The giraffe is Africa's tallest mammal, reaching a full height of more than 5m. Males are larger and darker than females, have thicker horns and have a lump on the forehead. They are most active in the early morning and late afternoon but also feed at night in bright moonlight. They will drink water if its available but can survive without it. Femals, calves, and juveniles occur in herds of about 10 with only very loose social ties, and a flexible membership. Young males may form small bachelor groups. Mature bulls are nearly always alone.