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 males 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 tortoises 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 males plastron (bottom shell) is concave to fit over the females carapace (upper shell) during mating. The posterior of the males shell has an inward curve, which also helps during mating. The posterior portion of the females shell is flared outward, which might help in mating, but likely provides more room for eggs.|