Blue whales, the earth's largest animals, call to others of their kind, though exactly what these cries communicate remains a mystery. But these sounds have begun evolving (演变). Since at least the 1960s,the pitch (音调) of Antarctic blue whales has downshifted. Scientists have theories as to why: all involving humans.
The deepening of their sounds is not unique. Many blue whales around the world have also dropped their pitch. In a study last year that analyzed more than 1 million individual recordings of whale calls, scale shifts were found across species, and among populations that don't necessarily interact with one another. This is to say, whatever has caused the change doesn't seem to have a specific geographic origin.
The underwater noises caused by ocean traffic and at-sea industries might seem a likely criminal. However, scientists have identified lowered pitches even across populations of whales that live in seas without major shipping routes.
One possible explanation for the change is the achievements of global conservation efforts. As their populations have grown, then, the whales may have decreased their volume because they are more likely to be communicating over short distances.
Last year's study of whale calls also suggests a threatening reason for the drop in pitch, however. Perhaps whales don't need to be so loud because sound waves travel farther in oceans made acidic by the absorption of carbon dioxide.
Carbon dioxide in the atmosphere, meanwhile, may also indirectly influence whale voices in other ways. Recent study shows that, during the summer, the whales must use their top volume to be heard in the cracking ice — a natural sound increased by unnatural processes, as rising temperatures worsen ice-melt. So the impacts of a warming planet maymodulateanimal sounds even in remote places.
