TECHNICAL PAPER  




Abstract

We describe an automated system that uses passive integrated transponder (PIT) tags to track movements of animals past specific locations. The system was designed to operate maintenance free for several months, be secure from vandalism and environmental damage, and record the identity, date, and time of passage of animals past a 2.4-m wide area. We used the system to monitor effectively the movements of 172 desert tortoises (Gopherus agassizii) through 2 storm drain culverts that pass beneath a state highway in the Mojave Desert, California. Four tortoises entered or passed through the culverts on 60 occasions. The system can be easily adapted to other species.


Key words: desert tortoise, marking, movements, PIT tags, RF-ID, technology, Testudiniclae, tracking


Knowledge of individual movements is fundamental to understanding behavioral ecology of species. We describe an automated system for tracking movements of individuals past specific locations, such as burrows, caves, and water holes. The system, developed primarily by AVID, Inc. and Beigel Technology Corp. (Norco, Calif.), used passive integrated transponder (PIT) tags. Passive integrated transponder tags are used to mark and identify individual animals (Camper and Dixon 1988, Prentice et al. 1990a). Passive integrated transponder tags have been used to census fish passing through specially equipped pipes (Prentice et al. 1990b). However, PIT technology has not been used widely to track movements of terrestrial animals. We refer to trade names to provide examples of equipment that works for our application. Use of trade names does not imply endorsement by the federal government.


Passive integrated transponder technology

Passive integrated transponder tags use radio frequency identification (RFID) technology (Ames 1990, Prentice et al. 1990a) and consist of 4 components: a microchip, an antenna, a chip-capacitor, and a housing, typically made of biocompatible glass. Glass tags are usually 2.1-3.85 mm in diameter and 11-32.5 min in length. Tags also may be configured into a disk shape, typically 18 min in: diameter and 2 mm. thick.


PIT tag (disk) on tortoise marginal.
Passive integrated transponder tags lack an internal power source. They are energized on encountering an electromagnetic (E-M) field emitted from a transceiver (reader) tuned to a specific frequency (usually 125, 134.2, or 400 KHz). The tag derives its power and timing signal from the reader's field. The tag's unique identity code (ID) is programmed into the microchip's nonvolatile memory. When energized, the PIT tag emits its ID by modulating the reader's E-M field. The reader detects and decodes the modulations and, thereby, reconstructs the tag's ID. To ensure accuracy, an error detection code is transmitted as part of the number sequence.

Most RF-ID readers used for animal studies are handheld units intended for manually scanning tagged animals. For unattended, fixed-point monitoring systems, the geometry of a reading coil must be appropriate to the application, the specific type of tag, velocity of the tag-bearing animal, and tag orientation and distance. The interaction between the reader's E-M field and the receiving-transmitting pattern of the tag coil determines the effective distance for a particular orientation of the tag to the fixed-point reader. The design challenge is to maximize the probability of reading tagged animals moving past the coil at random orientations and velocities. Each application requires consideration of animal morphology, method of locomotion, power consumption, and environmental characteristics.


Automated reading system (ARS) for desert tortoises

We used this system to study the effectiveness of a barrier fence for reducing mortality in the desert tortoise (Gopherus agassizii), a state- and federally listed threatened species, on highways in the western Mojave Desert, California (Boatman and Sazaki 1994). We predicted desert tortoises would use storm-drain culverts to cross beneath highways when barrier fences were erected to keep animals from crossing directly. We focused on movements through 2 1.6-m diameter x 66-m long, round, corrugated-metal culverts. Because we expected few tortoises to use the new culverts for the first several years of the project, we required a tracking system that would remain operational when unattended for long periods, require minimal maintenance, and have a low per capita cost over several years of the study. Because we were investigating tortoise movements past specific points, we could use a short-range detection system, such as PIT tags. However, we first had to overcome several automated reading system (ARS) design constraints to our application.

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