by Whit Gibbons

December 9, 2002

When we saw #346278 appear on the small digital screen of the instrument Tony Mills held alongside the kingsnake, we were all surprised. The number revealed new ecological information: eastern kingsnakes eat black swamp snakes.

After catching the big kingsnake, Tony was preparing to give it a unique code before returning it to the capture site. Capturing animals, marking or tagging them in some way with a personal identification code, releasing them back into the wild, and then recapturing the same individual weeks, months, or years later provides some of the most valuable information ecologists can gather on certain wildlife species.

By recapturing a "marked" individual, data can be obtained on growth rate, the distance the animal has moved, and simply whether it has survived over the time interval. In this rare instance, the data showed that a predator, the kingsnake, had eaten the coded animal, the black swamp snake. The information on predation was revealed only because of a special marking technology known as "PIT-tagging" that had been used on the swamp snake, a marking system different from the standard ones normally used on snakes.

The use of PIT tags has revolutionized some types of mark recapture field studies. A PIT tag is a tiny sliver of metal about a half inch long and the diameter of a thick paper clip that is inserted into the body cavity with a syringe. PIT stands for "passive integrated transponder." The implication of the term passive is that the metal is dormant until activated by a handheld reader with which the researcher can scan an animal and, if a tag is inside, read the code number. The process is similar to scanning bar codes in a grocery store. Ideally, once captured and PIT-tagged an animal will have a unique and permanent identification code for life. The PIT tag reader screen displays messages such as, "Searching," "No code located," or the code number.

PIT tagging has been more useful in the study of snakes than most other animals because of the difficulties in permanently marking individuals. Individual codes can be given to deer with ear tags, to turtles with notching along the sides of the shell, and to birds with color?coded and numbered leg bands. Not so with snakes.

One of the most impressive ecological studies using PIT tags on snakes that I am aware of is the long-term study of rat snakes and wood ducks begun in 1980 by Bobby Kennamer of the Savannah River Ecology Laboratory.

Wood ducks are the only species of hunted waterfowl that characteristically nest in the Deep South rather than farther north. To provide nesting habitats, conservationists use a duck box, which is a wooden structure with a duck-sized hole to mimic the tree holes where wood ducks lay and brood their eggs. For more than two decades, Bobby has checked wood duck boxes to record the number of birds nesting and eggs laid. During his first year of study, he found 19 rat snakes in the boxes where eggs were, or had been.

Being a scientist, he decided that rather than removing the snakes, he would try to find out more about their behavior in relation to the ducks. He injects each snake found in a box with a PIT tag and records the code number. By the end of the nesting period in 2002, Bobby had caught 159 rat snakes in and around his wood duck boxes and subsequently recaptured many of them a total of 228 times.

Such information as how far rat snakes move and how often they return to the same box in different years can be valuable in wood duck management programs. For example, the data provide evidence that some snakes are persistent about returning to particular duck boxes in subsequent years, suggesting that some are calculating predators. Rat snakes are indeed successful predators of wood duck eggs but most of the eggs eaten would not have survived the 70-day period after hatching. The numerous other natural causes of death to ducklings far exceed the impact from rat snakes.

PIT tags, a nondestructive sampling technique, allow scientists to track individual animals for many years. And occasionally they result in scientific serendipity.

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