The beaver's sex organs are inside the body, and the male's penis has a cartilaginous baculum. They have a single opening, a cloaca, that contains the genital, digestive and excretory openings. The cloaca evolved secondarily, as mammals have lost this feature, and may reduce the area vulnerable to infection when swimming in dirty water. The beaver's intestine is six times longer than its body, and the caecum is twice the volume of its stomach. Microorganisms in the caecum allow them to digest around 30 percent of the cellulose they consume. The beaver's feces take the form of balls of sawdust, which it deposits into the water. Female beavers have four mammary glands; these produce milk with 19 percent fat, higher than in other rodents. Beavers have a pair of castor sacs between the kidneys and urinary bladder that empty into the urethra and anal glands. The castor sacs secrete castoreum, a urine-based substance used mainly for marking territory. Anal glands produce an oily substance which beavers rub on their fur to make it waterproof. This plays a role in individual and family recognition. Anal secretions are darker in females than males among Eurasian beavers, while the reverse is true for the North American species.