The booming melodies sung by fin whales are low-frequency love balladsdesigned to attract a mate, according to new US research. Dick Ahlstromreports
Fin whales use booming songs to help them find receptive mates but noise pollution in the deep oceans may be thwarting their efforts. Ships, military sonar and acoustic research may serve to drown out the love songs, putting the future of these amazing animals in doubt.
The low frequency vocalisations of fin and blue whales are the most powerful and ubiquitous biological sounds in the ocean. Difficulty in pinpointing the animal responsible for the songs made it hard, however, to know for sure what all the singing was about.
Dr Donald Croll of the University of California, Santa Cruz and colleagues decided to get to the bottom of it by towing microphones and sound
localisation equipment near fin whales and then checking the sex of the singer. Their results are published this morning in the journal,
Nature.
The key discovery was that all of the songsters were males. "This finding indicates that (the vocalisations) may function as male breeding displays, and will help to focus concern on the impact of human-generated low-frequency sounds on recovering whale populations", they wrote.
The low-frequency component in the songs of both fin and blue whales is extremely powerful. The 15 to 30 Hz (cycles per second) vocal sequences
of fin whales, Balanenoptera physalus, can reach intensities of 184 to 186 decibels.
This allows the music to travel far and wide in the hope of attracting a distant mate.
Unlike the closely related humpback whale, fin and blue whales do not tend to return to specific areas for breeding. They choose instead to congregate where the food supplies, mainly krill, are plentiful.
This is a particular disadvantage for this species. With no "local" where prospective partners can meet, mating pairs can find one another if the distinctive whale song is heard.
Low frequency sounds are particularly effective for long range underwater communications, ideal for the male fin whale hoping to entice a visit
by a receptive female.
The researchers had to prove, however, that these sounds were being made by males and not by all whales.
The source of the sound was tricky to identify but the sound localisation equipment pinpointed the animal making the noise. The researchers then dispatched a skiff to take a quick biopsy, a tissue sample that would confirm the sex of the singer.
"Vocalisations were produced only by male fin whales, despite a one-to-one overall sex ration in the area. This sexual dichotomy in vocal behaviour supports the idea that the patterned sounds of fin whales are male breeding
displays," the authors report.
"We propose that these displays serve to attract females from great distances to aggregations of patchily distributed prey (Krill)."
Human activity, therefore, spells trouble for the fin whales on two fronts. Populations were reduced considerably over the years due to commercial whaling, which greatly reduced the likelihood of finding a mate.
Now underwater sound pollution may be causing so much background noise that the females won't be able to hear the singing males.
"Our results help to focus growing concern over the effects of human-produced sound on Balaenoptera," the authors write. Sound levels
from commercial ships, military sonar, seismic surveys and ocean acoustic research are "extremely high", between 190 and 250 decibels.
In addition, there has also been an increase in the amount of human produced sound in the 15 to 30Hz frequency range favoured by the whales.
"An increase in ambient noise could thus reduce the distance over which receptive females might hear the vocalisations of males," the authors
conclude. "To the extent that growth of Balaenoptera populations is limited by the encounter rate of receptive females with singing males, the recovery of fin and blue whale populations from past exploitation could be impeded by low-frequency sounds generated by human activity."