Aerial visual acuity and underwater visual acuity were measured in two sea lions (Zalophus californianus) by training them to emit click bursts if they saw a striped target or to remain silent if they saw a gray target. The closest grating spacings that could be resolved both in air and under water subtended a visual angle of 5.5 minutes of arc at a distance of 5.5 meters.
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Schusterman et al. (1970) studied this question.
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