Visual signals

Biological Science 3
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
• The brain processes these
signals to form a ‘picture’
Light forms part of the electromagnetic
• 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?
• 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
• 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- prey
• Structural changes
• 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
• 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
• Jellyfish use their transparency to avoid predators.
Many jellyfish are also bioluminescent.