Desert tortoises are slow-growing creatures, and reach sexual maturity somewhere between 15 and 20 years of age. During the summer rainy season tortoises are especially active, spending much of their time eating to replace fat stores and drinking water, which allows them to empty their bladders of concentrated urine.

The hungry and thirsty tortoises are also eager to mate. Male tortoise engage in combat using their gulars, which are extensions of the bottom shell (plastron). The males attempt to knock each other on their backs to compete for females. This activity is dangerous, for if the loser cannot right itself it may overheat and die.

The male’s plastron has a concave curve towards the back legs, which allows him to mount the female. After mating, the female stores the sperm in her cloaca. During hibernation, fertilization takes place and the embryos begin to develop. In late May to early June, the female excavates a shallow hole and lays somewhere between 2 and 15 eggs, with the average number being about 6. If not predated by a raccoon or gila monster, the eggs hatch in late September or early October, just as the adults are about to hibernate. Often the tiny (usually less than two inch long) hatchlings do not feed, but find a place to hibernate shortly after hatching. They can overwinter on fat stored while in the egg.

Baby tortoises are especially susceptible to predation, as their shells are relatively soft. Coyotes, skunks and ravens are all animals that take advantage of the easy meal. As a result, very few hatchlings survive to adulthood, possibly as few as one tortoise in every 20 nests! If the young tortoises can survive to about 6 years of age, they are more likely to reach adulthood, as they have reached a size where few predators (except mountain lions!) can break through their tough shells. A tortoise that reaches adulthood is likely to live another 20 years or more.

To better understand reproduction of Sonoran Desert populations of desert tortoises, researchers at Saguaro National Park have been x-raying female tortoises. The x-rays are taken using a portable x-ray machine in the field(above left) at 6 week intervals during summer months to determine how many eggs are being laid and when they are being laid. On an x-ray, the eggs show up as small circles (above right). The white spot to the left of the tortoise's head is the radio transmitter.

The picture on the left is an ultrasound of a female tortoise. The round shape at the top of the picture is a follicle, or unfertilized, egg. If a tortoise’s physical condition or weather conditions are not satisfactory, a female tortoise can reabsorb the follicle into her body tissues. Those tortoises with follicles will later be x-rayed to see if any developed into eggs.
These diagrams show the difference in shells of male and female tortoises. The male’s plastron (bottom shell) is concave to fit over the females carapace (upper shell) during mating. The posterior of the male’s shell has an inward curve, which also helps during mating. The posterior portion of the female’s shell is flared outward, which might help in mating, but likely provides more room for eggs.