The Power of a Seal’s Whiskers

Seals are born with fully formed whiskers, which are highly sensitive to water movement. These whiskers develop in the womb. While functional at birth, pups gradually learn to use them effectively for hunting prey. For example, seals hunt fish by following the tracks and eddies that fish leave as they swim. Their sensitive whiskers can detect even the weakest currents in the water. However, fish make their job difficult by simultaneously creating three jets of water of different sizes.

New research by Yvonne Krüger, Wolf Hanke, Lars Miersch, and Guido Dehnhardt from the University of Rostock, Institute for Biological Sciences, Sensory and Cognitive Ecology, investigates “Sensitivity of the mystacial vibrissal system of harbour seals (Phoca vitulina) to size differences of single vortex rings.” Published at the end of September, the study reminds us of the hidden sensory world beneath the waves.

We spoke with the corresponding study author, Yvonne Krüger, but Filou completely stole the show.

As Yvonne Krüger explains, during escape, the fish generates three jets; two of them form into vortex rings, while the third, created in the final movement of the fish, has a less organized structure and does not develop into a vortex ring. She notes that all three jets, regardless of their shape, travel in different directions. “In contrast, during continuous swimming, the vortex rings generated by the fish all travel in the same direction, namely in the opposite direction of the fish’s swimming direction,” Krüger says.

How does it work?

The fish swims in one direction, then an escape reaction occurs, and it bends in the shape of the letter C (the so-called C-start) and moves in the other direction. When the seal gets close to the fish, it notices three different jets, which makes it difficult for it to determine where the jets are coming from.

“These multiple flow signals make it harder to determine which jet truly indicates the fish’s escape direction. By ignoring the non-vortex ring structure and comparing the two vortex rings regarding their size, it can determine which corresponds to the actual escape direction — the larger vortex ring being the one traveling directly opposite to the fish’s final escape path. This allows the predator to potentially reorient itself and continue the pursuit successfully,” adds Krüger.

Meet a 19-year-old Filou

This 19-year-old male harbor seal (Phoca vitulina), born in captivity and named Filou, was the star of the study. The name, which in French carries the sense of a ‘naughty child’ or someone, let’s say, resourceful, reflects his playful and clever nature. Filou carefully explored the rings in the water through his whiskers, learning to follow their direction. He was never deprived of food, and the experiments were conducted once a day, five days a week.

Thanks to Filou, scientists have found that seals can sense size differences of up to 17.6 mm using their whiskers, which helps them determine the direction of the fish’s escape. To prove this, Filou was trained to identify which of two vortex rings was larger while his eyes were covered by a latex mask. He had to touch one of two balls, depending on which side had the larger vortex, and was rewarded with a cut-off piece of herring if he guessed correctly.

After thousands of trials, Filou successfully distinguished between vortex rings of different sizes—even when the researchers switched the pairs so that one ring that was previously smaller (68.0 mm) was now larger than the other. And he correctly identified it then. Because the seals can distinguish these vortex rings very precisely, they can detect the direction of the fish’s escape and react quickly, outsmarting the fish’s tricks.

16 Years of Trust, Care, and Friendship with Filou

Yvonne Krüger: I started working with Filou when he was just 3 years old – he is now 19. Over time, we’ve built a very close bond. Working with him daily is something I truly enjoy. He is a constant in my life. The trust in me allows me to do everything from experimental sessions, including covering his eyes, to medical training and handling when he isn’t feeling well…I just returned from five months of parental leave last week, and as soon as he saw me, he immediately came over and stayed close for quite a while, without expecting a reward.

Harbour seals aren’t cuddly animals at all, but once a year, always in summer, he voluntarily approaches me on the haul-out area, snuggles into my lap, and stays there until his fur is completely dry, letting me pet him the whole time. That day is always the best day of my entire year – because he spends his spare time with me, completely voluntarily, and lets me pet him without the prospect of a fishy reward.
I think the vibrissal system is, in my view, one of the most fascinating sensory systems, not just because we humans don’t have it, but because it functions in complete darkness (whether that’s due to night, deep water, murkiness, or even vision loss).

How can this help in the research of other species and, for example, their escape strategies from predators?

Harbour seals can reach 1.85 m in length and weigh up to 168 kg. Females outlive males, typically living 30–35 years compared to 20–25 years for males. Pups can weigh up to 16 kg, and interestingly, they can swim and dive within a few hours of birth. About six weeks later, they begin to become more independent and start hunting for prey on their own.

However, as our knowledge of these lovable seals continues to grow, it can provide us with valuable insights that may also be applied to other species, for example, in studying their predator avoidance strategies. “I believe it would be incredibly interesting to extend our research to other species. At our facility, we also house two California sea lions and a South African fur seal – there’s definitely potential for more comparative studies,” concluded Krüger.

Image: Filou, Marine Science Center