VISUAL SIGNALSBiological Science 3

2011

What is vision?• Light signals are detected by photoreceptors in the eye (in mammals, located on the retina at the back of the eye)

• Electrical signals are transmitted to the brain via nerves

• The brain processes these signals to form a ‘picture’

Light forms part of the electromagnetic spectrum

• The electromagnetic spectrum describes the spectrum of energy waves of different wavelengths

What do animals use vision for?

• Avoiding predators• Looking for resources/food/water/prey• Communication within groups

• Establishing dominance• Avoiding conflict• Mating rituals• Co-operation in hunting or other tasks

• Few animals have good colour vision. Most animals either see no colours, or only see the world as shades of blue and yellows.

• Squirrels and some primates can see red and green. • Why do many organisms not require colour vision? What do they

use instead?• What is the significance of the colour yellow in biology?

Ultra-violet• Some birds not only see more hues than we can, but they can also see ultraviolet colours, too.

• Insects can also see the ultra-violet part of the spectrum as well as yellow and blue light- so flowers often have markings in these wavelengths.

Infra-red• Nocturnal animals have no colour

vision but some of them are able to ‘see’ the part of the spectrum called the infra-red. These animals can ‘see’ a picture of warm objects.

• Snakes detect infra-red via special sense organs- small pits located on the head

Vision & behaviour within groups

• Communication• Showing dominance/submission• Communicating within group

• Courtship behaviour (plumage, changes in colour)

Why are some organisms brightly coloured, while others are not?• Reasons for dull colourations

• Camouflage, either as predators or prey

• Reasons for bright colourations:• To attract members of the opposite sex• To show poisonous taste• Batesian or Mullerian mimicry

Visual signals as a defence• Camouflage

• Passive (permanent)• Active- changes colours

• Illusions• Disruptive patterning• Appearing larger/threatening

Camouflage- predators• Give 2 other

examples of predators using camouflage.

Camouflage- prey• Structural changes

Camouflage• Also called cryptic colouration• May be passive or active

• Many fish have different colourings on their dorsal and ventral (top & bottom) sides, so that they are camouflaged when seen from below or above, as do penguins

• Prey species have paler bellies to give the appearance of being thinner

Disruptive patterning• Zebras- predators (e.g. lions) are unable to distinguish

individuals in the large group

• Some organisms make themselves appear larger & more threatening. Many mammals erect their hairs to appear larger and more dangerous. e.g. Frill necked lizards display their frill to intimidate predators (it also helps with temperature regulation)

• Porcupines make their quills stand up when threatened to draw attention to them.

Visual display- mating rituals

• Male birds develop bright plumage and perform mating displays to attract females. • Why are the females of the species generally less brightly coloured?

• Female baboons indicate oestrus

with changes in colour/swelling• Male fireflies send out flashes of light while in flight as a

signal to the female fireflies. Different species of firefly use different flashing codes!

• Some fish which live in deep water with very little light have light-producing organs on the sides of their bodies. The light is used to attract prey or scare away predators.

• Jellyfish use their transparency to avoid predators. Many jellyfish are also bioluminescent.