Effects of toe-clipping and PIT-tagging on growth and survival in metamorphic Ambystoma opacum

Jeannine A. Ott and David E. Scott

 A basic goal of ecological studies is to understand patterns in nature. In the case of wildlife, ecologists are often interested in population patterns, such as what causes the number of individuals in a population to fluctuate up in some years and down in others? Researchers who try to answer questions like this must learn as much as possible about individual animals in the population being studied. In a way the process is not unlike the census of our human population. Census-takers need to know names, ages, addresses, number of children, and many other details, all so that they can describe the population in the present and predict its status and trends in the future.

In wildlife studies, giving each individual in a population an identifying name or number (ID) is called “marking” the animal. By capturing an animal, taking lots of measurements, marking it, and then releasing it, biologists are able to gather detailed information on growth, movement, age, diet, and other characteristics each time the animal is recaptured. When the wildlife being studied is a small animal like an amphibian (frogs, toads, and salamanders), however, then just being able to give a “name” to each animal in the population can be a challenge. Historically a marking technique called toe-clipping has been used on amphibians to give individuals an ID. A number of problems exist with this method, and so researchers at SREL investigated an alternative technique to assess the feasibility of its use on small amphibians. The new technique is called PIT-tagging.

A PIT tag (Passive Integrated Transponder) is a relatively new, high-tech device that is receiving increased use in wildlife studies and in veterinary clinics. The tag is small, about the size of a rice grain, and is inserted under the skin or into the body cavity of an animal. Using the same technology as a bar code reader at a grocery store checkout counter, a PIT-tag reader is used to identify an animal’s code—thus, each animal can have a unique ID. SREL researchers examined the use of PIT tags on small salamanders, with an interest in comparing the PIT-tag technique to toe-clipping in terms of effects (if any) on salamanders, ease of use in the field, and expense.

The SREL researchers captured about 250 small marbled salamanders in the spring, just as the animals were leaving the wetland where they had developed as babies and were headed into the wooded uplands. One-third of these animals were marked using PIT tags, a third were toe-clipped, and a third were left unmarked. The salamanders were held in large, natural terrestrial enclosures, and monitored monthly over the next six months. At the end of the experiment, all survivors were captured, weighed, and measured. Over 95% of the salamanders in all three groups lived. In addition, the groups had similar growth rates; that is, neither PIT-tagging nor toe-clipping negatively affected the salamanders in any way compared to the control animals.

The results demonstrate that PIT-tagging is a viable technique, even in salamanders only a few inches long. However, the SREL researchers noted that it takes far longer to insert the PIT tags into small salamanders (tags had to be surgically implanted) than it takes to mark an animal using the toe-clip technique. Also, PIT tags cost more than $4.00 per tag, and salamander populations can be tens of thousands of animals. Because a large fraction (>50%) of marked animals are never seen again, the thought of watching $80,000 walk off into the woods is not an appealing one. Consequently, the researchers concluded that: 1) although both techniques have problems, both are useful under different circumstances, and 2) for the time being, an understanding of population fluctuations in pond-breeding salamanders will require the combined use of toe-clipping, PIT-tagging, and other techniques.