The family Leporidae, consisting primarily of rabbits and hares, includes 54 species from 11 different genera. Leporids range in mass from 300 grams (1.4 lbs) in pygmy rabbits to 5 kilograms (11 lbs) in arctic hares. Adult head and body length ranges from 250 to 700 mm. Unlike most mammals, females are usually larger than males. Color patterns vary between species and across seasons, and range from black to reddish brown to white. Leporids are widely distributed and have adapted to a broad range of habitat types. They can be found throughout the world with very few exceptions. Habitat type affects pelage color as well as litter size. Some leporids are extremely social, living in large communal dens, while others are solitary, coming together in groups or pairs for mating purposes only. The term 'true hares' includes hares and jackrabbits and consists of those species in the genus Lepus; all remaining species are referred to as rabbits. While hares are well adapted for running long distances, rabbits run in short bursts and have modified limbs adapted for digging. Hares have long muscle fibers in contrast to the short fibers found in rabbit muscle. Hares are often larger than rabbits, have black tipped ears, and have distinctly different skull morphologies.