Aposematism
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[edit] Definition
[edit] Types of aposematism
The most familiar form of aposematism is warning coloration. We think of yellow and black as the typical aposematic colors, but in fact the whole range of colors are found in nature, with the exception of camouflage colors such as green and brown. As a general rule we may say that if (as is almost always the case) an aposematic creature displays more than one color, they will be in striking contrast and have sharp boundaries: examples may be seen in the selection of Nudibranchia to the right of this article.
Smell is another common form of aposematic signal: for example, a ladybird will defensively undergo "reflex bleeding", the emission of yellowish gunk which you may sometimes have observed when handling this insect. This gunk is rich in pyrazines, a class of molecules common in poisonous plants and animals, which smell and taste bad and deter predators.
A plant, having a more robust physiology than an animal, can afford to lose a leaf or two to a predator to convince the predator that it's not worth eating: in this case, unpalatability is the aposematic signal.
[edit] How aposematism deters predation
There are two mechanisms by which predators avoid aposematic species. First, they may do so by instinct. For example, the Great Kiskadee will avoid coral snakes even when they are "naive" birds which have been hand-reared by humans (see S.M Smith, 1977, Coral snake pattern rejection and stimulus generalisation by naive great kiskadees, Nature, 265). In the same way, naive hand-reared chicks of the domestic hen prefer olive-colored mealworms to yellow-and-black, red-and-black, or red mealworms (see Roper and Cook, 1989 Responses of chicks to brightly colored insect prey, Behaviour 110)
Secondly, predators can learn to associate the appearance of potential prey with its noxious properties; furthermore, the learning process is faster when the prey is conspicuous rather than cryptic (see, for example, G. Gamberale-Stille, Benefit by contrast: an experiment with live aposematic prey,Behavioral Ecology Vol. 12 No. 6).
[edit] Evolution of aposematism
Aposematism is an evolutionary trade-off: aposematic organisms lose out by being visible, or detectable by scent, from greater distances, but gain from being recognisably members of a toxic species over short distances. Since a predator has to get up close to its prey to eat it, this is obviously a trade well worth making.
So it's easy to see how aposematism is maintained by natural selection once it has evolved; it is a little harder to see how such a state of affairs could arise in the first place. For a poisonous or unpalatable animal benefits from being poisonous or unpalatable by resembling a member of its own species, all of which are also poisonous or unpalatable. It would seem on that basis that any variation in any superficial trait, including a tendency to aposematism, ought to have a selective disadvantage.
However, this need not be the case, if some variation arises which makes the variant organism resemble a member of its own species more than a typical member of its species does. This suggestion may seem paradoxical: let us give a more concrete example. Consider a beetle which, if eaten, causes a digestive upset. The beetle will have a generic sort of beetle smell, which will attract predators, and a smell distinctive of its own species, which predators will learn to associate with nausea. Any variation which increases the production of the specific odor will make the variant individual easier to identify, by its smell, as being a member of its species. This will also make the individual more easily found by predators that hunt by smell; natural selection will favor such a variation just so long as the disadvantage of being easier to find is outweighed by the increased reluctance of predators to eat the beetle once they've found its scent.
Experimental work has shown[1] that increased contrast in the patterns of prey insects does indeed improve predator aversion learning and predator memory retention of the aversive response.
To summarise this hypothesis: a toxic or unpalatable species which is not aposematic will nonetheless have characteristics by which predators can recognise it, and which by that very fact serve as warning signals, although they did not evolve for this purpose. Aposematism then evolves because natural selection favors the intensification of these distinguishing characteristics, so long as the advantage of being recognisable as the member of a poisonous or unpalatable species outweighs the disadvantage of being easier to locate.
Mathematical models show that given the right circumstances, natural selection will indeed favor the evolution of aposematism: favorable circumstances include a high probability of detection of the non-aposematic variety, and a strong predator response to the development of aposematism.[2]
[edit] Aposematism and sexual selection
Any characteristic which identifies a species can be used to advertise to potential mates as well as to predators. For example, there is evidence that the pyrazines secreted by ladybirds to deter predators, as mentioned above, also serve to attract other ladybirds[3]. Hence, aposematic characteristics may be favored by sexual selection. However, we can distinguish between secondary sexual coloration and coloration which also plays an aposematic role: sexual coloration will tend to be confined to one sex, and to fluctuate seasonally, as it is useful only in the mating season: aposematic coloration is useful to both sexes all the year round.
[edit] Aposematism and mimicry
Aposematism is associated with two kinds of mimicry. In Batesian mimicry, a harmless species benefits from a resemblance to a harmful aposematic species. For example, the viceroy butterfly, edible to birds, bears a close resemblance to the monarch butterfly, which is unpalatable.
In Müllerian mimicry, two or more noxious aposematic species resemble one another: this benefits both species because a predator having learned to avoid one will avoid the other. For more information, see the main article on camouflage and mimicry.
[edit] Evolution and the discovery of aposematism
From a historical view, it is interesting to notice how the theory of evolution led to the discovery that aposematic coloration was, in fact, aposematic in function. Naturalists in the nineteenth century didn't realise that it served as a warning signal: the whole question of its function would not occur to any naturalist who believed that bright colors were God's way of making his creation look pretty. To Darwin, however, it was a puzzle: perhaps some coloring could be attributed to purposes of sexual display, but this would hardly account for it in species such as bees, with a social system in which sexual display serves no function. It was Darwin's co-discoverer of evolution, Alfred Russel Wallace, who first realised the role of aposematic coloring as such; this led directly to the pioneering work of Bates and Müller on aposematism and mimicry.
