Dolphin

Dolphins are the animal most people feel certain are intelligent without being able to say exactly why. The reasons turn out to be specific and well-documented: they have names, they sleep in shifts, they can see inside things and share what they see, and their social politics rival anything in the primate world.

Facts you didn't know

• 1

  Dolphins give each other names. Each individual develops a unique 'signature whistle' in infancy that functions as a personal identifier — other dolphins address them by mimicking their whistle, and individuals respond to their own signature even when played back artificially.

• 2

  Dolphins sleep with one brain hemisphere at a time. The two halves take turns in 2-hour cycles, allowing continuous swimming, breathing, and predator monitoring. The eye connected to the resting hemisphere closes; the alert eye stays open.

• 3

  Dolphin echolocation creates a three-dimensional acoustic image of objects — including their interiors — and dolphins can transmit these images to other dolphins, effectively 'showing' companions what they're seeing inside something. They have a sense that lets them see through objects, and they can share that vision.

• 4

  Dolphins in some populations use marine sponges as foraging tools — carrying them on their rostrums to probe the seafloor without injury. The practice is learned from mother to calf and is specific to certain communities, making it a genuine cultural behavior transmitted between generations.

• 5

  Bottlenose dolphins recognize themselves in mirrors faster than any other non-human species tested, showing curiosity about their own appearance and manipulating their bodies in front of mirrors to examine different angles.