zebras, and donkeys. Equines also communicate with visual displays, and the flexibility of their lips allows them to make complex facial expressions. Visual displays also incorporate the positions of the head, ears, and tail. An equine may signal an intention to kick by laying back its ears and sometimes lashing the tail. Flattened ears, bared teeth, and abrupt movement of the heads may be used as threatening gestures, particularly among stallions. Reproduction and parenting Among harem-holding species, the adult females mate only with their harem stallion, while in other species, mating is more promiscuous and the males have larger testes for sperm competition. Estrus in female equines lasts 5–10 days; physical signs include frequent urination, flowing muscus, and swollen, everted labia. In addition, estrous females 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 the flehmen response and the female will solicit mating by backing in. Length of gestation varies by species; it is roughly 11–13 months, and most mares come into estrus again within a few days after foaling, depending on conditions. Usually, only a single foal is born, which is capable of running within an hour. Within a few weeks, foals attempt to graze, but may continue to nurse for 8–13 months. Species in arid habitats, like Grévy's zebra, have longer nursing intervals and do not drink water until they are three months old. Among harem-holding species, foals are cared for mostly by their mothers, but if threatened by predators, the entire group works together to protect all the young. The group forms a protective front with the foals in the center and the stallion will rush at predators that come too close. In territory-holding species, mothers may gather into small groups and leave their young in "kindergartens" under the guard of a territorial male while searching for water. Grévy's zebra stallions may look after a foal in his territory to ensure that the mother stays, though it may not be his. Human relations The earliest archaeological evidence for the domestication of the horse comes from sites in Ukraine and Kazakhstan, dating to around 4000–3500 BCE. By 3000 BCE, the horse was completely domesticated, and by 2000 BCE, a sharp increase occurred in the number of horse bones found in human settlements in northwestern Europe, indicating the spread of domesticated horses throughout