Daniel M. Mulcahy, PhD, DVM, DACZM
Wildlife telemetry came into common use in the latter half of the last century. The factors that allowed for its increased use were the development of solid-state electronics, allowing for miniaturization of components, and the development of smaller, more powerful batteries, particularly lithium batteries. Initially, the focus was on VHF transmitters, but satellite transmitters were first used on wild animals in the early 1970s.
Transmitters have been attached to a wide variety of wild animals, ranging from insects to whales. Currently, the smallest, commercially available VHF radio transmitter weighs 0.35 g (LB-2N, Holohil Systems Ltd., ON, Canada) and has a transmission lifetime of 12 days. The primary use of VHF transmitters remains the determination of the location of the host animal. However, there is an increasing application of telemetry for measurement of physiologic variables, including heart rate, ECG, body temperature, EKG, electromyelography, and blood pressure. Telemetric measurement of heart rates has particular application in determining metabolic rates and in the study of human disturbance of free-ranging animals. Besides biologic information, environmental parameters such as temperature and pressure can be measured, and recently it has been suggested that marine mammals could function as mobile oceanographic monitoring platforms.
Satellite telemetry was a technologic forward leap which permitted following animals far beyond the range of human tracking. Oceans, mountain ranges, deserts, ice sheets, and rain forest were no longer impediments for animal tracking. Satellite transmitters designed for external mounting now weigh as little as 18–20 g and can be equipped with solar panel rechargers (PTT-100—Pico PTT™, Microwave Telemetry, Inc., Columbia, MD), giving them a lifetime measured in years. Satellite transmitters designed for implantation into an animal weigh 40 g (PTT-100—Implantable™, Microwave Telemetry, Inc., Columbia, MD), and have a lifetime of slightly more than one year (depending on the duty cycle selected). Several manufacturers offer programming that limits transmissions to periods when the satellites are overhead, preserving battery power. Implantable satellite transmitters typically send a record of the body temperature of the host animal, which can be used to determine survival of the host. Recently, a pressure reading has become available from implanted satellite transmitters. The transmission of additional physiologic parameters must await expansion of the message length allowed for satellite transmitters.
Implantable VHF and satellite transmitters have overcome some of the problems encountered using external attachments. This has been especially true for diving birds, where more than 20 species have been implanted with transmitters having a percutaneous antenna. In addition to allowing for placing instruments in some birds that will not tolerate an external transmitter, implantable transmitters permit a higher loading factor, with some transmitter-to-body weight ratios reaching 7%.
Transmitters, especially satellite transmitters, have been made smaller in part by using smaller batteries or eliminating one or more of a bank of batteries. Unfortunately, there is almost no ongoing research on battery technology that will permit either a decrease in the size of a battery without reducing its output, or an increase in the duration of output of an existing battery of small size. Likewise, it is doubtful that the spontaneous depletion of lithium batteries that occurs at elevated temperatures (such as those experienced inside birds) can be further slowed or eliminated. In effect, this means that it is very unlikely that satellite transmitters will be decreased in mass much below 20 g, and that decreases in transmitter mass will come at the expense of the duration of battery life.
Recent and planned launches of additional satellites will extend the coverage of the planets. The new satellites have multiple gates which allow for simultaneous detection of more transmitters. Global Positioning System satellites have permitted the development of new generations of positioning instruments accurate to within 30 m of the actual location.
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