Whales mature sexually between their third and seventh years. Toothed cetaceans attain sexual maturity later than filter-feeding whales. A blue whale is sexually mature at 5 years, whereas porpoises require at least 7 years. Most filtering whales are sexually mature in 2 or 3 years. Whales are not fully grown at sexual maturity, but they continue to grow for years. In most mammals growth stops with sexual maturity. Female whales generally can be expected to produce a baby every other year, for the gestation period is approximately one year. Babies are nursed for about 9 months. At birth the baby is completely formed and active, but lacking baleen, must nurse. A blue whale baby at birth weighs approximately 8 tons, about 1/12th of the weight of the mother. The mother provides the baby with 50 gallons of milk a day. Since the nursing is done under water, and the baby must surface frequently to breathe, the act of nursing is very brief. Muscles in the breasts of the mother force the milk into the baby’s mouth in large amounts. The baby will double its length in 7 months, which averages to a daily weight gain of 220 pounds. During all this time the mother must fatten for the winter ahead, and perhaps continue to grow herself. Life Span It is not known for certain how long a whale may live after completing its growth. At the present time, commercially important species seldom attain physical maturity before being captured. Many whales which are captured are measured for scientific study. Such measurements also keep the whalers from taking undersized juveniles. Whales apparently do not live to be very old. Fifty years appears to be the best current estimate of a life span. [Illustration: Back of a gray whale showing barnacles and barnacle scars. _Courtesy of Scripps Institute of Oceanography._] Whale Intelligence Whales are apparently very intelligent animals. Whalers have remarked how difficult it is to approach whales which have previously escaped. The gray whales were observed actually avoiding the coast after shore whaling had been carried out for a few seasons. Animal trainers have found the toothed whales particularly apt pupils, and these animals are the stellar attraction of the various oceanaria. Whale Senses _Sight_ Little is known, however, about the capabilities of the various sensory organs. Certainly the eyes are very important and are effective under water. It is not likely that the eye is very effective out of water, even though whales do elevate the head out of the water for a look around. The behavior has been appropriately called “spyhopping,” and it is manifested usually in the ice floes. Killer whales are believed to search the sea’s surface and the edge of the icebergs for seals and birds. _Hearing_