Aristotle, the fourth century BCE Greek philosopher, is considered the earliest experimental naturalist. Unlike others in his time, he made careful observations and even dissected animals.

Darwin published his theory of natural selection, On the Origin of Species, in 1859, but it wasn’t until his works The Descent of Man (1871) and The Expression of Emotions in Man and Animals (1872) that he linked the minds of human and nonhuman animals. Interested in reading some of Darwin’s ideas? Check out Darwin online.

Darwin had a lifelong fascination with earthworms. His last book was entitled On the Formation of the Vegetable Mould by the Action of Worms, which he published in 1882. Read it at this site. He wondered if earthworms could be distracted or surprised, which would demonstrate an ability to pay attention—one measure of intelligence. One of his experiments involved blowing a horn at them.

Alex the African Gray Parrot was the research subject of Dr. Irene Pepperberg, an animal cognition scientist at the University of Arizona. Alex died in 2007. Watch this video of Alex in action. Then visit PBS and watch a full-length documentary on him and Dr. Pepperberg.

Karl von Frisch, an Austrian scientist, discovered the “waggle dance” of bees in 1927. The waggle dance showed definitively that bees use their “dance” or movements to communicate the direction of a food source, as well as its relative quality. More recently, as seen in this article, scientists have also shown that bees make choices: when presented with multiple options, bees avoid difficult tasks if they don’t have the information needed to solve them.

“Theory of mind” is another measure scientists use to assess nonhuman animal intelligence. Theory of mind, sometimes called “mind reading” or “mental modeling,” is the ability to perceive and respond to the mental state of another, such as when you know what someone else is thinking. Theory of mind is key to empathy, or putting yourself in someone else’s shoes. Though studies of theory of mind in nonhuman animals are not conclusive, cognitive scientist Dr. Alexandra Horowitz, in her book Inside of a Dog, has given evidence that dogs might possess it. For more from Dr. Horowitz, watch her TED-Ed lesson, ”How do dogs “see” with their noses?”

The philosopher of mind Thomas Nagel wrote an influential paper in 1974 called “What Is It Like to Be a Bat?” He chose bats because, while they are mammals like us, they experience the world with a physical sense that we don’t have: echolocation, which allows them to use sound to produce a mental image of the world. He wondered if the distinctive physiology of bats would mean that we could never know the world from its perspective. For Nagel, all nonhuman animals experience the world in their own ways, and this experience represents the limits of scientific investigation.