Gazing into the distance, we adjust our focus and unconsciously scan our eyes back and forth across objects that capture our attention. So what do echolocating animals do when performing the acoustic equivalent? Explaining that echolocating porpoises produce intermittent echolocation clicks – listening for the reflection before clicking again – Danuta Wisniewska from Aarhus University, Denmark, adds that stationary porpoises can adjust the depth of their ‘acoustic gaze’ by decreasing the click rate as the target is moved farther away and increasing the volume. In other words, they can adjust their gaze by matching the click rate and volume to the distance from an object. However, echolocating animals are rarely stationary in the wild, ‘So I thought it would be an interesting problem to see how the animals use their echolocation while performing a more natural task’, Wisniewska says. Fortunately, Aarhus is just a two hour drive from the Fjord&Bælt centre, home to three trained porpoises – Freja, Eigil and Sif – who are old hands at working with research scientists, so Wisniewska and her colleagues Kristian Beedholm, Magnus Wahlberg and Peter Madsen travelled to the aquarium to find out whether the moving animals adjust their click rate and volume to match their acoustic gaze to their distance from objects (p. 4358).
‘The nice thing about Fjord&Bælt is that the porpoises live in a rich natural environment, so they have maintained their echolocation’, explains Wisniewska. The Fjord&Bælt trainers patiently taught each of the porpoises to approach and recognise an aluminium sphere suspended in the water with their eyes covered. Then the team trained the blindfolded animals to distinguish between the submerged aluminium sphere and another sphere (made from Plexiglas, PVC, brass or steel), with stainless steel the most difficult to distinguish because of the similarity in density to aluminium.
Once the porpoises were comfortable with the task, Wisniewska began recording their echolocation clicks to discover whether the porpoises directed their gaze and matched the volume and click rate to their distance from the target object. Positioning two small hydrophones just above each of the spheres to record the incoming echolocation clicks, the team also attached a digital tag – designed by Mark Johnson – to the back of each porpoise to record the acoustic reflections. In addition, they filmed the porpoises so that they knew the animals’ locations at all times to allow Wisniewska to estimate the distance of the animals to the targets.
After painstakingly synchronising all three systems, Wisniewska eventually recorded 95 successful trials. When she analysed the click rates and volumes, Wisniewska could see that the animals were controlling the direction of their echolocation beams and the acoustic gaze with high precision. Scanning the acoustic beam back and forth across the two spheres as they approached, the porpoises accurately adjusted the click rate and volume to match their gaze to their distance from the targets before switching to a continual buzz of clicks during the final moments as they closed in on, and correctly identified, the aluminium sphere.
However, the porpoises didn’t always get it right. Wisniewska explains that on some occasions, the animals mistook the sphere made from one of the other materials for the target aluminium sphere. However, having realised its mistake at the final moment, the porpoise was able to swiftly turn its gaze back to the aluminium sphere, instantly selecting the correct click rate for that distance. ‘They remember where the object is, they use spatial memory and they control the click rate to match their gaze to the distance from the object’, explains Wisniewska.