Biotelemetry and Animal Migration Studies

Biotelemetry enables humans to track migrating animals, once the device has been attached, continuously and, non-invasively in the wild (Bridger and Booth 2003; Bowlin et al. 2005; Axelsson et al. 2007). Telemetric devices can be attached externally or they can be implanted, which tends to reduce stress associated with protruding or physically confining tracking devices (Axelsson et al. 2007). Biotelemetry allows remote tracking of migratory movements in animals over a wide range of body sizes. Moreover, transmitters weighing less than 1 g are able to measure physiological parameters such as heart rate, respiration rate, and wing beat frequency in birds (Cooke et al. 2004; Bowlin et al. 2005).

Biotelemetry equipment now exists for invertebrates, fish, birds, and mammals (Figure 1). Transmitters differ in their ability to transmit signals depending on the environment. For studies on land or in freshwater, radio transmitters are used, but ultrasonic devices are often more reliable in saltwater (Cooke et al. 2004). Radio frequencies of radio transmitters are superior to ultrasonic signals in complexity and distance of signal transmissions, but for short-range transmissions, ultrasonic communication is used (Wolcott 1995; Cooke et al. 2004).

The amount of data that can be generated with biotelemetry devices is astounding. Internal variables can be monitored, such as salinity of body fluids and temperature (Lydersen et al. 2002; Cooke et al. 2004); tiny microphones record sound information, which can be used to infer feeding and social activities; and ever-evolving camera equipment provides visual information (Wolcott 1995; Cooke et al. 2004). Furthermore, swimming depth of aquatic organisms can be estimated based on water pressure (Cooke et al. 2004). Sensors can also document abiotic variables in the migrants' external environments, such as ambient temperature and light conditions, and the salinity of aquatic environments (Bridger and Booth 2003; Cooke et al. 2004).

In 2007, Axelsson et al. developed innovative implantable biotelemetry technology, which enabled researchers to record precise measurements of blood flow and blood pressure in animals. This technology was a ground-breaking approach to recording vital physiological measures with minimal disturbance to the animal. Scientists are now able to record cardiovascular processes in a variety of species, which will likely lead to a deeper understanding of the energy cost and cardiovascular strain of migration on organisms.

Biotelemetry Applications in Research

Crustaceans present both special opportunities and challenges when using biotelemetry. Transmitters can easily be fastened to the hard exoskeleton, enabling researchers to document an individual's position and its movements. It is also possible to access the muscle tissue that is located just below the outer shell, and record its activity. Challenges arise when trying to monitor muscular activity in internal tissues. The hard exoskeleton makes it extremely difficult to surgically implant electrodes in the muscles. The implantation of sensors would require a stressful and invasive surgery, which could damage the exoskeleton or the circulatory system. Furthermore, many crustaceans molt, shedding their exoskeleton and thus any transmitters attached to it (Wolcott 1995).

In spite of its limitations, biotelemetry has provided some fascinating information on underwater crustaceans. One study by Wolcott (1980) followed ghost crabs (Ocypode quadrata) underwater, where direct observations are impossible. Biotelemetry radio transmitters enabled the researchers to obtain data on activity budgets, thermoregulation, movements, and heart rate (Wolcott 1980; Wolcott & Hines 1989).

Fish are another example of taxa that are difficult, if not impossible, to track visually. Kaseloo et al. (1992) refined biotelemetric techniques employing electromyograms to measure the strength and duration of muscle contractions in fish. Using such data in conjunction with tail beat frequency, they were able to effectively document the intensity of swimming under various environmental conditions. The researchers also took measurements of oxygen consumption by fish when swimming to obtain an estimate of the metabolic requirements of this movement in their natural environment. More recently, studies have turned to monitoring large-scale movements of sharks. An interesting study by Weng et al. (2005) employed Smart Position Only Tags to follow movement patterns of salmon shark (Lamna ditropis) over many years (Figure 2). The transmitter is relatively small and the tag can be fastened to the sharks' dorsal fins. The transmitter sends signals to an Argos satellite, which subsequently transmits the information, allowing remote tracking of the sharks (Voegeli et al. 2001; Weng et al. 2005). Using the Smart Position Only Tags revealed that their migratory routes often span subarctic to subtropical regions; some individuals have been documented traveling over 18,000 km in less than two years (Weng et al. 2005).

In a study designed to investigate high-altitude bird flight, lightweight transmitters were attached to wild adult barnacle geese (Branta leucopsis) to determine physiological changes during fall migration. Interestingly, the geese preened the satellite transmitters underneath their flight feathers, thereby reducing any drag from the telemetric equipment. The researchers concluded that heart rate of barnacle geese while flying under natural conditions was substantially lower than previous estimates using doubly-labeled water or wind tunnels (Butler et al. 1998).

Leon et al. (2004) investigated the effect of surgically implanted biotelemetry devices on growth patterns and circadian rhythms in rodents. The results indicated that the anaesthetic used to implant the transmitter may initially affect body mass; however, the authors suggested that this effect may be species-specific. In recent years, biotelemetry has provided some intriguing insight into remote tracking of endangered wildlife. If researchers can safely access at-risk species, and attach biotelemetry transmitters, this may allow scientists to collect quantifiable information regarding the physiological status of the organisms and potential clues about environmental threats. In particular, biotelemetry is being used to acquire information on the behaviour, biology, movement patterns, and survival of animals. Biotelemetry techniques have already been employed in conservation efforts for African wild dogs (Lycaon pictus), coyotes (Canis latrans), bobcats (Lynx rufus), the Lower Keys marsh rabbit (Sylvilagus palustris hefneri), the Iberian lynx (Lynx pardinus), and woodland caribou (Rangifer tarandus caribou), among others (Cooke 2008). It is likely that biotelemetry will continue to become more prevalent in conservation studies as the methods and technology become more refined.

Possible Limitations

Most scientists agree that in order to reduce the impact of transmitters on energy budget and behaviour radio transmitter size should not exceed 2-5% of an organism's body mass (Adams et al. 1998; Cooke et al. 2004). In a study investigating the effect of biotelemetry transmitter implantation on behaviour and physiology of juvenile Chinook salmon (Oncorhynchus tshawytscha), researchers found that adverse effects of transmitter attachment may correlate with both transmitter mass and developmental stage of the organism (Adams et al. 1998). This study demonstrated that biotelemetry transmitters weighing from 4 to 10 % of the study animal's body mass were inappropriate, and significantly impaired swimming ability. It also showed differences in how the devices were attached, and suggested that surgically implanted transmitters may be less detrimental to fish than transmitters inserted into the stomach. Presumably, gastrically implanted transmitters may affect eating behaviour, which would in turn inhibit energy intake and affect the ability to swim (Adams et al. 1998).

Transmitter size also impacts the study of insects, which has thus far been restricted to larger individuals. While battery size is often a limiting factory in transmitter size, there is of course an inherent trade-off between mass and battery life; tiny batteries are required to study small organisms, but are likely to be exhausted quickly. Despite this setback, researchers have been able to gather data on a number of species, such as the desert locust (Schistocerca gregaria), which may provide valuable information for parasite and pest control (Cooke et al. 2004).

There are several reasons for loss of contact with a tagged individual. In larger animals, failure of biotelemetry radio transmitters may be caused by battery depletion. Further, it is not uncommon for transmitters to be destroyed when migrants are hunted and harvested. Loss of contact presents a dilemma for biologists because there is no way for a researcher to conclusively determine the source of radio failure without finding the individual animal (White 1983).

Conclusion

Biotelemetry has provided insight into the mesmerizing world of animal migration. In offering opportunities to track organisms remotely in their natural habitat, this technology has already proven that objective, quantifiable data can best be obtained from organisms navigating through their natural environments, with minimal human interference. As technologies improve, it is likely that biotelemetric devices will become smaller and will acquire the ability to extract even more physiological data. In turn, scientists can anticipate expanding this method to studies of smaller organisms and more challenging scenarios.