The devices are capable of detecting the vocalizations of right whales up to 30 kilometers (19 miles) distant.
For 25 days in May and June 2000, the pop-up recorders automatically detected sounds for a handful of minutes in every hour and stored the information on a hard drive.
When Clapham and Clark analyzed the data, they were in for a surprise. Alongside some recordings of right whales, Clark said, "mostly what we found was [one or more] humpback whale [songs], 24 hours a day for almost the entire spring."
The recording devices had been deployed in an important humpback feeding ground east of Cape Cod. "This was the first time that anyone had continuously monitored one of these areas for a period of several weeks, using devices of this range," Clark said.
The biologists believe that the consistent nature of the whale song indicates that mating continues long into the spring migration, even after humpbacks have arrived in summer feeding grounds. Clapham said the recordings hinted "that the humpback's mating system is rather more flexible than we thought."
At the time, the pair knew they would need more evidence to back up that claim. Since whale pregnancies last approximately 12 months, most calves are born in the same tropical breeding grounds in which they were conceived the prior year. Matings outside of the winter season should therefore result in later births.
To find the proof they required, Clapham and Clark examined whaling records from the 20th century. Whalers had collected demographic data on slaughtered whales, including the sizeand, by extrapolation, the developmental stageof fetuses.
"By looking at data on the size of fetuses you can get a pretty good idea of how births are distributed across the year," Clapham said.
As expected, the whaling records revealed that most calves were born between December and April. However, there were rare examples of unseasonably large or small calves that would have been born in spring or even summer, Clapham noted.
John Calambokidis, a whale expert with Cascadia Research in Olympia, Washington, commented that while occasional whale singing in feeding grounds in autumn and spring was known to researchers, "there has not been good documentation of to what degree this occurs, how late or early it extends, and the possible explanations for this."
Calambokidis said the "important contribution" by Clapham and Clark does a good job addressing such questions.
Clark and Clapham said they plan to monitor whale feeding grounds throughout the summer to see if song tapers off after spring.
For more whale news stories and information, scroll down.
SOURCES AND RELATED WEB SITES