Tortoise Group Information Sheet #2
Determining the Age and Sex of Your Desert Tortoise
Unless the date of hatching is known, accurately determining the age of a wild or captive tortoise is not possible. This is because the rate of growth depends upon the amount of body building food eaten. Both wild and domesticated tortoises develop none to several growth rings on each plate of the shell per year depending upon the quality and quantity of food eaten. For this reason age cannot be determined by counting growth rings. Eventually, rings wear and weather. A tortoise with a smooth shell is "old" but "how old"? Tortoises live about as long as human beings.
It is possible to raise a captive tortoise to sexual maturity in four years with a very nutritious diet, but this is not typical of most pet tortoises. Sexual maturity is a matter of size rather than age. For a female this is usually when she reaches 7"-8". This size is the shell length when measured down the middle of the shell front to back as a straight, not curved, line. See Information Sheet #3 Measuring Tortoise Size.
One of the ways of telling the sexes apart is by the presence or absence of a concavity (depression) in the posterior third of the plastron (lower shell). The hatchling has a flat plastron and for males it remains flat until his is about 61/2" to 7" long. Gradually the male's plastron becomes concave. The plastron of females remains flat or almost flat throughout life.
Males have longer tails and a longer and usually upturned gular horn that projects from the front of the plastron. The horn is not developed in juveniles and by the time it can be used to separate the sexes, the presence or absence of the plastral concavity can be used. Males usually attain a larger size than females but all tortoises seem to grow, even in minute amounts, throughout life.
As males mature they develop chin glands, one on either side near the front of the lower edge of the lower jaw. The enlargement of these glands is seasonal. At some times of the year the glands of the males may be as small as those of females.
The nails on the hind feet of older females tend to appear unusually long. This is normal. It may be an adaptation, as nests are dug with the hind feet. Until you have examined several tortoises, these features may be difficult to use for determining the sex of a tortoise by itself.
Recently, researchers determined experimentally that the sex of desert tortoises, like that of some other turtle species, is determined by the incubation temperature. Incubation temperatures of 91 to 95.5 degrees Fahrenheit produced females, 78.8 to 87 degrees produced males. At about 89.2 both sexes were produced.