Indian Ocean whales are changing their caller tunes

Scientists are baffled by the changes in the frequencies of the blue whale calls in the Diego Garcia region of the Indian Ocean. The niggling worry, though unsubstantiated yet, is that it could be due to the noise made by ships.

For some years, Tarun K Chandrayadula, assistant professor, Department of Ocean Engineering, IIT Madras, has been studying blue whale movement by tuning into the mammals’ calls in the Indian Ocean using the hydrophones deployed by the Vienna-based Comprehensive Test Ban Treaty Organization (whose objective is to detect nuclear tests.)

One of the great whales, recorded frequently near Diego Garcia, has been changing its calls a great deal. The whale call consists of multiple frequencies of 35-45 Hz. By deploying signal processing techniques, the researchers identified that some of the call frequencies decreased over the years, and some increased.

While it is not for the first time that scientists observe a change in call frequencies, this instance is more intriguing because of its variation — an increase and decrease in frequencies — which makes it difficult to theorise a reason.

There is worldwide concern on the increasing ship traffic and anthropogenic activity in the oceans, and their effect on marine life. Whales help maintain ecosystem balance, as also the carbon pump.

Some of the other causes looked at for the changing frequencies were changes in whale population, and selection pressure caused by mating or food. “However, we could not pin down a specific cause,” says Chandrayadula. “It could be a combination of these or something else. Studying the animal in the field will hopefully reveal the answer.”

Life in acoustics

Blue whales are the biggest mammals alive. They reach sizes more than 25 m, which would equal five elephants standing side-by-side. They are great travellers, and move across ocean basins.

Acoustics are a big part of their life. The whales use low-frequency calls to communicate and look for food. They also make some of the lowest frequency sounds, around 100 Hz and less, a lot of which you would not be able to hear. Human sounds are at frequencies 40 times that of whales.

The frequency depends on the type of whale. For example, the Antarctic blue whale makes sounds at 27-28 Hz. And the Sri Lankan blue whales make sounds around 100 Hz.

The type of whale Chandrayadula has been observing using acoustics is still unidentified. It is found in Diego Garcia and not elsewhere. Also, unlike other blue whales, this whale is unique with call frequencies showing both increases and decreases.

If the purpose of the call is to communicate among the group for social cohesion, then there is not much incentive to change call structure. On the other hand, if the call is a mating display, then there is incentive to advertise higher sexual fitness than the others. Lower frequencies imply a bigger lung, and thus a larger body.

While this explains the decrease in frequencies, it still does not explain the increase.

“Is it because of ship noise, increase in whale populations, or whales trying to outdo each other in playing new tones… we don’t have a single answer,” Chandrayadula says.

COMMENT NOW