The great, echoing moans of blue whales have gradually grown lower in pitch over the last few decades, and scientists say that could be a good sign for this endangered species.

Coinciding with the change in tune was a growth in overall population, leading researchers to believe the calls of the whales don’t have to travel as far to attract potential mates. Lower frequencies aren’t as loud, but take less energy to produce.

"The basic style of singing is the same, the tones are there, but the animal is shifting the frequency down over time. The more recent it is, the lower the frequency the animal is singing in, and we have found that in every song we have data for," says John Hildebrand of Scripps Oceanography.

Both climate change and an increase in human-produced noise in the ocean were examined as possible reasons for the change in pitch, or frequency.

But researchers believe that a much more positive force is at work — efforts to protect blue whales and grow their numbers. Blue whale populations have increased following international bans on commercial whaling.

"It may be that when (blue whale) densities go up, it's not so far to get to the closest female, whereas back when they were depleted it may have been that the closest female was a long way away," said Hildebrand.

In fact, the lower pitch may even be a way for male whales to show off to the ladies, says Hildebrand.

"When they make these songs they need to use most of the air in their lungs," he explained to Science Daily.

"It's like an opera singer that sees how long he can hold a note. The (male) songs are made to impress the females and/or other males, so I think that's how the boy blue whales are impressing the girls, or are showing off to other boys: by making a loud and long song."