Near the end of his book, “Are Dolphin’s Really Smart? The Mammal Behind the Myth,” Justin Gregg tells a story about traveling on an eco-tour vessel as part of his research on bottlenose dolphins.

He was snorkeling with some tourists when the group encountered an aggressive dolphin snapping its jaws and charging other males. Back on the boat, one of the tourists described the “friendly and playful dolphins” she’d seen. Gregg told her that the dolphin had not been playful at all — in fact, it had been possibly dangerous — but the woman ignored him, charmed by the creature’s “playful smile.”

(Wilfredo Lee/Associated Press)

“Oh, you mean the smile that . . . is permanently frozen in place because dolphins can’t change their facial expressions?” Gregg writes in “Are Dolphins Really Smart?” “The smile that the dolphin had as it rammed that other dolphin and probably broke its ribs?” Um, yes.

Gregg is no dolphin hater: He is a research associate with the Dolphin Communication Project and co-editor of the journal Aquatic Mammals. But he is willing to challenge the popular perception of dolphins as uniquely smart, friendly and communicative.

Do dolphins have meta-cognition — that is, are they self-aware? Is dolphin communication a sort of language? How does their intelligence compare with that of chimpanzees? His book presents a great deal of research, often ending in “we can’t know for sure.”

Warning: If you truly love dolphins, skip the part about “porpicide.”