Tape recordings were made of the underwater noises of captive sea lions swimming in a concrete pool at night. When approaching pieces of fish that were thrown into the water, the sea lions emitted trains of sound signals like those of the bat and the porpoise. A detailed analysis of these noises shows that they meet the criteria of a pulse-modulated sonar system and, in fact, reveal an amazing sophistication so far as echo ranging is concerned.
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Thomas C. Poulter (1963) studied this question.
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