Whale Songs Hint That Mating's Not Just for Mating Season
for National Geographic News
|April 20, 2004|
Humpback whales regularly break into "song" much later in the year than expected, according to a new study. The finding contradicts the idea that the songs are only associated with breedingand that the species' mating and feeding activities take place in completely separate time periods.
Humpback males were thought to sing mostly during the winter breeding season (October to March, in the Northern Hemisphere), when the up to 40-ton beasts migrate to tropical waters near Hawaii, the West Indies, and elsewhere.
The animals' haunting moans and long complex songsemitted only by malesare apparently used to woo females and to competitively display to rival males.
But now inadvertent recordingstaken off the coast of Cape Cod, Massachusetts, during May and Junereveal that humpback males also serenade potential lovers throughout spring, as they migrate to polar and other high-latitude feeding grounds.
As a result, humpback calves may sometimes be conceived and born outside of tropical waters, write researchers behind the chance discovery.
"It was a real surprise to find animals singing wall-to-wall, 24 hours a day through several weeks of spring monitoring," said Phil Clapham, a whale biologist behind the discovery. Clapham works at the Northeast Fisheries Science Center in Woods Hole, Massachusetts.
"Although it's clear that most mating takes place in winter in the tropics, the activity apparently doesn't end there," he said.
Humpback whale (Megaptera novangliae) song has rarely been recorded in high-latitude feeding areas. When it has, most recordings were made during early spring or late autumn.
In contrast, whale singing can be recorded endlessly throughout the species's entire winter breeding season in tropical waters. "Once in a while people had recorded song on feeding grounds," Clapham said. "But most experts thought it was very sporadic."
Clapham and Cornell University whale biologist Chris Clark, based in Ithaca, New York, made the discovery while collecting data for a study on the occurrence of critically endangered North Atlantic right whales (Eubalaena glacialis). Clapham and Clark's results will be published in an upcoming print edition of the science journal Proceedings: Biological Sciences.
Clark deployed self-surfacing recording devices three kilometers (five miles) apart from one another, 150 to 200 meters (500 to 650 feet) deep on the seafloor. (An acoustic signal from a nearby boat would trigger the 1-meter-diameter/3.2-foot-diameter glass orbs to automatically detach from the seafloor and float to the surface for retrieval.)
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.
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