Indirect observations

Many behaviors can be indirectly inferred from sign indexes and density estimates. Habitat selection is often measured through indexes that compare an animal's use between habitats or seasons. When indexes are used as a surrogate for direct observations, the closer the index is to the target behavior the less chance of error. For example, a browse index based on the number of buds clipped per tree is directly re lated to ungulate feeding, while pellet or track counts are more indirect indexes.

Radio telemetry, a means of indirect observation, is the most important advance in the last 50 years for the study of wild mammals. Before radio telemetry, researchers were limited by their ability to follow animals or detect their presence. Indirect indexes of activity and movement, such as sign counts, were the best means to measure habitat selection. Behavioral observations were often limited to sites near feeding or watering holes where animals could be observed from established blinds; when an animal disappeared it was often impossible to know if it had died, dispersed, or merely stopped coming to the observation site. Radio telemetry allows a researcher to locate a specific animal when it needs to be recaptured, observed, or its movements monitored. Radio telemetry allows a researcher to remotely track a specific animal's movements and survival with a minimal disturbance to its behavior. These abilities have opened observations into animal ecology that were unavailable to earlier workers.

Two components of radio telemetry are the receiver and transmitter. The receiver can pick up a range of frequencies and can be set to detect each unique transmitter. Traditionally, the animal carries the transmitter and the receiver is either a hand-held device or an orbiting satellite. The hand-held receiver can be moved on foot, by car, boat, or plane. To locate the animal, it can be approached directly, triangulated from a number of bearings from known locations, or by using the principles of the Doppler effect in the case of the satellite. More recent global positioning system (GPS) collars have the animal fitted with the receiver and the transmitters are aboard orbiting satellites. The receiver calculates its position based on the known position of the satellites and the time it takes a signal to travel between the satellite and receiver. The optimal arrangement is a package that contains a combination of units, so the animal's position can be determined through multiple means.

Traditional tradeoffs in radio telemetry are between power output and battery weight. Attempts to increase the range or duration of the signal are matched by increases in unit weight. Combining types of radio telemetry units into one collar also increases package weight. A general rule is that a package weight's should be less than 5% of the animal's body weight, but there are enough exceptions to this rule to warrant a pilot study before attaching packages to wild animals. Advances in battery technology and computer software have made weight considerations less important, especially for larger mammals. When the battery power cannot be sustained for the length of the project, microchips can regulate when the unit is active. Weight limitations are still serious concerns for species weighing less than 2.2 lb (1 kg).

A limitation of radio telemetry is that the observer must capture the animal to attach the telemetry unit. Most animals can be captured, but the time and labor involved can consume a large part of a project's budget. Once a telemetry unit is attached, the logistics of recovery are simpler. The telemetry unit can lead to the animal for application of anesthesia, or a remotely triggered tranquilizer dart can be inserted in the collar. When the study has terminated, the unit can be released,

A game warden collects fluids from a dead elephant bull to determine its cause of death. (Photo by Rudi van Aarde. Reproduced by permission.)

(i.e., arboreal, volant, terrestrial, subterranean, aquatic), and its behavior (i.e., cryptic, nocturnal, social, vocal). General guidelines can be given to the limitations imposed by each type of mammal.

0 0

Post a comment