Lions are the laziest of the big cats. They usually spend 16 to 20 hours a day sleeping and resting, devoting the remaining hours to hunting, courting or protecting their territory. They keep in contact with one another by roaring loud enough to be heard up to five miles away. The pride usually remains intact until the males are challenged and successfully driven away or killed by other males, who then take over. Not all lions live in prides. At maturity, young males leave the units of their birth and spend several years as nomads before they become strong enough to take over a pride of their own. Some never stop wandering and continue to follow migrating herds; but the nomadic life is much more difficult, with little time for resting or reproducing. Within the pride, the territorial males are the fathers of all the cubs. When a lioness is in heat, a male will join her, staying with her constantly. The pair usually mates for less than a minute, but it does so about every 15 to 30 minutes over a period of four to five days. Lions may hunt at any hour, but they typically go after large prey at night. They hunt together to increase their success rate, since prey can be difficult to catch and can outrun a single lion. The lions fan out along a broad front or semicircle to creep up on prey. Once with within striking distance, they bound in among the startled animals, knock one down and kill it with a bite to the neck or throat. Hunts are successful about half the time.