To emit a hollow, loud, animal cry, as a bull or cow. (Dictionary.com)
To roar; bawl: (Dictionary.com)
To shout in a loud voice, or (of a cow or large animal to make a loud, deep, sound. (Cambridge Dictionary)
To make the loud deep hollow sound characteristic of a bull. (Merriam – Webster)
To shout in a deep voice. (Merriam – Webster)
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In both love and war, alligators signal size by bellowing.