab to measure metabolic rate and water loss rates in tortoises under a wide range of environmental conditions,” Eric Riddell, an assistant professor in the biology department at UNC–Chapel Hill, told Mongabay in an email interview. “We will then incorporate this data into models which will simulate how organisms interact with the environment across realistic landscapes.”
The final goal, he said, is to be able to get the model to predict when and where desert tortoises may experience conditions that threaten their survival. As the impacts of climate change worsen over the years, Riddell said he hopes this information will be used for conservation efforts to breed or translocate the animals to places that are predicted to be more suitable for them.
“Physiology gives us a direct window into the conditions necessary for survival,” he said. “By coupling physiology with these complex simulations, we can give these animals the greatest chance of survival on our warming planet.”
Banner image: A Mojave Desert tortoise with an iButton attached to it. Image courtesy of San Diego Zoo Wildlife Alliance.