Twenty years ago I wrote a profile of primatologist and author Frans de Waal for another magazine. As a placeholder for the title until we could think of a better one, I came up with “The Writing on de Waal.” I'm both delighted and appalled to say that either by an act of editorial commission or by one of omission, the piece was published with that title. Now I have the chance to write on de Waal again, in the context of the arrival of his latest book, Are We Smart Enough to Know How Smart Animals Are? The answer: sometimes, with great effort.

Imagine judging Michael Jordan's basketball skills by watching him hit .202 when he played Minor League Baseball. Gauging an animal's brightness poses similar issues: “We need to familiarize ourselves with all facets of the animal and its natural history before trying to figure out its mental level,” de Waal writes. “And instead of testing animals on abilities that we are particularly good at … why not test them on their specialized skills?” De Waal thus prefers the term “evolutionary cognition” to “animal intelligence” for this field of study. “It seems highly unfair to ask if a squirrel can count to ten,” de Waal writes, “if counting is not really what a squirrel's life is about.” You could even say it's nuts.

Consider the story in the book about a test administered to gibbons, pretty good apes. (Not great apes. They're technically lesser or smaller apes.) The tree dwellers were asked to reach out with a thin stick to move a banana close enough to their enclosure to pick it up. Chimpanzees and some monkeys could get their bananas lickety-split. But gibbons flopped and were thus considered intellectually backward. Until a researcher named Benjamin Beck realized that their hooklike hands, excellent for traveling among the tree limbs, were miserable at the kind of manipulations the task required. When Beck redesigned the experiment so that the provided tools were in the subjects' anatomical wheelhouse, well, blue ribbons for the gibbons.

De Waal reviews numerous studies involving crows, dolphins, whales, bats, sheep and other fauna showing off their brainpower. But as a longtime chimp researcher—he is director of the Living Links Center at Emory University's Yerkes National Primate Research Center—he devotes the lion's share of his time to our close cousins. One anecdote involves a colony of 25 chimps at Burgers' Zoo in Arnhem, Netherlands, which de Waal studied for six years, starting in 1975. The chimps spent nights inside but were let out onto an adjoining island all day. One morning he and some colleagues carried a crate overflowing with grapefruits past the watchful chimps and out to the island—the first time either group of primates had ever engaged in this behavior. “We thought we would get a reaction from them, but they sort of ignored the grapefruits,” de Waal told me when he visited New York City in April.

The researchers then hid the grapefruits on the island to study how the chimps would search for them later. “And then we came back with an empty crate,” he said. “And that's when they reacted. They saw an empty crate, and they started jumping around and hollering and slapping one another on the back. And I've never seen animals so excited for no fruit.... They must have deduced that we cannot go out with a crate of grapefruits and come back with it empty without these things staying” on the island, where they would soon be able to party hearty on the citrus snacks.

But there's more. When the troop got to the island, some went right past the site of a few grapefruits buried not quite completely in the sand. The human observers assumed all the chimps had overlooked the cache. But later, when his Pan pals were taking a siesta, a low-ranking male nonchalantly returned to the buried fruit. “He knew exactly where they were,” de Waal said to me, “but he had decided not to react at the moment that he saw them.” Presumably because if he had, higher-ranking individuals would have pilfered his produce. That's some quick, strategic thinking that shows off impressive evolutionary cognition.

So when somebody says they don't believe that humans evolved from ape ancestors, I tell myself I'm better off talking to de Waal.