Scientists have long figured that the song of the blue whale, largest animal on the planet, had something to do with mating.

Now they know for sure, thanks to a pair of related studies that researchers at Scripps Institution of Oceanography at UC San Diego spearheaded.

Scripps postdoctoral researcher Erin Oleson and scientist John Hildebrand also studied recorded whale calls from throughout the world's oceans and determined that there are nine distinct sets of blue whale populations.

Published in the Jan. 25 issue of the Marine Ecology Progress Series journal, their work is the first that documented in detail what blue whales were doing, and how deep they were in the water, when they were singing. The scientists accomplished that by pulling up close to whales in a boat and attaching -- with the aid of a long pole -- radio-transmitting probes with suction cups onto the animals' backs, Oleson said.

Between 2001 and 2005, the scientists placed probes on 35 blue whales off the coast of California -- and a third of those were recorded making sounds, Oleson said.

The majority were males trying to catch the attention of females who they hoped had wandered into their underwater neighborhoods, she said. They were singing songs for minutes or hours at a time. The songs were characterized by two alternating notes, each lasting 15 to 20 seconds, and tended to be quite loud.

"It is a way of advertising themselves to the female," Hildebrand said. "He is trying to convince her that he is a big dog and she would be a fool to go out with any other male in the blue whale bar."

Hildebrand suggested that it is not difficult to see why serenading is important.

"You'd be hard-pressed to see even the tail under water," he said. "You might as well have another cue, such as sound, to judge the size of the male and whether he is fit to mate with."

In three cases, sounds were made for other reasons.

Oleson said two females and a male were observed making short, isolated, single-note calls lasting no more than five seconds to communicate with other blue whales known to be swimming nearby. That so-called downswept call is often used by a group of whales to keep track of one another's locations during feeding, for example.

"It's like shouting to someone across the room," Hildebrand said.

When the scientists compared the song of coastal California blue whales with the recordings of cousins in other parts of the world, they found that the local sound was distinct.

"If you go out into deep water, there is a totally different type of song," Hildebrand said. "And if you go down to South America, there is yet another type of song."

Interestingly, he said, the song of the coastal blues was more complex than the songs of whales inhabiting deep parts of the ocean. He said that is probably because coastal whales tend to stick closer together, so sound doesn't have to travel as far to reach an intended target.

"And they can afford to be a little fancier," Hildebrand said.

The scientists said that about 2,000 blue whales swim and frolic off the coast of California part of the year. They spend winters in the warm waters off Central America, then journey north in spring to feast in the krill-rich waters near the Channel Islands. The massive mammal's diet consists mostly of the small, shrimplike fish.

"For being such a big animal, they are kind of picky eaters," Hildebrand said.

"When they arrive here, they are kind of skinny," he added. "You can see their backbones sometimes."

Once fattened up, the whales -- which can grow as large as 100 feet long, or as long as a Boeing 737 -- turn their attention to mating and composing songs.

Through the different songs, the scientists were able to separate blue whales into the nine distinct population groups they call stocks, each with about 1,000 animals, Hildebrand said.

The scientists' findings have major implications for conservation efforts because each group -- not just the overall population -- will need to be protected, he said.

"It's a very significant thing to remove even one animal from one of these stocks," Hildebrand said.

At one point, the entire population was at serious risk. Before whaling began on a large scale in the 1800s, the blues numbered about 300,000 worldwide, Hildebrand said. By the mid-1960s, their numbers had dwindled to 2,000.

"That was a result of large-scale commercial whaling by many different countries," Hildebrand said. "They took it down to the point that it nearly went extinct."

Thanks to the 1966 worldwide whaling ban, populations have rebounded to 10,000, he said.

Contact staff writer Dave Downey at (760) 740-5442 or ddowney@nctimes.com.