Gaming seals reveal how cloudy water provides sense of direction

Open water swimming can be strangely claustrophobic. Immerse your face in cloudy water and your view might dwindle to a few centimetres. Yet, harbour seals (Phoca vitulina) manage to negotiate the turbid coastal waters in which they reside with ease, detecting their surroundings through various senses, including their dextrous whiskers. But Frederike Hanke from University of Rostock, Germany, wondered whether the resourceful creatures may also to use their vision to determine which direction they are manoeuvring in, despite the opaque view. ‘We wanted to know whether harbour seals can determine their heading from optic flow fields – the pattern of motion on the retina generated as a seal moves past visible objects, including particles in the water, in their surroundings’, she says. But to do so, she and her colleagues, Laura-Marie Sandow, Ann-Kathrin Thimian (both from University of Rostock) and Markus Lappe (University of Münster, Germany), would have to set some harbour seals a gaming challenge. They publish their discovery in Journal of Experimental Biology that images of particles in cloudy water moving across the retina at the back of they eye are sufficient to tell diving seals which direction they are moving in.

The team designed three computer simulations to show their seal ‘colleagues’: one which simulated a seal moving through the open sea with blocks of dots streaming toward them out of the screen; the second simulating the seabed passing beneath them with a plane of dots rushing toward the viewer; and the final one simulating the surface of the sea flowing above the viewer’s head with another plane of dots. Then, the team encouraged three seals, Nick, Luca and Miro, to shuffle into place in front of a large screen showing the simulations so that they could indicate which side, left or right, they appeared to be moving toward, based on the movement of the dots, by touching a red ball on either side of their heads in return for a reward of tasty sprats. Fortunately, Nick and Luca were gaming veterans, so picked up the task with ease, but Miro was a novice, so it took a while for him to learn – but Hanke recalls that Miro is a very open-minded seal, ‘so coped with all new situations easily’.

Once the seals were comfortable with the game, Sandow and Thimian kept track of the seals’ choices as they watched the dot simulations, which appeared to show the seal heading in a direction that was 22, 18, 14, 10, 6 or 2 deg to the left, or similarly positions to the right. Then, the team plotted the animals’ successes as they determined which direction they appeared to be travelling in, although Hanke explains that they sometimes made mistakes. ‘These are living animals, not robots. Errors are most likely due to inattentiveness or sometimes a drop in motivation’, she chuckles. And when the team plotted the seals’ successes, it was clear that the animals were perfectly capable of determining which direction they were travelling in based simply on the dots streaming in their view; exactly as the cloudy water, seabed or water surface would appear to move when they are swimming.

Even in the faintest light, harbour seals are able to use their vision and take advantage of cloudy water to determine which direction they are travelling in based on the apparent movement of objects and particles in the water streaming past their eyes. And now Hanke is keen to find out whether the skilful creatures are able to use this visual effect to determine how far they have travelled, ‘which is interesting in the context of navigation’, she says.

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REFERENCE: Sandow, L.-M., Thimian, A.-K., Lappe, M. and Hanke, F. D. (2025). Optic flow, a rich source of optic information for harbour seals (Phoca vitulina). J. Exp. Biol. 228, jeb250168. doi:10.1242/jeb.250168

DOI: 10.1242/jeb.250168

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