What can a humpback whale hear?

What can a humpback whale hear?

Crickets’ ears are located on their knees; snakes’ tongues are sensitive to sound vibrations; and elephants use low-frequency sounds beyond the range of human hearing to communicate with each other long distance.

We know that whales sing, but can they hear?

So much is known about how most animals hear, but when it comes to humpback whales, there’s still a surprising dearth of knowledge, says Rebecca Dunlop, Associate Professor in Physiology within the School of the Environment at the University of Queensland.

“With all of the baleen whales, we can predict what they can hear, but we don’t know for sure,” says Dunlop.

“We don’t know their hearing range, in terms of what frequencies they can hear, and we don’t know how sensitive they are at each frequency.”

The sheer size of these animals compared with seals and dolphins, has so far limited scrutiny.

“With something like a humpback whale, they’re the size of a double-decker bus and that’s the small ones,” says Dunlop.

“You can’t catch them, you can’t train them and you can’t look at their brain function when sounds are being played.”

Last week Dunlop and the team were braving a small boat on the seas off Peregian Beach on Queensland’s Sunshine Coast to unravel the secrets of humpback hearing as these giants of the ocean migrate from the Great Barrier Reef towards the Southern Ocean.

The project is funded by the US Navy’s Living Marine Resources program, of which one of the primary aims is to study the impact of man-made underwater noise on whale behaviour.

It’s gruelling field work which involves getting up at the crack of dawn for six long weeks to track down the massive marine mammals.

We know that whales sing, but can they hear?

“When whales migrate, they tend to pick a certain distance offshore and a certain direction and they don’t really budge from that – they just migrate down the coast,” says Dunlop.

The researchers steer a boat ahead of the predicted pathway, anchor up and play sounds at nominated frequencies.

In 2021, the research team established that whales hear frequencies of 1, 4, and 16 kHz; in 2022 they added 250 Hz; and this year they will be investigating whether they can hear up to 22 kHz and down to 63 Hz. 

(By comparison, adult humans’ hearing is usually 15 kHz and to 20 Hz.)

“Instead of training (the whales), we’re assuming that if we play a sound at them, and they change their behaviour, then they must have heard that sound,” Dunlop says.

The changes in behaviour are usually very subtle – for example, the whales might pause, change course slightly, or come over to investigate the source of the noise.

It seems to be that they first stop briefly to assess the sound, then carry on migrating in a slightly different direction and it’s these subtle changes in behaviour that we are trying to pick up.

Dr Rebecca Dunlop

“As soon as we detect that they’ve changed their speed, or changed their course in any way, that’s when we assume that they’ve heard the signal,” she says.

Some of the whales are tagged as well, with an instrument that provides a three-dimensional profile of how they are moving through the water. 

The tag, which picks up even more subtle changes in behaviour, is held on by suction cups and falls off approximately three hours after it is put on. It floats until the team come and retrieve it.

“What you’ll see from the tag is they’ll go down to the bottom of the ocean, they might roll a bit to try to get a sense of what the signal is, or they might come up to the surface,” Dunlop says.

“It seems to be that they first stop briefly to assess the sound, then carry on migrating in a slightly different direction and it’s these subtle changes in behaviour that we are trying to pick up.”

But most of the whales aren’t tagged. 

Approaching the animals with a suction cap tag on the end of a long pole is “hit and miss” process, with only one whale being tagged every three days on average, says Dunlop.

A volunteer team of 12 marine science students also keep a lookout and collect behavioural data from the top of Emu Mountain, between Peregian Beach and Coolum.

“The way we’ve designed our experiments is that the tagging is the icing on the cake, but the real data comes from the hill,” Dr Dunlop says.

“The volunteers have a laptop and track the whales as they move down the coast, and their position is fed into a networked tracking program, developed by surveyor Dr Eric Kniest.

“I can see what the volunteers are doing on my laptop when I’m on the boat, and use this information to position the other boat that plays the sound.”

But there are plenty of other challenges inherent in the research – like inclement weather.

“As soon as white caps appear (with winds of 8-10 knots) it becomes very hard to actually see and keep track of the whales,” Dunlop says.

“And I’m in the back of a very small boat with a computer – and taking waves over the side is not really good for the computer – or my seasickness,” she says. 

Then there’s noise from other boats, including whale watching vessels, that can cause whole reams of data to be thrown out.

“There are factors that you can’t control that make life quite difficult,” she says.

In 2010 to 2015, Dr Dunlop was part of a team that discovered air guns used for marine exploration were loud enough to affect humpback whales up to three kilometres away, potentially changing their movement patterns as they migrate.

“The ocean is a very noisy place, particularly with oil and gas exploration and naval sonar,” Dr Dunlop says.

“We have a lot to learn about how these sources of noise affect whales.

It’s still unknown as to whether it stops them feeding or breeding, or is responsible for mass beaching.

Dr Dunlop’s research is used to inform government policy.

“Now, instead of guessing what whales can hear, we can come back with real data and say, ‘Here’s where they’re most sensitive. Here’s the frequency ranges that they can hear’,” she says.

The ocean is a very noisy place, particularly with oil and gas exploration and naval sonar

Dr Rebecca Dunlop

UQ’s unofficial whale watching team is also looking at many other aspects of whale behaviour.

In May research published in Proceedings of the Royal Society B revealed that humpback whales sing louder when the wind is noisy, but don’t have the same reaction to boat engines.

Research lead Dr Elisa Girola from UQ’s Faculty of Science suggested that this quirk of whale evolution could have consequences for breeding and behaviour.

“Humpback whales evolved over millions of years with noise from natural sources but noise from man-made vessels is foreign to their instincts,” Girola says.

“It’s a surprising finding given engine noise has a similar frequency range to the wind.

“It’s possible the whales are picking out other differences such as wind noise being broadband and the same over large areas, while vessel noise is generated by a single-point source with specific peaks in frequency.

“We don’t know yet if this lack of response to boat noise is making whales communicate less effectively or making breeding practices more difficult.

“Male humpback whale singing is probably used to mediate reproductive interactions, but we can’t say if vessel noise is interfering.”

It’s still unknown whether this finding has arisen due to whales being able to discriminate between audio signals coming from different directions, or other factors.

“There’s still so much more research to be done,” Girola says.

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