How seals read a fish’s escape plan
In the dim world beneath the waves, predators and prey play a high-stakes game of pursuit. Seals have long been admired for their ability to track fish by sensing the water’s flow—especially the wakes and spinning vortices that hint at a fish’s direction. A new study from the University of Rostock reveals a more cunning layer to this chase: escaping fish may emit two jets that generate two vortex rings of different sizes. If a seal can discern which ring is larger, it could infer where the fish is heading next, potentially turning a tail-chase into a successful ambush.
The discovery: whiskers that read vortex rings
Researchers Krüger, Hanke, Miersch and Dehnhardt report that the mystacial vibrissae—those sensitive whiskers along a seal’s snout—can detect minute width differences between vortex rings in a wake. Their experiments show that, by relying solely on whisker input, a seal could distinguish rings that differ by as little as 17.6 millimeters. This capacity would let a predator anticipate a fleeing fish’s escape direction even when two simultaneous jets create nearly simultaneous, opposing forces in the water.
The experiment: training Filou to feel the difference
In Rostock, a harbour seal named Filou lived at the Marine Science Centre and was trained to submerge his head while blindfolded. The scientists introduced a spinning vortex ring on one side of his head, sometimes visualizing the ring with a harmless dye. Shortly after, they released a second ring on the opposite side—either larger or smaller than the first. Filou indicated which side produced the larger ring by tapping one of two green balls positioned near his head, earning a fish reward when correct. As Krüger notes, the concept took time to grasp, but the whisker-driven task became a robust test of tactile perception.
Measuring the threshold
The team started with rings ranging from 89.9 mm to 45.8 mm in diameter, reporting that Filou achieved accuracy well above 80% in identifying the larger ring using only his whiskers. To confirm the finding, the researchers swapped the pairs so that a previously smaller ring became larger, and Filou still chose correctly. Across months and thousands of trials, the seal demonstrated reliable discrimination at a minimum difference of 17.6 mm, supporting the claim that the mystacial vibrissal system is exquisitely tuned to detect subtle hydrodynamic cues.
Why this matters for predators and prey
The practical implication is that a predator equipped with such whisker sensitivity can “see” through a potential decoy. If an escaping fish squirts water in two directions, creating rings of different sizes, the seal could latch onto the larger vortex as the strongest clue to the fish’s intended path. In other words, whiskers may provide an edge in a game where the slightest cue can determine whether a hunt ends in a successful capture or a glancing miss.
Where to read the original work
The findings are reported in the Journal of Experimental Biology (2025) by Krüger, Hanke, Miersch and Dehnhardt. The study, titled “Sensitivity of the mystacial vibrissal system of harbour seals to size differences of single vortex rings,” demonstrates that the whiskers are capable of fine-grained hydrodynamic perception. Journal reference and DOI: Krüger, Y., Hanke, W., Miersch, L. and Dehnhardt, G. (2025). Sensitivity of the mystacial vibrissal system of harbour seals (Phoca vitulina) to size differences of single vortex rings. J. Exp. Biol. 228, jeb249258. doi:10.1242/jeb.249258. For reporting online, see https://journals.biologists.com/jeb. Note: this article is posted for advance access and full attribution is required; online reports should include a link to https://journals.biologists.com/jeb and mention the source.
Embargo details: This story is release-appropriate with permission and attribution to the Journal of Experimental Biology. See the cited DOI for the official record.
