What is a signature whistle in the common bottlenose dolphin?

Imagine being put into a dark room with ten people in it and asked, by voice alone, to call one of them by name. You say their first name. If they recognise you, they answer. The mechanism seems so obvious that we forget how sophisticated it is: it presupposes a stable proper name, an ability to recognise voices and, for the caller, the possibility of uttering that name at will. This ability, long held to be exclusively human, exists in the common bottlenose dolphin as well. During its first year of life, each individual develops a personal whistle, its signature whistle, which it will use for the rest of its life. And, remarkably, other dolphins can imitate that signature in order to call by name the individual it designates. For most researchers it is the closest acoustic equivalent to the human first name ever documented in the animal kingdom.

The discovery (1965)

The phenomenon was identified in the mid-1960s by Melba and David Caldwell, biologists at Marineland of Florida. They were recording the vocalisations of captive bottlenose dolphins and noticed that each animal produced, under stress (in particular during veterinary handling), a frequency-modulated whistle with very consistent characteristics. That whistle was different from one individual to another, yet stable over time in the same individual.

Their founding paper, published in Nature in 1965, proposes the term “signature whistle” and sets out for the first time the hypothesis that this is an identity signal, the equivalent of a name. At the time the hypothesis was met with scepticism. Fifty years later, after hundreds of further studies, it is the subject of a consensus.

How it is acquired

During the first months of its life, the young dolphin listens intently to the sounds around it. It tries out various modulations, much as a human baby babbles before speaking. At around one year of age, its signature crystallises and becomes fixed. It will not change again for the rest of its life, except, in certain particular cases, at times of major social change (the death of a close companion, joining a new group).

Is the signature learned or innate? Several studies (Sayigh et al., 1990; Fripp et al., 2005) have shown that it is learned socially, from the young animal’s vocal environment. Female calves may partly copy their mother’s signature, but most often young dolphins develop a signature that is original and distinct from those of their close companions. If a calf is raised in captivity with unrelated dolphins, it will still compose a unique signature that does not take up any of the others. This capacity to generate a new acoustic signal, known as productive vocal learning, is extremely rare among mammals. It is found in humans, in bats, in some pinnipeds and, to a lesser degree, in some primates and elephants. Most other mammals, our chimpanzee cousins included, can produce only genetically programmed vocalisations.

Stephanie King’s experiment (2013)

The decisive criterion for calling a signature whistle a true “first name” is its capacity to be used referentially, that is, to be uttered by another individual in order to designate precisely the animal it names. Stephanie King and Vincent Janik, at the University of St Andrews in Scotland, provided that demonstration in a paper published in PNAS in 2013.

Their experimental protocol is elegant. They recorded the signature whistle of one dolphin (“individual A”), then played it to another dolphin (“individual B”) that knew it. A’s signature was uttered by the recording, but also, under certain conditions, by other dolphins in the group in a slightly modified form. The result: dolphins respond preferentially when their own signature is played, and they do not respond when another’s is played. More than that, B sometimes copied A’s signature in order to address it, exactly as a human would call a friend by name.

King and Janik showed that the signature carries enough identity information for a dolphin to recognise a companion even after ten years of separation. That is a feat of long-term memory rare in a non-human mammal.

When and how it is used

Signature whistles are observed in several natural contexts. The best documented is the reunion of separated individuals: after a dive, a lull, a temporary dispersal of the group, dolphins exchange their signatures in order to locate one another and come back together. The signature carries several hundred metres in calm conditions, which makes it an effective cohesion signal.

Mothers and their calves exchange signatures intensively during the first years, rather as a human pair multiplies the uses of a first name. That is also the period when the calf’s signature settles, probably through interaction with the mother’s.

Finally, when captured or in distress, a dolphin emits its signature at an increased rate, behaviour that recalls, by functional analogy, a human calling for help by saying their own name.

Beyond the dolphin

Comparable phenomena, though not quite equivalent, have been suggested in other species. Sperm whales have their individual codas, sequences of clicks that could have an identity function (see our page on codas). Killer whales have stereotyped calls passed on along family lines, which identify the pod rather than the individual. Marmosets and some parrots also show recognisable individual calls. But the signature whistle of the common bottlenose dolphin remains the best documented and the most convincing of the acoustic analogies with the human first name.

Sources

• Caldwell, M.C. & Caldwell, D.K. (1965). “Individualized whistle contours in bottlenose dolphins (Tursiops truncatus)”. Nature, 207.
• Janik, V.M. & Sayigh, L.S. (2013). “Communication in bottlenose dolphins: 50 years of signature whistle research”. Journal of Comparative Physiology A, 199.
• King, S.L. & Janik, V.M. (2013). “Bottlenose dolphins can use learned vocal labels to address each other”. PNAS, 110.
• Sayigh, L.S., Tyack, P.L., Wells, R.S. & Scott, M.D. (1990). “Signature whistles of free-ranging bottlenose dolphins”. Behavioral Ecology and Sociobiology, 26.
• Fripp, D. et al. (2005). “Bottlenose dolphin (Tursiops truncatus) calves appear to model their signature whistles on the signature whistles of community members”. Animal Cognition, 8.

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Further reading