Which trait shared by dolphins and bats possibly led to the evolution of echolocation in these two animal groups?

Dolphins and bats both need to find their way and locate prey in environments where visibility can be limited: dolphins underwater and bats at night. Echolocation helps them navigate and detect objects using returning sound waves, so the shared need to move through dark environments may have favored its evolution. Sending sound through air or water is not a trait they share, since dolphins live underwater and bats fly in air.

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