Researchers Use Rocket Scanner To Give Minke Whale A 'Hearing Test'

The 2018 Experimental Biology Conference taking place this week in San Diego, California, has certainly proved to be extremely interesting so far. Following the April 21 revelations regarding the hidden threats of artificial sweeteners and the April 22 symposium on hominin bipedalism that showed early humans learned how to walk upright before Homo sapiens sapiens even appeared, an exciting presentation held on April 23 revealed the intricate biomechanisms that allow whales to hear sounds.

Based on research by evolutionary biologists at San Diego State University, the presentation focused on baleen whales and how they are able to pick up low-frequency sounds underwater.

Since sound is the most efficient way of passing on information underwater and it also alerts marine creatures to approaching predators, the team wanted to see how baleen whales receive and process the low-frequency sounds that they use to communicate with each other.

In order to get the big picture (pun intended), the team performed the first-ever CT scan of an entire minke whale, reports. And, because of the sheer size of this imposing animal, they had to do it using an industrial rocket scanner, designed for solid-fuel rocket motors.

"You can imagine that it is nearly impossible to give a hearing test to a whale, one of the largest animals in the world," said lead researcher Ted Cranford in advance of Monday's presentation.

Baleen whales are among the planet's largest cetaceans, and although minke whales are the second smallest in the group, they still can grow up to 35 feet in length. This posed some challenges to the scanning process, which the researchers said it took more than two years to schedule because, as you'd expect, a specialized rocket scanner is not easy to come by.

In order to reduce the time required to scan the entire whale, the team resorted to cutting the specimen in half and scanned both pieces at the same time, then reconstructed the complete specimen on a computer, notes.

No Whales Were Harmed In Process Of This Experiment

That is because the minke whale specimen, provided by the Smithsonian Institution Support Center in Suitland, Maryland, was borrowed from a freezer where it had been stored since 2012.

The CT scans, performed on an 11-foot minke whale calf, were later combined with custom-developed computer simulation, which used an advanced technique called finite element modeling (FEM) to add tissue elasticity measurements to the bone density data revealed by the rocker scanner.

According to Cranford, this allowed his team "to simulate the biomechanical processes of sound reception" in minke whales and to estimate their audiogram, or hearing curve, based on their anatomic makeup.

"Scanning an entire minke whale and applying our FEM tools allowed us to predict how well the whale can hear across a range of frequencies," he explained.

This is the first time that anyone has done this, states, mentioning that Cranford's team predicted audiograms for both minke whales and fin whales, another species of the baleen group.

According to the results, the skulls of baleen whales play an important role in the biomechanism that allows them to hear low-frequency sounds. The team ran simulations based on the CT scans and found out that sound waves vibrate along the skull and then travel through the soft tissue of the head.

Minke whale swimming under water.
Minke whale swimming underwater.

Another important discovery was that minke whales best pick up low-frequency sounds that come from directly ahead. The team believes this might be the first piece of evidence ever uncovered that suggests whales actually have directional hearing to help them locate sound sources, such as other whales or oncoming ships.

The researchers were equally puzzled to uncover that minke whales are also surprisingly sensitive to high-frequency sounds. Until now, it was believed they pick up frequencies of up to 1-2 kilohertz in order to hear each other. However, the simulation revealed that they can actually hear high-frequency sounds ranging between 10 to 40 kilohertz.

Since this was only found in the minke whale audiogram and doesn't occur in fin whales, the team speculates this increased sensitivity to high-frequency sounds might help them hear the sounds produced by killer whales, their main predator. The much larger fin whales have no need to be alerted to killer whale sounds, notes, because they're not part of the menu.