by Whit Gibbons

July 20, 2014

Everybody loves tortoises. And a new book has lots of advice about how we can protect these engaging reptiles. "Biology and Conservation of North American Tortoises" edited by David C. Rostal, Earl D. McCoy, and Henry R. Mushinsky (Johns Hopkins University Press, 2014) answers myriad questions about tortoises. In fact, the 33 experts who wrote the 18 chapters of the book answer more questions about the conservation and ecology of tortoises than the general public has even thought of.

Most people have a pretty clear picture of this distinctive group of turtles when they think of the giant Galapagos tortoises. In the southeastern United States, particularly parts of Alabama, Georgia, Florida, and South Carolina, many folks are familiar with the native gopher tortoises, which dig long underground burrows that are often used for shelter by other animals. Out West, in arid regions of southern California, Nevada, Utah, and Arizona, desert tortoises are natural inhabitants with which many people are familiar. A less well known U.S. species is the Texas tortoise, aka Berlandier's tortoise, found in southern Texas. Desert tortoises and Texas tortoises also extend south of the border into the Mexican deserts. As indicated in the title, the book covers North America, hence the inclusion of the Bolson tortoise, which is restricted to the Chihuahuan Desert in Mexico.

This scholarly work addresses topics of interest to research biologists who study tortoises: the fossil record, reproductive traits, growth rates, movement patterns, and general life history. A chapter on social behavior of these captivating creatures is of particular interest. The authors consider in what ways the communication patterns of these "highly social vertebrates" affect how they distribute themselves within a tortoise colony, what the mating structure will be, and what strategies they use for avoiding predators. The final chapter, "Threats and Conservation Needs for North American Tortoises," addresses tough questions about what must be done to ensure the survival of these marvelous animals.

Two ecological facts noted in the book make for some interesting speculation. One is related to the incubation period of eggs of desert and gopher tortoises. Both have been studied extensively enough for scientists to know that the eggs can incubate and hatch in as few as 10 weeks if temperatures are high. Eggs incubating at cooler temperatures can take more than four months to hatch. Another set of data for North American tortoises confirms that eggs incubating at high temperatures are more likely to produce females whereas hatchlings from eggs experiencing low temperatures are more likely to be males. Thus the actual sex ratio in a tortoise population could become highly skewed because of changes in environmental temperatures. Extensive variation can occur with such biological phenomena. However, the implications of how tortoise populations might respond to climate change scenarios that could result in higher temperatures during incubation are worth considering.

Some might consider global warming a major danger to wildlife in North America and throughout the world. But the most serious threats to tortoises - and indeed animals everywhere - are much more imminent and clearly identifiable as a result of habitat destruction. As the chapter on habitat characteristics of tortoises points out, "Most threats to tortoise habitat generally can be attributed directly to human causes that include urbanization, agriculture, and the building of highways." Another less tangible detrimental impact on natural populations of tortoises is the release into wild areas of pet tortoises. These discarded pets may carry contagious diseases, including an upper respiratory tract infection that can be passed on to wild tortoises.

The data give unquestionable proof that North American tortoises are declining at an alarming rate. Ironically, most "are protected in whole or in part by international, federal, and state regulations." However, as the authors note, these official designations have not "resulted in the regulations and enforcement essential" to actually protect the tortoises. The book is to be commended for providing evidence that more stringent conservation measures are needed on behalf of U.S. and Mexican tortoises.

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