nse of smell seven times stronger than that of the bloodhound, essential for locating food underground. Using their elongated claws, bears dig deep trenches in search of burrowing animals and nests as well as roots, bulbs, and insects. Bears can detect the scent of food from up to eighteen miles away; because of their immense size, they often scavenge new kills, driving away the predators (including packs of wolves and human hunters) in the process. The sense of smell is less developed in the catarrhine primates, and nonexistent in cetaceans, which compensate with a well-developed sense of taste. In some strepsirrhines, such as the red-bellied lemur, scent glands occur atop the head. In many species, smell is highly tuned to pheromones; a male silkworm moth, for example, can sense a single molecule of bombykol. Fish, too, have a well-developed sense of smell, even though they inhabit an aquatic environment.[citation needed] Salmon utilize their sense of smell to identify and return to their home stream waters. Catfish use their sense of smell to identify other individual catfish and to maintain a social hier