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Wildlife-radio telemetry is the process of using radio signals sent from a device on an animal to a receiver to track said animal's movements and to carry data about an animal's body condition and environment (Mech and Barber, 2002; Fuller and Fuller, 2012). Radio-tracking can also be used to estimate population abundance, fecundity, survival, and causes of mortality (Fuller and Fuller, 2012). Wildlife-radio telemetry is especially useful for gathering data on fast-moving, secretive, and/or wide-ranging animals, or to find animals for in-person observation (Fuller and Fuller, 2012). While wildlife-radio telemetry can be a great way to gather data about a population, radio-tracking can also cause a host of behavioral or physiological stress (Fuller and Fuller, 2012). Radio-tagging has been shown to increase mortality or reduce reproduction due to improper packaging and attachment of the radio-tag (Fuller and Fuller, 2012). Proper care must be taken to ensure as little of an impact on the individual as possible during studies utilizing wildlife-radio telemetry.

There are 3 main types of radio-tracking: VHF radio tracking, satellite tracking, and GPS tracking (Mech and Barber, 2002). The advantages and disadvantages of each type of wildlife radio telemetry are provided in each description. Telemetry bias is also discussed, and an example of using wildlife-radio telemetry to track feral hogs is provided.

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Very high-frequency radio tracking is the oldest method of radio telemetry and has been in use since the late-1950s according to Dahlgren, Kohl, and Messmer (2012). VHF radio waves are transmitted from a radio-collar worn by the animal at a predetermined rate (Dahlgren et al., 2018) and received with a handheld antenna (Farve, 2012). VHF radio tracking can also detect mortalities when motion sensors are incorporated into the radio tag/collar (Dahlgren et al., 2018). This method of radio-tracking requires the acquisition of the signal from three or more locations in order to triangulate the location of the animal wearing the device (Farve, 2012). The advantages of this method of radio-tracking is that it is relatively inexpensive, has a long life span, and accurate enough for most studies (Mech and Barber, 2002). A disadvantage of VHF radio tracking is that it requires a researcher to be in the field, which can be labor-intensive and difficult in harsh conditions (Mech and Barber, 2002).

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US Fish and Wildlife service employee tracking tagged mountain lions by radio

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US Fish and Wildlife service employee tracking tagged mountain lions by radio

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Carmen, Luna (2013)

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Satellite tracking utilizes the two Argos satellites orbiting around the Earth's poles to receive Ultra-High Frequency signals from Platform Transmitter Terminals (Farve, 2012). While most PTTs are located on fixed land locations, buoys, or ships to transmit oceanographic/meteorological data, the miniaturization of PTTs in the 1990s allowed them to be attached to animals (Farve, 2012). These satellites use Doppler measurements of UHF signals to calculate PTT location and receive sensor data (Fuller and Fuller, 2012). Satellite tracking is advantageous when animal location, gross movement, or long-range movement are needed as this method does not require researchers to be in the field aside from placing the PTT on the animal (Mech and Barber, 2002). PTTs are able to transmit data from 8 or fewer sensors that can detect measurements of temperature, motion, pressure voltage, and more (Fuller and Fuller, 2012). Satellite tracking's main disadvantages are its higher initial cost, shorter lifespan, and lower accuracy compared to VHF tracking (Mech and Barber, 2002). Satellite tracking is also only useful in locations greater than 60 degrees in latitude due to the polar orbit of the Argos satellites (Farve, 2012).

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Mojica, Kynor Elizabeth (2008)

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Unlike VHF and Satellite tracking, GPS tracking uses a GPS receiver that receives signals instead of transmitting them (Mech and Barber, 2002; Farve, 2012). Using three satellites a GPS receiver can be located in two dimensions and with four satellites a GPS receiver can be located in three dimensions, with a location accuracy of 30 meters (Farve, 2012; Fuller and Fuller, 2012). The location of the GPS receiver can be stored on the GPS receiver itself which must be retrieved from the animal, sent to a handheld device within a certain distance from the receiver, or beamed to the Argos satellites using a combination of the GPS and Satellite tracking techniques (Mech and Barber, 2002; Farve, 2012). GPS/Satellite tracking can allow researchers to collect and access fine-scale data worldwide on a set schedule (Farve, 2012). GPS tracking can also allow biologists to collect more locations over time compared to VHF and satellite tracking (Dahlgren et al., 2018). Like satellite tracking, GPS tracking is more expensive and has a shorter lifespan than VHF tracking due to its high power requirements and requires a relatively large animal as the GPS receiver is usually fairly large (Mech and Barber, 2002; Fuller and Fuller, 2012).

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Campbell et al., 2013

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One common weakness between VHF, satellite, and GPS tracking is the occurrence of "telemetry bias". Telemetry bias" occurs when researchers put too much stock in the radio-marked individual's habits (Dahlgren et al., 2018). A common error in radio-tracked studies is to assign areas where the tracked animal frequented as important and unused areas as less important (Dahlgren et al., 2018). The unused areas may still be important to the rest of the population of the animal species/group and were simply unused by the tagged animal. Dahlgren et al. (2018) describe a study they conducted radio-tracking sage-grouse on Parker Mountain. According to the researchers' radio-tracking data, sage-grouse did not enter or quickly passed through experimental treatment plots. However, when the researchers examined the treatment areas conducting pellet counts and bird dog flush counts they found lots of unmarked grouse within the treatment plots. The researchers state that if they had only relied on their telemetry data to report their results they would have come to erroneous conclusions, indicating the importance of not relying on only radio-tagging to analyze a species. Dahlgren et al. (2018) suggest as many individuals within a population as possible be radio-tagged to represent more of the population and computer generated habitat selection models should be used to apply the collected telemetry data to the entire landscape where the species is found.

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Feral hogs are an invasive species reported in at least 35 states (History of Feral Swine, 2020). Feral hogs are the product of escaped food pigs introduced to the US in the 1500s mating with Eurasian boar introduced in the 1900s (History of Feral Swine, 2020). Thanks to their intelligence, lack of predators, ability to reproduce quickly and adapt to most habitats, and omnivorous diet, these destructive disease vectors have quickly spread (Feral Hogs, 2018). These hogs threaten native flora and fauna, change the chemistry and biology of the soil, and damage streams and wetlands (Feral Hogs, 2018).

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In Congaree National Park, feral hogs have become an increasing problem (Feral Hogs, 2018). One of the methods utilized to track feral hog movements was VHF radio telemetry (Feral Hogs, 2018). 23 feral hog were trapped, tranquilized, ear-tagged, and fitted with radio collars before being released (Feral Hogs, 2018). These hogs were then tracked over two years to determine the home range, habitat preferences, and factors affecting hog mortality (Feral Hogs, 2018).

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A tranqualized feral hog after being ear-tagged and collared

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A tranqualized feral hog after being ear-tagged and collared

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NPS photo

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According to the study, the home range of the feral hogs in Congaree National Park was found to be 470-540 acres with overlapping home ranges (Feral Hogs, 2018). The hogs preferred habitat with abundant oak trees which are presumed to be due to the availability of acorns. At the end of the study 13 of the 23 hogs had moved out of park boundaries and were hunted on adjacent lands and only 4 of the 23 collared feral hogs were confirmed alive. The researchers concluded that natural pressures within the park are not enough to reduce the overall population.

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Campbell, H. A., Dwyer, R. G., Irwin, T. R., & Franklin, C. E. (n.d.). Crocodylus
porosus with Gps-based satellite transmitter attached to the nuchal rosette.
Wikimedia Commons. photograph.
Luna, C. (n.d.). Us Fish and Wildlife service employee tracking tagged mountain
lions by radio. Wikimedia Commons. photograph.
Dahlgren, D., Kohl, M., & Messmer, T. (2018). WHAT WILDLIFE MANAGERS
SHOULD KNOW WHEN USING RADIO-TELEMETRY DATA. Utah State
University Extension, 1862.

Farve, R. Demonstration of Satellite/GPS Telemetry for Monitoring FineScale
Movements of Lesser Prairie-Chickens, Demonstration of Satellite/GPS
Telemetry for Monitoring FineScale Movements of Lesser Prairie-Chickens
(n.d.).

Feral Hogs at Congaree National Park. (n.d.). Retrieved August 28, 2018, from
https://www.nps.gov/rlc/ogbfrec/feral-hogs-at-congaree-national-park.htm

Fuller, M. R., & Fuller, T. (2012). Radio-Telemetry Equipment and Applications for
Carnivores. In Carnivore Ecology and Conservation: A Handbook of Techniques
(pp. 152–168). Oxford: Oxford University Press.

History of Feral Swine in the Americas. (n.d.). Retrieved January 28, 2020, from
https://www.aphis.usda.gov/aphis/ourfocus/wildlifedamage/operational
activities/feral-swine/feral-swine-history

Mech, L. D., & Barber, S. M. A critique of wildlife radio-tracking and its use in
national parks: a report to the U.S. National Park Service,  (2002).
Jamestown, ND: U.S. Geological Survey, Northern Prairie
Wildlife Research Center.
Mojica, E. K. (n.d.). Platform Transmitter Terminal (Ptt) attachment on a Whimbrel
that was captured along the lower Delmarva Penninsula in Virginia on 20 May
2008. Whimbrel Tracked with Satellite Transmitter on Migratory Flight across
North America (Vol. 115, pp. 119–121). photograph, Wader Study Group
Bulletin.