2.6 Senses and communication …

Light travels only a few hundred metres through clear water, so sight is of limited use to animals hunting in deeper waters.

Seals use their eyes and sensory whiskers to search for food in relatively shallow water. Elephant seals feed at depths of 300–700 m and have eyes that adapt to poor light faster than those of any mammal tested; some six minutes compared with 20 minutes for the human eye. Despite this capacity, vision is probably of little use at the bottom of their feeding range and they must rely on sensitive whiskers and hearing. In fact, the whiskers of seals and sea lions are so important that a special blood supply at their bases keeps them warm and flexible even in the deepest, coldest waters.

Eyesight is certainly of some use in making sense of the immediate environment, and sirenians need little else to find their food in shallow, clear waters. But when hunting at depth, as many toothed whales do, communication using sound is generally a better option. Sound travels five times faster in water than it does in air, and the toothed whales use a range of high-frequency squeaks, whistles and moans to communicate with each other and coordinate social behaviour, such as the coordinated driving of fish by a pod of bottlenose dolphins, which you will find more about in Section 2.7.

You’ll also be aware that dolphins (and many other toothed whales) use a sophisticated echolocation system – ‘seeing with sound’ – to make sense of their environment and detect prey.

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Figure 14 The generation and reception of sound in the head of a dolphin. The melon acts as an acoustic lens, focusing the clicks, while the oil-filled cavity of the lower jaw acts as an acoustic pipe, conducting sound to the ear

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A section through the head of a dolphin has been drawn to show that sounds generated in air passages around the blowhole are reflected off the skull to an area above the upper jaw called the melon, which focuses the outgoing clicks. Returning echoes travel down an oil-filled cavity below the lower jaw, to reach the auditory receptors.

Figure 14 The generation and reception of sound in the head of a dolphin. The melon acts as an acoustic lens, focusing the clicks, while ...

Dolphins produce a range of high-frequency clicks using air passages around the blow-hole (you can find many videos of dolphin clicks on youtube. This is one example of a video about how dolphin sounds work, with example audio [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] ). These clicks are focused by the front of the skull, which is shaped like a satellite dish, and pass out into the water through a waxy structure, the ‘melon’, which acts as an acoustic lens. In this way, the animal can focus and direct the stream of sound. The returning pulses are picked up by oil-filled cavities in the lower jaw and pass through to the inner ear, which is enclosed in a bone casing called the auditory bulla (Figure 14). This bone casing itself is surrounded by a mucus foam to isolate it from other vibrations. The whole system is sensitive enough to allow the animals to determine the size, shape and distance – even the internal structure – of objects as much as 800 m away. It is also powerful enough to allow them to stun, or even kill, their prey with a well directed burst of sound.

As well as travelling faster in water, sound also travels further and with less distortion than in air. The other suborder of cetaceans – the baleen whales – utilise this property in their use of low-frequency calls produced from the vocal cords to communicate with each other across the oceans. The song of the humpback whale is perhaps the best-known example. The song differs from population to population, and individual to individual, and it develops over time as the whales imitate each other and introduce variations. The whales also exploit a special property of the oceans to make themselves heard over great distances. At a depth of around 1000 m, there is a band of water in which temperature and pressure combine to reduce the speed of sound to a minimum. The low-frequency song of the whales can spread out through this ‘sound channel’ for distances of up to 1000 km, with little loss of energy to the water surrounding the channel.

Recent research on harbour seals and killer whales in the northeastern Pacific illustrates the sophistication of the sound world of marine mammals. The seals largely ignore a resident population of fish-eating killer whales, which use frequent echolocation clicks and communication calls. From time to time, however, the area is visited by transient pods of mammal-eating killer whales. These animals use few vocalisations, presumably to avoid alerting the seals, but the seals have learned to recognise the sounds that they do make and take evasive action.

Activity 4 Whale vocalisations

Timing: Allow about 10 minutes

To round off this section on senses and communication, the activity introduces you to some whale vocalisations and their possible function, or functions.

Now listen to the clicks produced by sperm whales at depth. The clicks are used for echolocation but might also be loud enough to ‘stun’ squid at close range. The clicks also have a social function. In the background, you can hear the high-pitched sounds of dolphins:

Download this audio clip.Audio player: Audio 1 Sperm whale clicks

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Audio 1 Sperm whale clicks

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Humpback whales produce a wide range of sounds that are very complex and often produced for long sessions of several hours. Listen to a short clip and marvel at the structure of the ‘song’:

Download this audio clip.Audio player: Audio 2 Humpback whale song

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Audio 2 Humpback whale song

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Hal Whitehead from the Department of Biology at Dalhousie University in Nova Scotia has a special interest in the social structure and cultural life of sperm whales, as he told the BBC’s Julian Hector:

Download this audio clip.Audio player: Audio 3 Hal Whitehead interview

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Transcript: Audio 3 Hal Whitehead interview

HAL WHITEHEAD

The best evidence is-- for sperm whales and for most whales and dolphins, comes from their sounds. Sounds are very important in the ocean, they travel very well, much better than light or smell or anything like that, and we can record the sounds much more easily than we can see the animals. And so we've been able to listen to the sounds that the different whales and groups of whales are making, and to chart those patterns, and when we do this, we find patterns which don't make sense, just from the basis of genes controlling these things. So, it's clear that the whales are learning their vocabulary, the sounds they make, from each other. Off the Galápagos, where I've done a lot of work, there are two very distinct clans of sperm whales, and one make patterns of clicks like 'Click click click click' or 'click click click click click'. We call them the 'regular clan'. And then there's another clan who live in the same waters, who go 'Click click click, click' or 'Click click click click, click.' We call them the 'plus one' clan. So they have these different ways of vocalizing, but there's much more to it than that. The different social units form groups, they get together and work together for periods of days. They only do this with members of their own clan. The different clans have distinctive ways of moving around the islands. They have different feeding success. In normal years, when the waters are quite cool off the Galápagos, then the regular clan does better. But when they warm up, you get El Niño coming in, then it's reversed, and everyone does badly, but now it's the plus one clan who are doing better. So it seems that these clans, which we – and probably the whales themselves – identify primarily through these sounds, their culture is a very important part of what they do, how they are and how they interact with the rest of the world.

JULIAN HECTOR

If they're relying a lot on information being handed from one individual to another, and they can even share information from one distant group to another, depending what the year is, what impact does hunting on those populations have, if those groups are disrupted?

HAL WHITEHEAD

The first idea is that there may be particular animals, and probably in particular older females, who have very important sets of knowledge, and as things in the ocean are varying over long timescales, the older animals' knowledge becomes particularly important, because they've been through a lot of the variations which happen very rarely. So, especially if you lose a lot of the older animals in your society, it can really affect the ones who are left. And we have indications of that in the Eastern Pacific, where the whaling between about the 1950s and the early 1980s was particularly intense, and there it's likely there aren't that many very older animals, and we find less success in raising calves than say, in the Western Atlantic where there wasn't that modern whaling. So that's one level of it, and another level is that, if you get a population down to very small size, then not only do you lose genes, genetic diversity, but you lose cultural diversity, so, some of the knowledge that has been important for that species may be lost, just because there's so few animals left, and those that are left have either forgotten or not learned that particular knowledge. So for instance, the right whales in the North Atlantic and North Pacific, which have been brought down to probably tens of animals, seem not to use many of the areas that traditionally they did use, and so in some ways perhaps they've lost important cultural knowledge of how to use their environment.

JULIAN HECTOR

So in your view, Hal, is this an argument to ban whaling or to limit whaling?

HAL WHITEHEAD

I think it's an argument to ban whaling, as well as possibly to limit whaling in some cases. I think the third argument which I didn't make then is the idea that, the culture is a good marker of these other cognitive traits, that are characteristic obviously of humans, but of a very few other animals which we're beginning to think maybe we should give special rights to. So I think we need to think about that – these are difficult issues, but if we're going to give special rights to chimpanzees, we're not going to chop them up for meat, then I think the same arguments hold for whales, and we shouldn't be doing the same for them.

End transcript: Audio 3 Hal Whitehead interview

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Audio 3 Hal Whitehead interview

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You can listen to recordings of lots of other whale species online (on Wikipedia for example).