Tortoises are personal pets that have gained immense popularity around the world in recent years. And as evidenced by the proliferation of books and websites catering to the subject of captive tortoise maintenance, keepers of tortoises are constantly seeking information about providing their pets with optimal conditions. Many species adapt well to captivity, but pet tortoises are very much wild animals. Knowledge about their wild counterparts, therefore, may help tortoise keepers go a long way in satisfying the needs of their pet tortoises. The purpose of this article is to discuss aspects of the biology of wild tortoises – including evolutionary history, life history traits, physiological ecology, and social behaviors and communication – that can help keepers of tortoises provide a better existence for both captive and wild tortoises. In my third post, I discuss physiological ecology of tortoises.
An understanding of wild tortoises, such as this wild adult male Agassiz’s desert tortoise, can give keepers of captive tortoises great insight into the lives of their pets. Photo by Michael Tuma.
Physiological Ecology
Tortoises exhibit unique physiological traits that have allowed them to adapt to a wide range of ecological conditions across the planet. Certain physiological traits, including those that help tortoises to manage water and metabolic processes, define how the different species articulate with their environments in the pursuit of food, water, and other resources. These aspects of physiological ecology not only determine how each species of tortoise lives within their respective environments, but are a critical consideration in providing their captive care.
With respect to physiologies, there are two groups of tortoises: grassland species and forest species. These groups occupy different types of environments and have divergent physiological traits that allow them to live and thrive in the different ecological conditions they experience. Grassland tortoises that are kept in captivity include the North American Gopherus tortoises, the Mediterranean Testudo tortoises, the Madagascan Astrochelys tortoises, the Asian star tortoises (Geochelone), the Aldabra tortoise (Aldabrachelys gigantea), the African spurred tortoise (Centrochelys sulcata), the pancake tortoise (Malacochersus tornieri), and the leopard tortoise (Stigmochelys pardalis). Forest tortoises include the South American red- and yellow-footed tortoises (Chelonoidis carbonaria and C. denticulata), the Asian elongated Indotestudo tortoises, the African Kinixys tortoises, and the Asian Manouria tortoises.
Grassland tortoises usually live in environments where water is available only on a seasonal basis; by contrast forest tortoises live in environments where water is generally plentiful year-round. Grassland tortoises are able to survive in mostly dry, seasonally wet habitats by using their urinary bladder to store water for metabolic processes. During periods of rain, grassland tortoises drink copiously and store the water in their urinary bladder through the dry season. It’s almost like having an internal Camelbak reservoir. In some species like the desert tortoise, the urinary bladder can hold up to 40% of the animal’s weight in water (Miller 1932). Forest tortoises also store water in their urinary bladders, but their bladders are comparatively smaller and incapable of storage of water for long periods.
Grassland tortoises, such as this leopard tortoise, are adapted to environments that are often seasonal in nature, especially with respect to water availability. Photo by Charles J. Sharp.
Grassland species produce uric acid as a by-product of metabolism. This is the solid, white, chalky substance that tortoises periodically expel from their cloacae. Forest species, on the other hand, produce urea, which is dissolvable in water and expelled in a water solution. The crystalline structure of uric acid requires more energy to make, but saves water. It should be no surprise, therefore, that many organisms that produce uric acid, such as tortoises, lizards, and birds, thrive in dry environments like grasslands and deserts. Urea uses less energy to make, but necessarily requires regular consumption of water. Accordingly, forest tortoises are never found too far from accessible sources of water, and they thrive in environments that are moist.
Forest species are omnivorous and can consume proteins and purines that are found in animal tissues and fungi. Diets of forest tortoises include carrion, insects, worms, gastropods, mushrooms, and fruits that they find on the forest floor, in addition to leaves and flowers. Their constant access to available water in their environment allows them to expel the metabolic wastes that accumulate quickly with this kind of diet. By comparison, grassland tortoises are predominantly strict herbivores. Because grassland tortoises rely on their urinary bladders for storing water, their bladders accumulate metabolic wastes during the dry season (Nagy and Medica 1986). Accordingly, grassland tortoises are restricted to eating plants that contain high amounts of water and protein, and comparatively low levels of potassium (Oftedal 2002). Deviations from this type of diet could result in rapid accumulation of lethal amounts of toxins in their bladders and bloodstreams during dry periods.
Forest tortoises, like this yellow-footed tortoise, are physiologically adapted to moist environments where water is nearly constantly available. Photo by Geoff Gallice.
Finally, grassland species actively thermoregulate, seeking sun and shade resources to control their body temperatures, whereas forest species generally do not. Grasslands and deserts are characterized by wide fluctuations in temperature both daily and seasonally, and the tortoises that occupy these environments exhibit specialized behaviors in adapting to these changes. Grassland tortoises occupy open, exposed environments, and they bask in the sun to warm themselves and retreat to burrows or forms to escape it. Some grassland tortoise species, including several Gopherus and Testudo species, occupy environments characterized by cool seasons, and they hibernate seasonally. Other grassland species, such as the African spurred tortoise, may aestivate during hot, dry seasons. In comparison, forest tortoises occupy relatively stable environments, with more homogenous temperature regimes and more subtle seasonality changes. Moreover, their environments are comparatively closed, with less access to the sun. Forest tortoises therefore are passive thermoregulators that typically do not bask, hibernate, or aestivate, but rather rely on relatively stable ambient temperatures encountered within their environments.
Grassland tortoises should be provided a diet of only leaves and flowers in captivity. Photo by Michael Tuma.
As pet tortoise keepers, it is important that we understand the environmental requirements of our tortoises in order to provide them with proper diets, temperature regimes, and access to water. Knowing the needs of grassland and forest tortoises is a critical first step in providing these conditions in captivity. Grassland tortoises should only be fed leaves and flowers, should have access to basking areas, and in some instances, should be allowed to hibernate or aestivate. Their enclosures should be primarily dry, with access to a relatively moist burrow or hide where they can control rates of evaporative water loss. They should be soaked periodically to allow them to void their urinary bladders of metabolic wastes and replenish them with fresh water. Forest tortoises should be offered a variety of other foods in addition to leaves and flowers, including animal protein, mushrooms, and fruit. They should also have constant access to water, be kept at relatively constant temperatures daily and seasonally, and be kept in relatively moist enclosures.
Literature Cited
Miller, L. 1932. Notes on the desert tortoise (Testudo agassizii). Transactions of the San Diego Society of Natural History 7:187–208.
Nagy, K. A. and P. A. Medica. 1986. Physiological ecology of desert tortoises in southern Nevada. Herpetologica 42:73–92.
Oftedal, O. T. 2002. Nutritional ecology of the desert tortoise in the Mojave and Sonoran Deserts. In T. R. van Devender (ed.), The Sonoran Desert Tortoise: Natural History, Biology, and Conservation, pp. 194–241. University of Arizona Press and the Arizona-Sonora Desert Museum, Tucson.