Tis the season of colour!
COLOUR AS A WARNING SIGNAL
Within nature there are many examples of conspicuous colouration, which function to signal an individual’s unprofitability to a predator, and is known as aposematism. For example, contrasting colours such as red, black, orange and yellow as stripes or spots, often indicate that a species is either poisonous or distasteful. Mushrooms are widely recognised for using this tactic (figure 2). This form of colouration is maintained by frequency dependant selection, with common aposematic colouration and patterns having the greatest fitness advantage. This is because, as a certain aposematic phenotype becomes more common, a predator will learn to avoid it. Many organisms will use aposematism coupled with other chemical and physical defences, in order to reinforce the signal and improve memorability of the signal by a predator. The Harlequin ladybird (Harmonia axyridis), is a very good example of aposematism (figure 1). It is a polymorphic species and the elytra can range from yellow-orange to red, with anything between zero and 19 black spots. When being attacked by avian predators, they also releases a strong pyrazine odour, as a memory reinforcement to predators.
Fig. 2. Turkey tail mushroom (Coriolus versicolor) found on a tree stump
COLOUR AS A SIGNAL IN SEXUAL SELECTION
Colour also plays an important role in sexual selection, by signalling attractive mate qualities and influencing mate choice. Many species are sexually dimorphic, with the male generally displaying bright colouration and the female showing none. This has been widely studied in birds. A mate will often chose an individual of the opposite sex based on their appearance and performance, because they can gain indirect (genetic) and direct (material) benefits for their offspring. This has been proven by studies which have shown links between colour intensity, genetic qualities and immunocompetence. Colouration vibrancy has also been shown to be a signal of a mate’s social status and ability to provide resources, among other things. The ornate plumage of the Blue Peafowl (Pavo cristatus), otherwise known as the peacock, is a classic example of extreme sexual dimorphism, driven by female choice (Figure 3). Males have vibrant iridescent plumage with long tail feathers which are used during displays. The condition of his plumage indicates how healthy he is. Tail length indicates his genetic quality, because tail length increases vulnerability to predators, and only those of high genetic quality will be able to grow long tails and still survive.
Fig. 3. A Male blue peafowl (Pavo cristatus)
COLOUR AS A FUNCTION OF CAMOUFLAGE
Fig. 4. Two Illioneus Giant Owl butterflies (Caligo illioneus), showing the dull cryptic eyespot colouration on the underside of their wings.
Another common function of colour in nature is for camouflage, otherwise known as cryptic colouration. Organisms employ a wide range of coloration patterns and strategies in order to disguise themselves and blend into their surrounding environment, to avoid predation or catch prey. The nature of a species camouflage will depend on several factors, such as physical characteristics like skin texture, species behaviour and behaviour of a species predator or prey. Many species use mimicry to avoid predation. For example, the Illioneus Giant Owl butterfly (Caligo illioneus), has two large eyespots that mimic that of an owl’s eye to deceive predators (figure 4). Batesian mimicry, where a harmless species mimics a poisonous one, and Müllerian mimicry, where two or more poisonous species with a common predator mimics each other, are other forms of mimicry that have been widely studied in butterflies. Other organisms use crypsis to blend into their environment. For example, background matching is a common form, where the use similar colours to their background environment allows an organism to be inconspicuous and avoid predation.
Countershading is another cryptic form,
which involves the top side of an
organism’s body being darker in colour
than its underside. Shark (superorder
Selachimorpha), colouration uses
countershading allowing then to blend
in with the dark ocean depths when
viewed from above, and blend in with
light surface water when viewed from
below. Similarly, disruptive colouration
is another tactic. This involves a
certain colouration pattern that causes
predators to misidentify what they are
seeing. A brilliant example of this is
seen in the Swainson's Lorikeet
(Trichoglossus haematodus moluccanus)
(Figure 5). Outside of its environment
this brightly coloured bird would appear
to be a very conspicuous individual.
Within its tropical environment though,
its bright disruptive colouration pattern renders flocks of these birds virtually impossible to see. This is because the orange and red colours blend in as fruit on trees, whilst its blue head and wings reduces contrast against blue skies. Fig.5. A male Swainson's Lorikeet
(Trichoglossus haematodus
moluccanus), showing his
disruptive colouration.
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COLOUR AS A DISTRACTION
Some organisms use colour and patterns to distract predators. Organisms
use mimetic and cryptic camouflage on surfaces displayed to potential predators in order to blend in to the environment when static. However, in instances where predators discover the organism, they reveal brightly coloured markings in order to stun and distract the predator whilst they try to escape. This is seen the Blue Morpho Butterfly (Morpho peleides) (figure 6), where a dull ‘eyespot’ colouration is displayed when wings are closed, mimicking the eyes of birds, but a bright blue colour is displayed
Fig. 6. A Blue Morpho Butterfly when they open their wings to
(Morpho peleides) showing the vibrant fly.
blue colouration of the inside of its wings.
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Fig. 7. The bright red colouring of a Belladonna lily (Amaryllis belladonna).
COLOUR IN POLLINATION
Floral colouration plays a large role in pollination by organisms. Wind-pollinating
plant species rarely use colour, but plants that rely on pollinators use brightly coloured flowers, pollen and fruits, in order to stand out from green foliage and lure them in (Figures 7 & 8). They not only use visual colouration, but they also use colours on the ultraviolet and infra-red spectrum. As a result of this strategy, pollinators have coevolved to have a specialist vision in order to efficiently locate certain flowers.
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Fig. 8. Honey bee (Apis mellifera) pollinating a flower
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References.
Blount, J. D., Rowland, H. M., Drijfhout, F. P., Endler, J. A., Inger, R., Sloggett, J. J., Hurst, G. D. D., Hodgson, D. J. & Speed, M. P. 2012. How the ladybird got its spots: effects of resource limitation on the honesty of aposematic signals. Functional Ecology, 26, 334–342.
Greenwood, J.J.D., Cotton, P.A. and Wilson, D.M. 1989. Frequency dependent selection on aposematic prey – some experiments. Biological Journal of the Linnean Society, 36, 213-226.
Hamilton, W. D. & Zuk, M. 1982. Heritable true fitness and bright birds: A role for parasites? Science, 218, 384–387.
Linville, S. U.; Breitwisch, R.; Schilling, A. J. 1998. Plumage brightness as an indicator of parental care in northern cardinals. Animal Behavior, 55, 119–127.
Merilaita, S, Schaefer ,H. M. & Dimitrova, M. 2013. What is camouflage through distractive markings?Behavioural Ecology, 24, e1271-e1272.
Speed, M. P. 2000. Warning signals, receiver psychology and predator memory. Animal Behaviour, 60, 269–278.
Stevens, M., Cuthill, I. C., Windsor, A. M. M. & Walker, H. J. 2006. Disruptive contrast in animal camouflage. Proceeding of the Royal Society of Biological Sciences, 273, 2433–2438.
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