Part two, entitled "Deep Thinkers," splits its time between dolphins and whales. Bottle-nosed dolphins are the stars, playing with bubble rings in central America, hydroplaning in western Australia, and beating mud to trap fish in Florida. A big focus of the dolphins' intelligence concerns getting food, as many of them have developed ingenious ways to do so. In the case of the Floridian variety, their talent has been passed down for generations, making it an impressive tradition. These skills are practical, but there are also some purely fun experiments, like how dolphins have learned to recognize themselves in the mirror at an aquarium.
When thinking of the smartest animals on the planet, dolphins naturally leap to mind. Whales, not so much, but it turns out that this is a misconception. Humpback whales are featured, in that they trap herring, as several dolphin species do with their own prey fish. Some attention is also given to gray whales in Mexico and their empathy towards people. They will swim up to the boats and interact, seemingly for no gain of their own. It's quite an unexpectedly advanced trait. What's more, dolphins and whales have spindle cells, which is something formerly believed only humans possessed. The possibilities for their minds, with the presence of these cells, are extraordinary.
Finally, episode three is called "Voices of the Sea." Having established that some dolphins and whales show signs of higher intelligence than most species on the planet, attention turns to some of the abilities they have that humans do not. There is a lot here about echolocation and bioelectical signals. The systems these animals have are quite elaborate. It has long been known that these sound waves can be used for communication, but they can also be used to hunt prey. This comes in quite handy, especially, for the boto dolphins who live in the muddy Amazon river.
What isn't known is exactly how the signals work, or how they can malfunction, which leads to sperm whales beaching themselves in Australia. Also a mystery is why humpbacks sing near Hawaii. Some answers will likely remain unknown to the human race, the two species being so different, without a lot of further study.