Signature whistles can be heard in a variety of dolphin species.
Suppose I had an eight-month old baby who liked to say something like gork, and I told you that one day I heard the baby's two-year-old brother make a perfect imitation of the gork sound, to which the baby responded, hello. Would you conclude from this evidence that the baby is already using language? Careful, for it seems that bottlenose dolphins can participate in these sorts of exchanges.
While most people agree that only people use language, there does not appear to be nearly as much agreement on what makes language so special. One common test is the use of words or names. A famous challenge to this idea comes from the vervet monkey. These monkeys of the African woodlands have different calls to warn of approaching predators, depending on whether they are birds, leopards, or snakes. So do they have a three word language? Few people want to say yes to that one, and they point to the monkey's response as justification for the no answer. Typically the vervet looks around (up for a bird, at the tree itself for a snake, or more generally for a leopard) and, upon sighting the predator, begin making exactly the same call. You can end up with an acacia tree full of monkeys, all looking at an approaching leopard and making the same warning call. The scene appears reflexive and lacking the spirit of conversational give and take we associate with language. On the other hand, human verbal exchanges can also be pretty reflexive and banal:
— "Nice day."
— "Maybe global warming is a hoax after all."
One thing in that human exchange worth noting is the use of names. We use them to identify particular individuals. It turns out that bottlenose dolphins also learn something like individual names.
These dolphins use what are called "signature whistles," that is a distinctive sound that is unique to one dolphin. Take two dolphins, call them Marie and Mary. When Marie swims along she periodically whistles Marie. Mary regularly whistles Mary. Neither responds to the other's whistle. Sometimes, however, Mary whistles Marie, and upon hearing that, Marie does respond with a stereotypical sound. If we anthropomorphize a bit, the exchange goes something like this:
—MARY: Mary. [Call that a signature whistle.]
—MARY: Marie. [Call that a matching whistle.]
—MARIE: Hi there. [Call that a response whistle.]
The presence of signature and matching whistles among captive dolphins has been known for decades. More recently they have been proven to exist in the wild as well. The matching whistle, of course, must be learned. This fact alone is enough to distinguish it from vervet calls, which are probably innate and are plainly reflexive. The use of learned signals puts the dolphins in limited company. Humans, some songbirds, parrots and toothed whales (a category that includes dolphins) learn the vocal signals they use.
The response whistle, from a human perspective, is a bit ambiguous. Is it an aggressive signal or a friendly one? Male songbirds sometimes use song matching to challenge a territorial claim. A recently published paper ("The Role of Signature Whistles in Bottlenose Dolphins, Tursiops truncatus": abstract) in Animal Behaviourby Stephanie L. King, Heidi E. Harley and Vincent M. Janik reports that "signature whistle matching is an affiliative signal that allows bottlenose dolphins to address social companions." [p. 79] The whistle exchange allows members of a group [pod] of dolphins to address one another individually.
In the theory developed on this blog, vocalization went through two very broad stages: (1) vocalization as a form of bonding to replace the grooming system lost when the human lineage lost most of its body hair; and (2) the rise of language. The dolphins appear to be in stage (1), which does not mean that a stage (2) is inevitable, but it might be available if some new crisis in the natural history of dolphins were to develop.
P.S. Why don't I credit whistle matching as a form of simple language? On page 85 of the paper the authors refer to the exchanges as "dyadic [paired] interactions," but I have said many times on this blog that language depends on a triadic [triangular] relation. Along with the speaker and the auditor you need a topic. Language is about something. Yes, it can be a banal, "Hi, Blair," and we will call it language, but only if you can move on to a subject. So far, dolphins seem to have nothing to say about anything.