Giant pandas reach sexual maturity between the ages of four and eight, and may be reproductive until age 20. The mating season is between March and May, when a female goes into estrus, which lasts for two or three days and only occurs once a year. When mating, the female is in a crouching, head-down position as the male mounts her from behind. Copulation time ranges from 30 seconds to five minutes, but the male may mount her repeatedly to ensure successful fertilisation. The gestation period is somewhere between 95 and 160 days - the variability is due to the fact that the fertilized egg may linger in the reproductive system for awhile before implanting on the uterine wall.