The secret chatter of dolphins
COVER STORY These mammals have come up with a cryptic form of conversation
In oceans around the world, marine animals are making quite a noise. It’s long been known that cetaceans such as whales and dolphins use sound to locate food, to navigate and to communicate, but new research is revealing fascinating other uses for this underwater orchestra – from evading predators to ‘whispering’ to their young.
The science of locating objects by sound is known as echolocation (or biosonar). Many species, including toothed whales and dolphins, bats, swiftlets, oilbirds and shrews, use the behaviour to ‘picture’ their environment via sound. Echolocating animals emit calls, locating and identifying objects through the returning echoes. It’s a highly specialised system that can be used for navigation, foraging and hunting. Whales and dolphins echolocate by throwing out beams of high-frequency clicks in the direction they are facing, much like sonar on a submarine. (Frequency relates to how high or low a sound is. For example, the beat of a bass drum and a rumble of thunder are low-frequency sounds; a piercing whistle and a child’s squeal are high-frequency sounds.)
These clicks are created by passing air through the skull, specifically through the bony nares (nostrils) and across the phonic lips (structures that project into the nasal passage). When air passes through the phonic lips, the surrounding tissue vibrates, producing the sound (see box on p71).
In dolphins, the clicks are then reflected by a concave bone and air sac in the cranium. The sound is fine-tuned by a large fatty organ in the head, known as the melon, situated in front of the blowhole. This organ acts as an acoustic ‘lens’ that further enhances the clicks into
a highly focused sound beam, which is then emitted towards the dolphin’s object of interest.
Echoes that bounce off the object in question are received through the dolphin’s lower jaw, which contains complex fatty structures. These sound waves are then transmitted to the middle and inner ear via a continuous body of fat. Nerves connect from the inner ear directly to the brain, where the sound is translated into an image.
Echolocation works in a similar way to ultrasound. Dolphins can explore their environment in three dimensions with an incredible degree of accuracy. Bottlenose dolphins, for instance, can identify an object as small as a golf ball from a distance equivalent to the length of a football field.
See inside
But dolphin biosonar has evolved to go beyond mere identification: the cetaceans are able to ‘see’ what’s inside an object as well. When a dolphin echolocates on a seal, for example, it is able to visualise muscle tissue, bone, fishing hooks – even subtle features such as scar tissue.
Sophisticated biosonar does have one downside, though: it’s a rather noisy affair. The low-frequency clicks and whistles used to communicate with fellow pod members across several kilometres, for instance, is helpful for communicating, but it also puts the group on the radar of eavesdropping orcas – an apex ocean predator whose prey species regularly includes other dolphins and whales.
Some species have come up with a novel way to get around the problem. They have, independently, evolved to produce narrowband high-frequency (NBHF) echolocation signals with a strikingly similar waveform and frequency of about 125kHz. Because orcas can’t hear anything above 100kHz, these highly adaptive NBHF species can potentially avoid predation by keeping their chatter out of earshot. This strategy is termed ‘acoustic crypsis’.
Changing the frequency of their clicks, does, however, come with significant disadvantages. The signal repertoire (the number of sound variations they can use) and complexity of the communication between pod members becomes extremely limited. Certain sounds, such as whistling – which is like giving a shout-out to orcas across the swells – disappear entirely. This has a tremendous effect on social interactions, because signals vary only slightly and can become confusing for the receiver. It’s a whole new language, with such tiny differences in pitch and volume that can be misinterpreted when a dolphin is too far from its buddy.
NBHF clicks are highly directional – they travel in the direction that the dolphin’s head is pointing, like a focused laser beam. But as high-frequency clicks suffer from increased sound absorption in the water, they also weaken quickly. Members of the same dolphin group thus need to be close together (less than 1km apart) and facing the sender in order to detect the signals. So foraging and interaction across vast areas is much more challenging. Scientists have now discovered that certain species are adapting even further to compensate for these social disadvantages.
Sound barriers
Bioacoustics researchers have been analysing clicks and frequencies for decades. Clicks are usually emitted in a series, known as a ‘click-train’, and at different tempos, resulting in a plethora of sounds from squeals to growls.
Of particular interest with NBHF species are two types of click-trains that occur in rapid succession: these are the buzz and the burst pulse. Both have noticeably short inter-click intervals (ICIs) – the duration
Low-frequency clicks and whistles put dolphins on the radar of eavesdropping orcas.
of the pause between each click. A buzz occurs when clicks are repeated with increasing speed, and the ICIs shorten. It ultimately forms a continual buzzing sound and is often heard when dolphins hunt.
“When dolphins find a potential prey item, they tend to increase the rate of clicks so they can hone in on it. In these cases, echolocation turns into a foraging buzz – where clicks are produced at very fast rates,” says Tess Gridley, founder of the African Bioacoustics Community and codirector of Sea Search.
A burst-pulse has a more consistent click rate of more than 600 clicks per second and is most commonly used when dolphins are engaged in socialising behaviour. These rapid clicks are not only emitted at a fast rate, but are decoded just as quickly by the receiver.
Burst-pulses are also heard during courtship, aggression and aerial displays (leaping, backflipping and tail slapping). It appears that the rate of burst-pulses reflects the degree of heightened emotion in the signalling dolphin.
So how on earth do experts study these different sounds? It’s not as if you can just hop in the water and join the conversation. Funnily enough, though, scientists do just that. For starters, they use an underwater microphone (known as a hydrophone) to make recordings while observing the dolphins’ behaviour. They start to link specific click frequencies to behaviour above and below the water. To explore the click language even further, scientists play back different sounds to the dolphins and watch their response.