An important aspect of understanding animal migration is the ability to track and recognize individuals over time and space (Boarman et al. 1998). Technological advances made over the past decades now allow us to digitally track our study animals in ways that just a generation ago would have seemed unfathomable. Radio and satellite telemetry have revealed a lot about animal movements, but are limited by the need for batteries, whose weight sets a ceiling on the size of the animal that can be tracked (Roussel et al. 2000).

External tags (e.g., ear and leg bands, dart tags, painted marks, scale clipping) have also facilitated animal tracking. These are relatively cost-effective, but their external nature makes them susceptible to being lost, scratched, rendered illegible upon recapture, and also subject to human error while being read (Gibbons & Andrews 2004). Branding, which has been used on such animals as elephant seals [Mirounga leonina]), often causes the animals pain and has thus been viewed as inhumane (Galimberti & Sanvito 2000). So what should you do if you want to recognize an individual animal over the course of years? Here, we review use, benefits, and limitations of internal passive integrated transponder (PIT) tagging to study animal movement.

Internal Identification Tags

The use of internal PIT tags began in the mid-1980s with scientists measuring fish movement, but has since been expanded to include the study of mammal, amphibians, reptile, bird, and invertebrate movement (Gibbons & Andrews 2004). PIT tags are also used by veterinarians for identifying household pets and for tracking livestock and zoo animals. A PIT tag consists of an integrated circuit chip, capacitor, and antenna coil encased in glass (Roussel et al. 2000). PIT tags vary in size and shape depending on the study animal. Generally, tags are cylindrical in shape, about 8-32 mm long, and 1-4 mm in diameter, but they also exist as disks.

Essentially, PIT tags act as a lifetime barcode for an individual animal, analogous to a Social Security number and, provided they can be scanned, are as reliable as a fingerprint (Gibbons & Andrews 2004). PIT tags are dormant until activated; they therefore do not require any internal source of power throughout their lifespan. To activate the tag, a low-frequency radio signal is emitted by a scanning device that generates a close-range electromagnetic field. The tag then sends a unique alpha-numeric code back to the reader (Keck 1994). Scanners are available as handheld, portable, battery-powered models and as stationary, automated models that are usually used for automated scanning.

Internal PIT tags are inserted via large-gauge needles or surgically implanted either subcutaneously or into a body cavity. Roussel et al. (2000) inserted PIT tags into the peritoneal cavity of Atlantic salmon (Salmo salar) and reported no post-surgery mortality or tag shedding, provided fish were of a minimum size and sutures were used. Wagner et al. (2011) recommend standardizing implantation techniques for different types and size classes of animals to minimize tissue damage and infection rates, and to maximize the speed of wound healing.

Uses of PIT Tags

PIT tagging can be used to answer questions regarding growth rates, survivorship, food webs, and movement patterns. A major advantage over mark-recapture methods is that marked animals do not need to be recaptured; they just need to pass by an automated reading system antenna. Such systems are of great use for colony-nesting species such as bats, and for animals that travel through natural structural bottlenecks like spawning salmonids. Roussel et al. (2000) used antennae hung over a shallow spawning stream and were able to obtain readings of Atlantic salmon at 1-m water depth.

Since humans have been altering landscapes used by migratory animals, pathways or corridors are sometimes created in an attempt to keep the animals out of harm's way. Highways, hydroelectric dams, and pipelines are common disruptions to migratory pathways. To test the usefulness of underpasses or fish ladders around dams, automated PIT tag readers are effective measures that do not require direct observation (Gibbons & Andrews 2004). Boarman et al. (1998) used an automated reading antenna to show that desert tortoises (Gopherus agassizii), a threatened species (IUCN 1996), were making good use of storm culverts built under desert highways (Figure 1), an example of a man-made structure that decreases turtle mortality. Another interesting use comes from the Committee on the International Trade in Endangered Species (CITES),which uses PIT tags to verify captive-bred animals to monitor illegal harvest and international trade of species at risk (Gibbons & Andrews 2004).

Answering Questions About Migration

Skov et al. (2008) were interested in determining migratory strategies of three small species of carp (Cyprinidae) that tried to avoid larger predators in a shallow lake ecosystem. Using PIT tags, they relied on automated scanning systems to provide information about which fish were traveling where, without the need to be present and handle fish throughout the 9-month study. Automated scanners were set up at stream entrances and exits to the lake to determine which species were passing out of the lake at different times throughout the winter. Skov et al. (2008) were thus able to determine that two of the three cyprinids were using streams as refuge from predatory fish. The third species was found to overwinter in the lake once they reached a certain size. Fish migration is often thought to take place on a large scale (e.g., salmonids), but PIT tagging has allowed researchers to show different migration strategies within a medium-sized ecosystem.

Salamanders often return to the same breeding pond year after year, but determining their whereabouts in the interim has been a challenge. Charney et al. (2009) used PIT tags to estimate movement rates of marbled salamanders (Ambystoma opacum), which are threatened in Massachusetts, USA. Wetland buffer zones have been established to protect processes and organisms that depend on these habitats, but research has revealed that the salamanders migrate past buffer zones to reach upland habitat, where they spend most of their lives. The researchers also showed that PIT-tagged individuals traveled faster than radio-tagged animals, which suggests a negative effect of radio tags. PIT tags were also used in revising estimates of the home ranges of European salamanders (Salamandra salamandra) (Schulte et al. 2007).

Benefits of PIT Tagging

Many studies have reported great success using PIT tags. Schooley et al. (1993) marked 1,200 Townsend's ground squirrels (Spermophilus townsendii) with PIT tags and found no mortality caused by tagging and little incidence of infection. The researchers saw very low rates of tag loss, most of which occurred within 10 days of implantation. This method was more appealing than external tagging because the squirrels have small ears (making it hard to attach ear tags) and because toe-clipping impairs burrowing ability. In newborn snakes, radio-transmitters and scale clipping increased winter mortality and decreased feeding rates, while internal PIT tagging caused no change in growth rate or movement speed, indicating that PIT tags are safe and reliable for marking individuals (Keck 1994).

With large animals, PIT tags are practically permanent when installed correctly, and have virtually no negative impacts on the animal. Tagging rarely results in infection, and upon death, tags can even be reused (Wright et al. 1997, Neubaum et al. 2005, Semmens et al. 2007). PIT tags can be recognized 24 hrs a day, which proved especially useful for monitoring nocturnal bats (Roussel et al. 2000). An advantage of PIT tag recognition is that scanning need not be done by a professional, so when octopuses are caught as by-catch by commercial fisherman, they may be scanned and data returned to the appropriate researcher (Semmens et al. 2007). Moreover, PIT tag reading can be automated, which eliminates the need for an observer to be present.

Pitfalls

PIT tagging has not been problem-free. High failure rates plagued PIT tags encased in polypropylene due to leakage of the casing, although there have been no reported leakages with glass encasements (the current industry standard) to date (Schooley et al. 1993). Glass cases have even been shown to function after a tagged snake was eaten by another snake (Gibbons & Andrews 2004). Improper implantation usually leads to loss or failure to read the tag, or can cause infection leading to tag shedding or mortality of the individual. Feldheim et al. (2002) reported a tag-shedding rate of 12% over a 5-year study on lemon sharks. Tag shedding is often attributed to incomplete healing prior to release and to insertion of tags in locations where they are more easily muscled out of place. Several studies on snakes have observed that shedding can result from the body recognizing the tag as a foreign object and expelling it (Gibbons & Andrews 2004). Tag loss may also be the result of aggressive behavior.

Once installed correctly, PIT tags have a high likelihood of staying within their animal for its lifetime. Care must be taken find a suitable location within the animal so that the tag will not move within the body, which could make it difficult to read (Gibbons & Andrews 2004). The small size of some animals may not allow the injection of tags, and sometimes a more invasive, laborious, and costly surgery is the only method of implantation. Animals must be in good health prior to these procedures to ensure healthy recovery.

After installation, PIT tags can be a non-invasive, lightweight method of recognizing individuals. Cases have been reported where the presence of a tag in small animals has interfered with normal function. The combined action of handling and tagging freshwater pearl mussels was shown to decrease burrowing activity, which could make the muscles more susceptible to predation or to being washed away (Wilson et al. 2010).

As technology improves, PIT tags will become more appealing to researchers seeking to track individuals over the long-term. In 2004, tags cost 6-7 $US each. Hand readers cost 800-1500 $US, which is currently more expensive than external tags but still cheaper than telemetry equipment. Automated scanning is very time-efficient compared to mark-recapture studies, providing the animals come close enough to the reader (Gibbons & Andrews 2004). Note that if two tagged animals pass the same reading station at the same time, the reader will not be able to read either tag. While radio and satellite telemetry can track animals over longer distances than PIT tags, as technology improves, detection distances for PIT tags are likely to increase. While most tag readers are sensitive to water, in time waterproof models will be developed to better study large marine migrants.

Future of PIT Tagging

With the design of waterproof scanners, PIT tags could be operated by scuba divers and snorkelers, providing a less invasive method of tracking marine creatures than the use of large vessels (Roussel et al. 2000). For animals migrating in herds, development of remote-controlled scanners would be very useful for individual recognition in aggregated populations. A small remote-controlled boat with a scanner would be much less disturbing than a large motor boat for scanning groups of manatees or walruses that may seasonally gather around a central location (Wright et al. 1997). Just as lay-people can report leg-banded birds, it may become possible in the future to involve eco-tourists in reporting tagged individuals.