When seen trundling slowly through hot sand, the desert tortoise (Gopherus agassizii) may look vulnerable to predators and extreme desert conditions, but this interesting reptile's adaptations are so successful that his form has changed little over the last 30 million years. His physical structure and behavior provide him with protection from attack and wide temperature fluctuations, and help him move through difficult terrain.
The desert tortoise's most noticeable adaptation is its shell. Threatened by many predators, such as snakes, foxes, bobcats, badgers, coyotes and golden eagles, a desert tortoise's shell provides effective protection when he draws in his head and legs. A desert tortoise attacked in its burrow retreats and wedges his shell against the roof of the burrow so that he can't be dragged out. One disadvantage of the desert tortoise's shell is that he finds it difficult to right himself if turned upside down during courtship fights with other males, sometimes resulting in death from suffocation or exposure.
To survive dry conditions, the desert tortoise has evolved a large, specialized bladder. At full capacity, it can hold more than 40 percent of the tortoise's weight in nitrogenous wastes, urea, uric acid and water, and the tortoise has the ability to extract water from it back into his bodily systems. His urinary waste is semi-solid to limit the amount of water lost. When conditions are wet, the desert tortoise drinks excessively, filling his bladder and excreting nitrogenous waste. If he's frightened, he can void his bladder, putting him at risk of dehydration, which is why it's important to never handle a tortoise in the wild unless he's in immediate danger.
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Desert tortoise legs are adapted to help him walk in sand and dig burrows. His hind legs are thick and stumpy, and his front legs are flattened. Both are equipped with sharp claws. The desert tortoise digs burrows with its front legs and often removes the dug sand from the burrow entrance, possibly to help hide his location. Females dig nest holes with their hind legs for laying eggs. Thick legs make walking in sand easier because wide feet help spread body weight evenly.
Adapted behavior patterns increase the desert tortoise's chances of survival in dry environments with extreme temperature variations. In the Mohave Desert, he digs burrows up to 35 feet long, which provide protection from predators, summer heat and freezing winter cold. In this burrow he enters periods of dormancy, when his heart and breathing slow down, and his body temperature drops to between 40 and 60 degrees Fahrenheit, about the same as the surrounding air. In this state, he can survive long periods with no food or water. The desert tortoise also digs depressions in the earth to catch rainwater.
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