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Reptile and Amphibian Life Span and Life History

How long do reptiles and amphibians live? Some species of lizards and frogs live short lives of only one or two years. It is relatively easy to study the life stages and longevity of these short-lived species. However, for long-lived species, much less is known about life span and age-related processes. Studying these animals requires monitoring individuals in the wild over many years. Most often this is expensive and hard to do. Many herps outlive the life of the study, and some even outlive the researchers. Therefore, much of the information on animal life spans comes from zoos that keep careful records.

Some of the largest turtles, such as giant tortoises, may survive for as long as 100 years. But size is not always related to life span. The small Blanding's turtle can live for more than 60 years and a common ring-necked snake that you encounter can be 15 years or older. Even some of the smallest salamanders can live for 10 to 15 years or more.

How can you tell the age of an animal in the wild? In the case of a turtle, its bony shell is covered with a series of dry scales called scutes. As the turtle grows, it adds a new layer underneath the old scute, only bigger. This results in layer upon layer of scutes. Some turtles shed outer scutes periodically, while other turtles retain their outer scutes until they are worn off by years of abrasion.

In either case, an interesting phenomenon occurs with turtles in colder climates. The turtles grow only during the warm weather. When the turtle's growth slows down in the fall, it forms a distinct line around the edge of the scute. As new scutes grow underneath, a remnant of past lines remains. Because one new line is added each winter, you can count the lines to get an estimate of the age of the turtle. This method does not work for soft-shelled turtles, seaturtles, and some other species. But if you look at a single scute of a painted turtle, snapping turtle, or diamond-backed terrapin, you can get a fairly good idea of the turtle's age.

Because it is possible to determine the age of turtles, they make ideal subjects for studying age-related biological processes. Scientists often want to know the number of individuals of different ages in a population of turtles. This allows them to determine whether the population – the number of turtles in a given area – is growing or declining. However, it is difficult to locate and age every turtle in a wild population. Therefore, the scientists carefully select a random sample of individuals in a population and then use statistics to analyze the data and to describe the larger populations.

When studying a population of turtles, scientists measure and weigh each animal sampled. When appropriate, they also count scute rings to estimate the turtle's age. They record the time and location of capture and often label each turtle on its plastron (lower shell) or carapace (upper shell). A very clever method of keeping track of individuals in a population is to photocopy their plastrons. Because each individual is unique, this is as effective as taking fingerprints of humans. Each time an animal is captured and measured throughout its life, a lot of important information can be learned about its biology, such as movement, growth rate, and life span.

When studying wild animals, and especially when making decisions about conservation and management, knowing the life span is not as important as knowing the relationship between the age of an animal and important events in its life. In particular, the average age at which the members of a species begin to reproduce, the number of offspring produced by each mother, and how often they reproduce throughout their lifetime are important measures of the species' ability to persist in nature. When births are few or far between, the importance of each individual increases. When a species takes a long time to reach maturity, the relative importance of adults in the population also increases. Animals that have low rates of reproduction often are vulnerable to environmental disruptions, and thus we should put greater effort into protecting individuals.

Many reptiles and amphibians take years just to reach sexual maturity. For example, a painted turtle may take five years to reach sexual maturity whereas a snapping turtle may remain immature for its first 10 to 15 years. A few decades ago, the American alligator was nearly driven to extinction even though only the older, larger animals were being hunted. What we found nearly too late was that no alligators were being allowed to get old enough to reproduce. Once we figured out how long it took an individual to reach maturity, we were able to successfully repopulate the American alligator across the southeastern United States. Thus, by understanding age-related processes such as life span and the timing of reproduction, we can design better strategies for conservation.

This background information can be used with the lesson plans Long-Lived Turtles and Estimating Turtle Size and Age.

Excerpted from Hands-On Herpetology.

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