Optical Tracking and Telemetry for Nocturnal Field Studies

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A miniaturized optical telemetry transmitter for small animals was designed, constructed, and field-tested on free-ranging nocturnal crabs. The device produces accurately timed flashes that can be used for individual tagging, tracking, and telemetry. When observed with a night vision scope, the flashes are visible for several hundred meters. Transmitters are inexpensive, relative simple, and rugged. Weight is less than 1 g, and working life is about 4 weeks with 4 g of hearing aid batteries. J. WILDL. MANAGE. 41(2):309-312 Little information exists concerning the behavior and ecology of nocturnal animals, due to the difficulty of observing unrestrained subjects in the dark. The optical device described here was developed to overcome some of the limitations of radiotelemetry for a study of ghost crabs (Ocypode quadrata). Radiotelemetry has been used successfully for long-range tracking (Cochran and Lord 1963), activity monitoring (Knowlton et al. 1968, Osgood and Weigl 1972) and tracing movements of mice (Rawson and Hartline 1964) and voles (Chute et al. 1974). However, the fixes provided by radio, even of animals held in an enclosure under an antenna grid (Chute et al. 1974) are approximate due to the 2-6 m wavelengths used. It is virtually impossible to localize precisely free-ranging animals by radiotriangulation without disturbing them, due to the necessity of taking bearings from several positions at close range. Exact localization and direct observation of nocturnal behavior require seeing in the dark. The image intensifiers (starlight scopes) developed recently make this possible even in faint moonlight. Unfortunately most nocturnal animals are cryptically colored. Starlight scopes cannot create image contras where there is no target contrast; in fact, starlight scopes lose existing chromatic contrast by producing a monochromatic image. I found that ghost crabs, which are hard to see on their natural background by daylight, were impossible to see at night unless moving, even with a good-quality s arlig t cope (Noctron IV, Varo, Inc.). The circuit described here overcomes this difficulty by producing a timed flashing light, visible to the unaided eye at close ranges, and capable of extending the useful range of a starlight scope to distances at which the subject animal would be total y undetectable. In actual use it survived exposure to surf and sand, and permit ed individual recognition and tracking of crabs from distances of up to 300 m in the field, for periods of several weeks. The circuit is quite flexible, and may be modified to telemeter I channel of data.