Mammals_Concept_7

Generally, it is believed that there are five senses — but this is not strictly true. Sight, smell, sound, taste and touch are the accepted five (although taste and smell are quite closely linked). But touch is not really just one sense: it is linked to the sense of acceleration (balance), the sense of direction and a few other, more unusual senses that particular animals exhibit. The ability of pigeons to sense changes in the local magnetic field is now being used for early warning of earthquakes- These are all ways in which it is possible to be aware of the local environment, so they are, strictly speaking, senses.

Vision

The mammalian eye (see Figure 11) is roughly spherical in shape, with a tough outer coating and a liquid-filled centre to prevent it from collapsing. Light enters the eye through a clear, scratch resistant membrane called the cornea. Directly behind that is a cavity filled with liquid, called the aqueous humour. Within this is a coloured membrane, the iris, which can expand or contract to let more or less light into the eye. The black circular gap made by the iris is known as the pupil; in some nocturnal mammals, it can be as big as the cornea. The pupil is the entrance to the optic lens, which is made of clear fibrous tissue that is able to expand and contract to make the lens thicker or thinner, thus changing its focal length. This allows the eye to focus voluntarily on near or far-away objects.

Most of the eye is filled with a viscous liquid called the vitreous humour, through which the light travels on its way to the retina. The retina is the lining of the back of the eye, composed of light-sensitive cells. Most mammals have two types of light-sensitive cell: rods and cones. The cones, which are sensitive to red, blue or green light, are used only in bright conditions and register the intensity of light in a particular colour. The rods are effective even in very dim conditions, and are only capable of identifying motion — which is why you cannot see colours in poor light. The information received by these cells is transmitted to the optical centre of the brain through the optic nerve.

So, what are the eyes capable of? The mammalian eye has quite a complex design: it can be adjusted for distance, brightness, colour and direction. However, the eye itself only registers the intensity of a particular colour of light and whereabouts on the retina it fell — the rest is accomplished in the brain. The brain puts all of the coloured dots together, adjusts the fens to ensure that the resulting picture is in focus, and adjusts the iris to ensure that sufficient light is being allowed in. An impressive bit of computing — but that's not all! As the cones are only capable of registering one of three colours (red, blue or green), all of the other colours and shades that you see are literally figments of your imagination. Your brain makes up what we see as orange by extrapolating from the intensities of red and green light (and the absence of blue) in the field of vision. Also, the brain has the knack of turning colours down' if it decides that there is too much of one colour hitting a particular patch on the retina.

Although most mammals have similar eye structures, the ability of different mammals to see varies. Some moles are all but blind. Gaiagos (bush babies) are nocturnal and have very large eyes to collect as much light as possible. Cattle have few cone cells, and are incapable of seeing colour (so bulls cannot 'see red' at ail).

The position of the eyes on the skull is important. Many herbivorous mammals, such as rabbits, have eyes on either side of their head, allowing a very wide field of vision to help them avoid predators. Predatory mammals, such as cats and dogs, have a greater need for good 3D vision in front of them, so their eyes are on the front of the head with a large degree of overlap (see Figure 12).

Hearing

Hearing is the ability to sense vibrations (small oscillating movements), Not all mammals have ears, but they are alt capable of hearing Dolphins, for example, need to have a streamlined body shape, and protruding ears would get in the way; so they hear through their lower jawbone rather than an outer ear. However, most mammals have a three-part ear system: the inner; middle and outer ear (see Figure 13). The outer ear (or pinna) collects the sound vibrations and directs them to the middle ear, where three small bones transmit them to the inner ear (or cochlea).This is a liquid-filled, rolled-up cone like the shell of a snail, covered on the inside by small hairs. These hairs are the actual sound sensors, and are connected directly to the brain via the auditory nerve. The hairs further up the cone are sensitive to higher frequencies of sound.

Having ears placed on either side of the head means that most mammals have very effective directional hearing. Some mammals, such as bats and whales, use echolocation as a means of sensing objects around them: they send out ‘vocalised clicks' and listen for echoes off hard objects. Whereas eyes are only capable of registering three different frequencies of light, ears are capable of detecting and distinguishing between many different frequencies of sound.

The ear also contains the balance organ which senses changes in position or movement, Within the inner ear; three liquid-filled semi-circular canals at right angles to each other register changes in the direction and rate of movement. The liquid in them 'sloshes around' as the animal changes speed, direction, or orientation relative to gravity; sensory hairs stimulated by the liquid transmit this information to the brain.

Smell

Smell is perhaps the least welt understood of the five main senses. It is closely linked to taste, and one often helps to reinforce the other. The organs of smell are very small hair-like sensors high up in the nasal cavity. When stimulated, they transmit information to the brain via the olfactory nerves. There are three primary colours of light to which mammalian eyes are sensitive, four primary tastes, and seven primary odours. Each different smell has a particular combination of these seven odours, and a different type of olfactory nerve registers each odour.

What scientists think happens (it is still poorly understood) is that the chemical pattern that represents each of these odours either sets off a chemical reaction in the olfactory nerve or locks onto the nerve ending (like a piece of a jigsaw). Either way, a nerve impulse is sent to the brain to register that odour. The brain then puts all of these nerve impulses together to make up an impression of a smell. Over-exposure to a particular smell will reduce the sensitivity of particular receptors, so that the smell seems to go away (for example, smokers may be unaware that they smell of smoke).

Taste

The four primary tastes are the same for all mammals, though the degree of sensitivity varies according to species, age and gender (human females have more taste buds than males). The receptors are the taste buds: small bumps along the tongue and at the back of the mouth near the roof. Different areas of the tongue were thought to be responsible for each of four different tastes. This has shown not to be the case and all areas of the tongue can sense different types of tastes, although to different degrees. There are also now thought to be five (not four) main tastes: sweet, salt, sour; bitter and umami. Umami is a Japanese word meaning ‘pleasant savoury taste', and has only recently been accepted as the -fifth taste’.

Taste sensitivity varies according to temperature, time of day and a range of other variables, all of which affect the efficiency of the 9000 or so taste buds on the human tongue. It is far from being the most exact of senses. Most tastes are predominantly smells: when the olfactory sensors are not working well because your nose is blocked, you wilt find it much more difficult to 'taste' food.

Touch

Touch is an unusual sense, in that the sensing organ (the skin) covers the body surface entirely. it is really four distinct senses: contact, temperature pressure and pain.

Specific receptors in the skin register contact with other substances. They can be used to detect the texture of a surface: rough, smooth, sticky and so on, These receptors are bunched together where they will be most effective — for example, human fingertips, the end of an elephant's trunk the whiskers of a cat and so on.

The skin also has receptors that register temperature. There are different receptors for hot and cold. In human skin, as With most other mammals, these receptors have two limitations. Firstly they tend to register temperature only relative to what the sensor has become used to, and so can give misleading signals. Secondly, outside a fairly narrow range of temperatures the pain receptors tend to do the sensing instead.

Pressure sensors allow the skin to register the degree of contact with another body, enabling us to grip and lift a bottle or lean against a wall.

Finally, other receptors register pain. They come into play when other 'touch' sensors have become overloaded (too much pressure, heat or cold). As with all of these receptors, there is a greater density of them in some skin areas than in others. There are further touch and pain receptors throughout the interior of the body.

Why you need to know these facts

MoMammals have a range of highly developed sense organs which they use to gain information about their surroundings, Although all mammals have the same range of sense organs, particular organs are specially developed in certain mammals — for example, hearing in bats, smell in dogs, touch in moles and motion sensing in cats. An appreciation of the importance of their senses and what they can learn from them is essential for the children to become effective observers and scientists..

Vocabulary

Cochlea — the sound-sensitive part of the inner ear.

Cones — the colour-sensitive cells in the retina, only responsive to bright light.

Cornea — the tough outer covering of the eye.

Echolocation — the use of echoed sounds to 'see' objects in the dark

Iris — the coloured outer ring of the pupil that regulates the amount of light entering the eye.

Optic lens — the part of the eye that focuses light onto the retina.

Pinna — the outer portion of the ear.

Pupil — the black centre of the eye that allows light through to the lens

Retina — the light-sensitive inner coating of the eyeball.

Rods — retinal cells that are sensitive to motion but not to colour,

Semicircular canals — the part of the inner ear that maintains balance by registering changes in position.

Taste buds — the taste-sensitive cells on the tongue and at the back of the mouth.

Amazing facts

Some children with particularly sensitive hearing (mostly asthma sufferers) can hear frequencies up to 20kHz — but dolphins can respond to frequencies well above 100kHz.

There are approximately 1 30 million light-sensitive cells on the retina of each human eyeball.

• In favourable conditions, wolves can detect the scent of their prey up to 2.5km away.

• When it is very dark cats use their whiskers to feel their way around.

Common Misconceptions

Bats are blind.

They are not blind, but they do find out more about their environment by building up a 'picture' from sound than by seeing. Their echolocation system is sensitive enough to enable them to catch a flying insect in the dark.

If you are colour blind, you only see in black and white.

Colour blindness usually means that one set of the light-sensitive cells is not working properly. The most common form is red— green blindness, which makes those two colours indistinguishable. In extremely rare cases, a person who is colour blind will only be able to see in shades of grey.

Questions

How does a dog track by smell?

The smell receptors in dogs (and cats as well — but they are much harder to train!) are very acute. The area within a dog's nose devoted to the sensing of odours is fifty times as large (with fifty times as many smell-sensitive cells) as the corresponding area in a human nose. Even the shape of a dog's nose is more conducive to the flow of smells than human nostrils. A dog is a thousand to a million times more sensitive to smells than a human.

Why do surgeons dress in green or blue?

When you are operating on someone, there tends to be quite a lot of blood around, so much of what the surgeons are looking at will be red. After a while, their eyes will become less sensitive to so much red, making it more difficult for them to see what they are doing. To turn the red-sensitive cones in the eyes back ^ton', the surgeons will look away from the operation for a brief moment and stare at something non-red — such as a green medical gown. When they look back to the operation, they will be able to see red once more.

Teaching ideas

Colour sensitivity (exploring, testing)

Ask the children to use coloured pens or coloured paper to make a cross in one colour (say red) on a white background.

Ask them to stare at the cross for 30 seconds, then blink and look away at a plain white background. They should see a faint 'after-image' of the cross, indicating the part of their retina where the red receptors have been 'turned down'*This after-image should be a blue-green colour called cyan, the 'opposite' of red The children can explore-what other after-images they can make themselves sees What colours are opposites? (Red and cyan; blue and orange; green and magenta.)

Sound location (exploring, modelling)

This activity is best done in a hall or gym. Blindfold a child and place him or her within a circle of other children. When you point to a child in the circle, they should click their fingers or tongue, Can the blindfolded child point to the source of the noise? This demonstrates how we can use our two ears to identify the direction of a sound.

Now blindfold a child and place him or her in a circle of children as above, Another child without a blindfold, should now slowly walk around inside the circle. The blindfolded child says 'Ping', to which the other child replies 'Pong'. The blindfolded child then sets off in pursuit, walking slowly and 'pinging' in an attempt to locate and catch the other child, who must always respond immediately. This demonstrates how bats catch insects in the dark by echolocation.

Taste and smell (testing, sorting)

NB Make sure that standards of hygiene are maintained, and be aware of any food allergies. Supply the children with a variety of different flavours of crisps, presented in bowls without any labelling. Can they distinguish between the different flavours? Can they do so as accurately while holding their noses?

Taste and sight (testing, sorting)

NB Make sure that standards of hygiene are maintained, and be aware of any food allergies. You will need powdered mashed potato, some clear food flavourings (such as vanilla and peppermint) and some flavourless food colourings. Randomly colour and flavour some samples of potato. Ask the children to find out whether different colours of the same flavour taste 'different'. They will probably find that they do (even though they are the same flavour) — which demonstrates that we use different senses together to gain

Smell matching (testing, sorting)

provide a selection of different 'smelly' substances (such as spices and soaps) in opaque but open containers. Ask children to smell each substance without looking inside the pots: can they recognise and match different smells?

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