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.
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.
PIT tag (disk) on tortoise marginal.
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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