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Critter Crossings: Linking Habitats and Reducing Roadkill

Tortoise Underpasses

[Photo of a Tortoise - Desert tortoises are found in the Mojave and Sohoran deserts of Arizona, California, Nevada, and Utah. The Mojave population is state - and federally listed as threatened. The desert tortoise is a large, terrestrial turtle with a light brown to very dark brown shell and thick, stumpy hind legs. Desert tortoises live a long time (50-70 years), mature late, and reproduce few offspring. They eat mostly native plants, including cacti. During inactive periods, they hibernate in underground burrows or caves.]

Imagine several hundred desert tortoises wearing name tags.

That was the scene a few years ago on Mojave Desert land along State Highway 58 in San Bernardino County, California. Locating and tagging the tortoises was the first step in a multi-agency project to monitor the tortoises' use of new storm-drain culverts spanning the width of the highway. Eight federal and state partners were involved-the California Department of Transportation, the Bureau of Land Management, the California Energy Commission, the U.S. Fish and Wildlife Service, the Federal Highway Administration, the Nevada Department of Transportation, the California Department of Fish and Game, and the U.S. Geological Survey.

Since desert tortoises move rather slowly and within a limited home range, finding them and attaching IDs to their shells was relatively easy. Developing a reliable system for tracking their passage into and out of culverts proved more difficult.

"We needed an automated system that could be left unattended from March through October, when the tortoises were not hibernating," says William Boarman, USGS biologist and project manager. "We also wanted it to be secure from vandalism and environmental damage."

The research team's solution: Passive Integrated Transponder (PIT) tags and a computerized reading system. The PIT-tag system, used primarily to census fish, was adapted by AVID, Inc. and Beigel Technology Corporation to meet the criteria of the desert tortoise project. Easy maintenance was achieved by using a renewable energy source-solar panels and a solar-rechargeable battery. Protection from theft and environmental damage (for example, high summer temperatures) was accomplished by burying the system's receiver and antennae underground and camouflaging them with desert soil. (Protecting the solar panels, which could not be hidden and which were frequently stolen, required more complicated measures).

Here's how the PIT-tag system worked: When a tortoise entered or left a culvert, it passed through an electromagnetic field emitted from a "reader" tuned to a specific radio frequency. The reader detected and decoded the tortoise's ID (programmed into the memory of the system's microchip), and a datalogger inside the reader recorded it along with the date and time of passage.

Throughout the research project, team members paid attention to the smallest details. They attached the PIT tags to the tortoises' shells with an Epoxy glue that would not hurt the shells...They programmed the reading system to turn itself off at night (desert tortoises are active only during the day)...They consulted a world-class surfboard designer to come up with a well-insulated "house" for the reader coil...They buried each reader in locked "nesting boxes"...They designed the reading coil to cover an 8-foot (2.4 m) area, overcoming the short-distance limitations of PIT-tag technology.

[Photo of a Tortoise Underpass - To record the movement of desert tortoises through this underpass, researchers adapted a computerized tracking system that until then had been used mostly to census fish.]

The system recorded five tortoises using the culverts on 75 occasions in 1995 and 1996. Since the small vertebrates have no identifiable "corridor" and move at a leisurely pace (it took one tortoise seven hours-stopping and starting--to get through a culvert), these numbers are likely to increase as the tortoises gradually discover the culverts.

The solar-powered PIT-tag system used on the Highway 58 project can be used on almost any narrow wildlife crossing, according to Michael Beigel of Beigel Technology. Says Beigel: "If you need more power, you can make simple adaptations--bigger batteries, additional solar panels, or moving solar panels."

The system is also easily adaptable to other species. "It can be used for virtually every animal you can catch," says Boarman.

For more information contact William Boarman at 858-637-6880 or

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Updated: 7/18/2012
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