Deep below the ocean's surface, blue whales are singing, and for the first time, scientists think they know why. Researchers from the Scripps Institution of Oceanography recorded the sounds and say they offer new insight into the behavior of the passenger jet-sized animals. Using tags suctioned to the whales' bodies, researchers tracked the whales and found that as they feed, they send out calls to let each other know where they are, each group employing a different sound. The noises play a similarly important role during mating season when males sing long, low-pitched songs to indicate their reproductive fitness to females. Females select mates based on size and estimate that by evaluating males' songs: Larger males can take in more air and hold notes longer. The research appears in the January 25 issue of the Marine Ecology Progress Series journal.