Bighorn sheep are named for the large, curved horns borne by the males, or rams. Females, or ewes, also have horns, but they are short with only a slight curvature. Sheep range in color from light brown to grayish or dark, chocolate brown, with a white rump and lining on the back of all four legs. Rocky Mountain bighorn females weigh up to 200 pounds, and males occasionally exceed 300 pounds. During the mating season or rut, occurring in November and December, the rams butt heads in apparent sparring for females. Rams' horns can weigh more than 40 pounds, and frequently show broken or broomed tips from repeated clashes. Lambs, usually only one per mother, are born in May and June. They graze on grasses and browse shrubby plants, particularly in fall and winter, and seek minerals at natural salt licks. Bighorns are well adapted to climbing steep terrain where they seek cover from predators such as coyotes, eagles, and mountain lions. They are susceptible to disease such as lungworm, and sometimes fall off cliffs.