Mating behaviour Vertebrates Mammals Mammals mate by vaginal copulation. To achieve this, the male usually mounts the female from behind. The female may exhibit lordosis in which she arches her back ventrally to facilitate entry of the penis. Amongst the land mammals, other than humans, only bonobos mate in a face-to-face position, as the females' anatomy seems to reflect, although ventro-ventral copulation has also been observed in Rhabdomys. Some sea mammals copulate in a belly-to-belly position. Some camelids mate in a lying-down position. In most mammals ejaculation occurs after multiple intromissions, but in most primates, copulation consists of one brief intromission. In most ruminant species, a single pelvic thrust occurs during copulation. In most deer species, a copulatory jump also occurs. During mating, a "copulatory tie" occurs in mammals such as fossas, canids and Japanese martens. A "copulatory lock" also occurs in some primate species, such as Galago senegalensis. The copulatory behaviour of many mammalian species is affected by sperm competition. Some females have concealed fertility, making it difficult for males to evaluate if a female is fertile. This is costly as ejaculation expends much energy. Invertebrates Invertebrates are often hermaphrodites. Some hermaphroditic land snails begin mating with an elaborate tactile courting ritual. The two snails circle around each other for up to six hours, touching with their tentacles, and biting lips and the area of the genital pore, which shows some preliminary signs of the eversion of the penis. As the snails approach mating, hydraulic pressure builds up in the blood sinus surrounding an organ housing a sharpened dart. The dart is made of calcium carbonate or chitin, and is called a love dart. Each snail manoeuvres to get its genital pore in the best position, close to the other snail's body. Then, when the body of one snail touches the other snail's genital pore, it triggers the firing of the love dart. After the snails have fired their darts, they copulate and exchange sperm as a separate part of the mating progression. The love darts are covered with a mucus that contains a hormone-like substance that facilitates the survival of the sperm. Penis fencing is a mating behaviour engaged in by certain species of flatworm, such as Pseudobiceros bedfordi. Species which engage in the practice are hermaphroditic, possessing both eggs and sperm-producing testes. The species "fence" using two-headed dagger-like penises which are pointed, and white in colour. One