Seals distinguish watery footprints
Harbour seals use their vibration-sensitive whiskers to detect the size and shape of objects that have passed through water, according to a study published this week in the Journal of Experimental Biology.
Seals often hunt in murky conditions, and previous research has shown that they can pick up the trail of water disturbance, the wake, from a fish even half a minute after it has passed. But scientists did not know if seals could tell what they were chasing, so they wanted to see if a seal could tell the difference between wakes from differently shaped objects.
To find out, they teamed up with the friendly harbour seal, Henry. The researchers moved differently shaped paddles - flat, undulating, cylindrical, or triangular - through the water in a special box, then released Henry so that he could investigate. Dr Wolf Hanke, one of the authors of the study, explained that the object was long gone before Henry reached the box. "Only the water movements were left behind." They trained Henry to press a target when he recognised a standard paddle, and to press a different target when he sensed the paddle was bigger, smaller, or differently shaped to the standard.
The results were striking. According to Hanke, "It could detect objects that differed by 4.4 cm in size". And Henry could distinguish all of the different paddles shapes from each other except the triangular paddle, which to him was too similar to the cylindrical or undulating shapes.
"The seal is blindfolded ... and the hearing is excluded by headphones" explained Hanke. It is highly likely that Henry was using his mystacial vibrissae, or whiskers, as covering these meant he was no longer able to distinguish wakes.
To visualise the patterns in the water that Henry was detecting, the used a technique called particle image velocimetry. This involves shining a laser beam through water that has been is 'seeded' with buoyant particles. The movement of these particles is measured, and the patterns of flow in the water can be analysed.
They found that, like the paddles, different fish produce very different wakes. "We applied the same laser technique to fish. And the fish showed very different patterns in their wakes. This is important because different fish species have different lipid content and different value for the nutrition of the seal." So this could be how seals recognise the best fish to chase and avoid wasting their energy on fruitless hunts.
In the future, Hanke and co-workers plan to use a vortex generator to produce more pure water disturbances that are easier to analyse.
"We don't yet know which part of the wake is most important for the seal. But this can be overcome by a new stimulus generator."
Dr Wolf Hanke, and author of the study, explains how they found that seals can distinguish different wakes using their whiskers: Interview with Dr Wolf Hanke