Researchers in California say the voices of male blue whales have gotten lower by 31 percent since the 1960s.
Blue whale songs are getting deeper, say baffled scientists.
Mark McDonald of WhaleAcoustics, which conducted the study with researchers from the Scripps Institution of Oceanography in California reported in the journal Endangered Species Research that recording devices off California had to be adjusted to capture the lower frequency calls.
The singers have always been found to be males traveling at relatively high speeds but why their voices have lowered is a mystery.
Mr McDonald said one theory was the impact of an increasingly noisy ocean. But he said if blue whales are struggling to make themselves heard over the sound of marine traffic, their voices should become higher, rather than lower.
It has also been suggested that climate change has led to the seas becoming more acidic and lowered voices could travel better in these waters. However, scientists said this still doesn't explain the extent of the drop.
The favoured theory is that males do not have to call as loudly to be heard by females because their numbers are slowly increasing following a whaling ban in 1966.