Team tracks tortoise habitat at YPG
With November here, the Yuma Proving Ground’s population of Sonoran Desert Tortoises are preparing for brumation, the reptilian equivalent to hibernation.
Humans responsible for their stewardship, however, are celebrating a year of discovery about the desert creatures.
“We learned more this season about tortoises in this region than has ever been known,” said Daniel Steward, YPG wildlife biologist.
To facilitate YPG’s important mission while at the same time conserving the proving ground’s wildlife population, wildlife biologists have actively sought to determine where populations of desert tortoises live, searching for the creatures in plots of land most likely to have them present. Steward says that, unlike the Mojave Tortoise, which isn’t found at YPG, Sonoran Tortoises prefer rocky areas with lots of shelter sites.
“Most of the tortoise activity is up on the mountains where YPG conducts less activity,” said Steward. “That reduces a great deal of conflict with our mission because most activities are down in the flats.”
Sonoran Desert Tortoises spend most of their lives in underground burrows. They can survive for more than one year without water, getting most of their liquids from the plants they eat. They are most active in the periods that immediately follow monsoons.
“Tortoises have a slow metabolism and are well adapted to this environment, so they only have to drink a few times a year,” said Steward. “They can store water in their bladder. One of the risks of people handling tortoises is that they will urinate, and when they urinate they are giving up vital water resources.”
To track the tortoises and study their habits, biologists attach small VHF transmitters and GPS data loggers to the shell of each tortoise they find. They also paint a unique number on the tortoise’s shell, and file a small notch through one of the keratin scutes at the thin rear edge of the shell, which has a consistency similar to a human fingernail. All this is done after an examination of the tortoise’s health and weight.
Coaxing one of the creatures out of their shelters can be a challenge: if they feel threatened, they oftentimes wedge themselves against the rear wall and ceiling of their miniature caves, which can be yards deep.
“They’re shockingly strong,” said Hillary Hoffman, a herpetologist with the Arizona Game and Fish Department who has been coming to YPG since 2009. “If they don’t want to come out, they’re not coming out.”
Once the data loggers are attached, biologists seek the tortoises out with an antenna to track the transmitters. To reduce the weight of each data logger to avoid hurting the tortoise, batteries must be changed on a monthly basis, at which time the biologists retrieve stored data.
“That data can then be used to look at how far these tortoises travel from their shelter sites,” said Steward. “Plus, a lot of times when you are following a tortoise in a good area with lots of shelter sites, it will guide you to other tortoises.”
So far this season, the biologists have found 20 tortoises, a dramatic and unexpected increase over the two they found in the last study three years ago. Steward thinks the selection of study plots helped the effort.
“It’s always been thought that overall population densities out here were low,” said Hoffman. “Perhaps there are pockets of high density, but range-wide the habitat is just not appropriate. It’s a dry, dry, dry part of the state.”
There have been persistent rumors in recent years that the Sonoran Tortoise could be added to the federal government’s Endangered Species List. According to Steward, this possibility should not interfere with YPG’s longstanding mission if it occurred.
“The Endangered Species Act is a process-driven law,” said Steward. “It wouldn’t affect what we do; it would affect our planning process. In any wildlife conservation activity, first you want to avoid impact, then minimize impact, lastly you mitigate impact. This research gives us the information we need to be able to assess future impacts on this species and allows us to better support YPG’s mission while ensuring tortoise conservation.
With YPG’s efforts to conserve tortoise and the interagency cooperation, Steward is hopeful the tortoise will not be federally listed. In the meantime, the search and tracking effort continues.
“Everything is bigger out West – Yuma Proving Ground is a monstrous range that is bigger than the state of Rhode Island,” said Steward with a smile. “We have a crew of people trying to determine the population, size and location of a small, subterranean reptile. That’s a pretty big feat.”