The five senses

Mammals in the land beneath the waves

Living in water holds many challenges for a mammal. Whales’ senses have had to adapt to this environment where sound, light and odour do not travel the same as in air.

Sight

As light penetrates water, the colour red is absorbed within the first 10 m, orange and yellow disappear between 10 and 20 m, green and blue between 20 and 30 m, leaving only violet. Light also loses its energy as it penetrates water; it gets darker the deeper you go.

Cetaceans are well adapted for seeing at depth: their photoreceptors, very dense and numerous, are particularly receptive to bluish light (the light that penetrates deepest in water). As well, they have a reflective layer behind the retina, the tapetum, that reflects light not initially absorbed. This is the same membrane that make a cat’s eyes shine in the night.

Unlike us, cetaceans also have good visual acuity under water, they see just as well close up as far away. In air they can resolve the problem of myopia (near-sightedness) by contracting the pupil to the size of a pin hole. Thus they have the same visual acuity in air as in water, as demonstrated by the precision of their jumps and their ability to leap accurately to targets placed above the surface.

Hearing

Sound travels between four and five times faster in water than in air. It is therefore understandable that whales make use of sound in their underwater universe. Sound can serve different purposes.

As an example, most toothed whales (sperm whales, dolphins, beluga whales, orcas, etc.) produce ultra-sonic sounds to "see" their surroundings and to find food, kind of like a very sophisticated sonar. This is known as echolocation. Belugas can also emit a wide range of sounds that help them to stay in contact with other members of their group.

Blue whales emit very low-frequency sounds (15 to 20 Hz). Some scientists believe that this allows them to perceive the relief of underwater terrain, again, functioning much like sonar. They may have an acoustic memory map of oceanic landscapes. These sounds travel over very long distances and could also serve as a form of communication between blue whales.

Finally, the humpback whale emits very melodious sounds. During the mating season, male humpbacks sing long, complex musical phrases which appear to be a way of charming the female humpbacks and intimidating other males during mating. As scientists continue their study of whales and their vocalizations they will likely discover other ways in which whales use sound.

Smell and taste

Some marine mammals, such as seals, use odours to send messages: for example, female seals recognize their pups by smell. This system is useful only on land. Olfactory messages travel slowly in water and seals move too fast to communicate efficiently by smell. For this reason, the sense of smell is absent in cetaceans which live exclusively in water. However, cetaceans and seals can taste what they eat.

Touch

The sense of touch is very important for cetaceans. Whales use their pectoral fins to caress each other. Female beluga or humpback whales often maintain constant physical contact with their young as they swim.

The sense of touch is particularly well developed in the head area, near the blowholes. This likely helps cetaceans determine when they have cleared the surface and when it is safe to breathe.

Baleen whales and freshwater dolphins also have "moustaches", or vibrissa, on the end of their rostrum (snout). This enables them to perceive the density of a patch of plankton or the presence of prey in the muddy bottom, for example.

The sixth sense

Several studies are beginning to reveal that cetaceans can detect variations in the planet’s magnetic field. This sense would allow them to orient on the north-south axis of the Earth, very useful during long migrations.