Seals judge size using their whiskers
Seals are able to judge the size of an object using their whiskers, according to research.
The mammals are known for their touch-sensitive whiskers but scientists wanted to know exactly how they size up their prey.
Researchers from Manchester Metropolitan University found that the animals move specialised whiskers towards an object to measure it.
The results also showed how quickly seals could judge size.
The findings are published in the Journal of Comparative Physiology A.
Dr Robyn Grant travelled to the marine science centre at the University of Rostock in Germany to study the whiskers of two harbour seals named Marco and Moe.
Recent hydrodynamic research showed how seals can size up fish based on the wake they leave as they move through the water.
"We thought it would be good to do more of an in-depth study to look at how they do it rather than just whether they can do it or not," explained Dr Grant.
Many species use whiskers as touch sensors and animals such as rats and mice move their whiskers around in order to get more information about their environment.
For seals, however, this would require a lot of energy expenditure, according to Dr Grant, because water is denser than air and takes more effort to move through.
"I thought they would probably do something more like humans," she told BBC Nature. "Humans detect size by calculating the span of their fingers."
But instead of using their whiskers in the same way as we use our fingers, the seals showed a specialised approach.
Wearing eyemasks and headphones to restrict their other senses, the seals were trained to take part in an experiment in which they had to sense two different size discs using just their whiskers.
To test whether they could tell the difference in size, the seals had to touch their nose to the correct object - the larger disc for Moe and the smaller disc for Marco - for a fish reward.
Dr Grant and colleagues filmed the interactions to understand precisely what was happening.
"They orientate to this special place on their muzzle which is where the whiskers are really small and really densely packed," said Dr Grant.
She explained that these whiskers acted as a "higher-resolution sampling space", meaning that the seals could gather lots of information from one spot without moving all of their whiskers.
"They can press [their muzzle] on [the object] and by the number of whiskers it contacts, they can work out whether it's a bigger or smaller thing."
This brushing technique allows the seals to gauge their prey in water where visibility is often poor.
Dr Grant suggested that it might also be a quicker way to work out the size of an object because it did not require more complicated span calculations.
"We've found that they do it super-quick - all their decisions are made in under half a second," she said.
The biologist proposed that comparative tests in humans might examine which method of measuring size was faster.