Dolphins are one of the most intelligent species on the planet, which makes them a very interesting animal to scientists. In their natural habitats, dolphins use various vocalization techniques. They whistle and squeak to recognize members of their pod, identify and protect their young, and call out warnings of danger. They also make clicking sounds used for echolocation to find food and obstacles in dark and murky waters.

Question 2: In line 2, the word vocalization is closest in meaning to _______.