2.6 Senses and communication …
Glance down at the second paragraph of this section, where you will find a sentence about the speed at which eyes adapt from bright light to poor light, and the statement that this process takes 20 minutes for the human eye. With your developing sense of scientific enquiry, that might lead you to wonder how it would be possible to test this assertion. Could you design an experiment to see if it does take 20 minutes for the human eye to adapt fully to poor light?
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 you'll appreciate that to hunt 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, of the type that you saw very dramatically in the coordinated driving of fish by a small pod of bottlenose dolphins off the coast of South Carolina (from 32.50-35.38 in the TV programme).
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.
Dolphins produce a range of high-frequency clicks using air passages around the blow-hole. 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 3). 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.