Rock hyraxes build dwelling holes in any type of rock with suitable cavities such as sedimentary rocks and soil. Rock hyraxes give birth to two or three young after a gestation period of 6–7 months (long, for their size). The young are well developed at birth with fully opened eyes and complete pelage. Young can ingest solid food after two weeks and are weaned at ten weeks. After 16 months, the rock hyraxes become sexually mature, they reach adult size at three years, and they typically live about ten years. Adults make use of at least 21 different vocal signals. Hyraxes are the first non-human species in which structural balance was described. They follow the "the friend of my friend is my friend" rule, and avoid unbalanced social configurations. Rock hyraxes produce large quantities of hyraceum—a sticky mass of dung and urine that has been employed[when?] as a South African folk remedy in the treatment of several medical disorders, including epilepsy and convulsions.[27] Hyraceum is now being used by perfumers who tincture it in alcohol to yield a natural animal musk.