Hares and jackrabbits are mammals belonging to the genus Lepus. They are herbivores, and live solitarily or in pairs. They nest in slight depressions called forms, and their young are able to fend for themselves shortly after birth. The genus includes the largest lagomorphs. Most are fast runners with long, powerful hind legs, and large ears to dissipate body heat. Hare species are native to Africa, Eurasia and North America. A hare less than one year old is called a "leveret". A group of hares is called a "husk", a "down" or a "drove". Members of the Lepus genus are considered true hares, distinguishing them from rabbits which make up the rest of the Leporidae family. However, there are five leporid species with "hare" in their common names which are not considered true hares: the hispid hare (Caprolagus hispidus), and four species known as red rock hares (comprising Pronolagus). Conversely, several Lepus species are called "jackrabbits", but classed as hares rather than rabbits. The pet known as the Belgian hare is a domesticated European rabbit which has been selectively bred to resemble a hare. Biology Hares are swift animals and can run up to over short distances. Over longer distances, the European hare (Lepus europaeus) can run up to . The five species of jackrabbits found in central and western North America are able to run at over longer distances, and can leap up to at a time. Normally a shy animal, the European brown hare changes its behavior in spring, when it can be seen in daytime chasing other hares. This appears to be competition between males to attain dominance for breeding. During this spring frenzy, animals of both sexes can be seen "boxing", one hare striking another with its paws. This behavior gives rise to the idiom "mad as a March hare". This is present not only in intermale competition, but also among females toward males to prevent copulation. Differences from rabbits Hares are generally larger than rabbits, with longer ears, and have black markings on their fur. Hares, like all leporids, have jointed, or kinetic, skulls, unique among mammals. They have 48 chromosomes, while rabbits have 44. Hares have not been domesticated, while some rabbits are raised for food and kept as pets. Most rabbits live underground in burrows or warrens, while hares live in simple nests above the ground, and usually do not live in groups.