Optic flow, a rich source of optic information for harbour seals (Phoca vitulina).

Marine mammal vision is often considered to only provide limited information, particularly underwater in low light levels and turbidity. However, when these animals move through turbid water optic flow is elicited. A past study has documented the harbour seal's (Phoca vitulina) ability to perceive deviations from heading from optic flow simulating movement through a volume of turbid water. Here, we asked whether harbour seals are also able to perceive and analyse surface optic flow. Thus, we simulated three optic flow environments and trained three harbour seals to determine the simulated heading. The harbour seals precisely indicated their heading with a mean (±s.d.) accuracy of 4.61±0.56 deg for volume optic flow, 4.96±0.74 deg for surface optic flow mimicking movement over a surface and 3.58±1.12 deg for surface optic flow mimicking movement underneath a surface. We conclude that harbour seals have access to and can thus rely on optic (flow) information whenever there is enough light for vision, thus refuting existing opinions about poor visual guidance in harbour seals or, more generally, in marine mammals. A detailed analysis of optic flow perception in (semi-) aquatic animals is expected to enhance our understanding of optic flow perception and vision in general.