also communicate with visual displays, and the flexibility of their lips allows them to make complex facial expressions. Visual displays also consist of head, ear, and tail postures. A zebra may signal an intention to kick by dropping back its ears and whipping its tail. Flattened ears, bared teeth and a waving head may be used as threatening gestures by stallions. Individuals may greet each other by rubbing and sniffing and then mutually rub their cheeks, and move along their bodies towards each other's genitals to sniff. They then may caress their shoulders against each other and lay their heads on one another. This greeting usually occurs between harem or territorial males or among bachelor males playing. Plains and mountain zebras strengthen their social bonds with grooming. Members of a harem nibble and rake along the neck, shoulder, and back with their teeth and lips. Grooming usually occurs between mothers and foals and between stallions and mares. Grooming establishes social rank and eases aggressive behaviour, although Grévy's zebras generally do not perform social grooming. Reproduction and parenting 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. Female zebras have five to ten day long oestrous cycles; physical signs include a swollen, everted (inside out) labia and copious flows of urine and mucus. Among reaching peak oestrous, mares spread-out their legs, lift their tails and open their mouths 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. Gestation is typically around a year. A few days to a month later, mares can return to oestrus. 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. Usually, a single foal is born, which is capable of running within an hour of birth. A newborn zebra will follow anything that moves, so new mothers prevent other mares from approaching their foals as they become more familiar with the mother's striping pattern, smell and voice. At a few weeks old, foals begin to graze, but may continue to nurse for eight to thirteen months. Living in an arid environment, Grévy's zebras have longer nursing intervals