In lieu of a precise definition for language, many experts and textbooks fall back on the work of the American linguist Charles Hockett, who in the 1950s and ’60s proposed a set of more than a dozen “design features” that characterize language, like semanticity — distinct sounds and symbols with specific meanings — and displacement, the ability to speak of things outside your immediate environment. He acknowledged that numerous animal-communication systems had at least some of these features but maintained that only human language boasted them all. For those who think that language is a prerequisite for consciousness, the unavoidable conclusion is that animals possess neither.

To many biologists and neuroscientists, however, this notion smacks of anthropocentrism. There is now a consensus that numerous species, including birds and mammals, as well as octopuses and honeybees, have some degree of consciousness, that is, a subjective experience of the world — they feel, think, remember, plan and in some cases possess a sense of self. In parallel, although few scientists are as ready as Slobodchikoff to proclaim the existence of nonhuman language, the idea that many species have language-like abilities, that animal communication is vastly more sophisticated than Hockett and his peers realized, is gaining credence. “It’s increasingly obvious just how much information is encoded in animal calls,” says Holly Root-Gutteridge, a bioacoustician at the University of Sussex. “There’s now a preponderance of evidence.”

In the 1990s, inspired in part by Slobodchikoff’s studies, the primatologist Klaus Zuberbühler began investigating monkey vocalizations in the dense and cacophonous forests of the Ivory Coast in Africa. Over the years, he and his colleagues discovered that adult male Campbell’s monkeys change the meaning of their screeches by combining distinct calls in specific sequences, adding or omitting an “oo” suffix. Krak exclusively warns of a leopard, but krak-oo is a generalized alarm call; isolated pairs of booms are a “Come this way!” command, but booms preceding krak-oos denote falling tree branches. Studies of songbirds have also uncovered similar complexity in their communication. Japanese great tits, for example, tell one another to scan for danger using one string of chirps and a different set of notes to encourage others to move closer to the caller. When researchers played the warning followed by the invitation, the birds combined the commands, approaching the speaker only after cautiously surveying the area. In the South Pacific, biologists have shown that humpback-whale songs are neither random nor innate: rather, migrating pods of humpback whales learn one another’s songs, which evolve over time and spread through the ocean in waves of “cultural revolution.” And baby bottlenose dolphins develop “signature whistles” that serve as their names in a kind of roll call among kin.

With the help of human tutors, some captive animals have developed especially impressive linguistic prowess. Dolphins have learned to mimic computer-generated whistles and use them as labels for objects like hoops and balls. A bonobo known as Kanzi communicates with a touch-screen displaying hundreds of lexigrams, occasionally combining the symbols with hand gestures to form simple phrases. And over the course of a 30-year research project, an African gray parrot named Alex learned to identify seven colors, five shapes, quantities up to eight and more than 50 objects; he could correctly pick out the number of, for instance, green wooden blocks on a tray with more than a dozen objects; he routinely said “no,” “come here” and “wanna go X” to get what he desired; and on occasion he spontaneously combined words from his growing vocabulary into descriptive phrases, like “yummy bread” for cake.