Among plains and mountain zebras, the adult females mate only with their harem stallion, while in Grévy's zebras, mating is more promiscuous and the males have larger testes for sperm competition. Oestrus in female zebras lasts five to ten days; physical signs include frequent urination, flowing mucus, and swollen, everted (inside out) labia. In addition, females in oestrous will stand with their hind legs spread and raise their tails when in the presence of a male. Males assess the female's reproductive state with a curled lip and bared teeth (flehmen response) and the female will solicit mating by backing in. The length of gestation varies by species; it is roughly 11–13 months, and most mares come into oestrus again within a few days after foaling, depending on conditions. In harem-holding species, oestrus in a female becomes less noticeable to outside males as she gets older, hence competition for older females is virtually nonexistent.