The primary objective of any species is to survive. Whether it be obtaining food (predator) or avoiding being eaten (prey), an individual must use the best means necessary to achieve the immediate goal of staying alive. In addition and in correlation with the primary objective is the need to safely proliferate. Some animals, such as the green sea turtle (Chelonia mydas), will travel thousands and thousands of miles to the site of their own birthplace before laying their eggs. Why does Chelonia mydas go to such effort to ensure the proper birth its offspring? The answer is natural selection. The theory of evolution by natural selection, proposed by Darwin and Wallace, revolves in part around the idea that only the strong survive. Defining strong in terms of survival is difficult in that contextual circumstances elicit a different definition. Strong in this context refers to a characteristic of a species that allows it to stand out above the rest. It is an adaptation that has allowed a particular species to gain an advantage over everyone else, allowing it to successfully proliferate. The primary concern of a species is not competition between neighboring individuals, rather competition between other levels of the food chain within its own niche.
At first glance, the prevailing characteristic of any species is their color. Ranging from the beautiful array of colors on the hind feathers of a peacock to the dull, olive drab green of the sea turtle, we have come to understand that these colors did not happen by accident. Endler (1992) proposed that the evolution of animal signals, including color patterns, is predictable from knowledge of: the environmental conditions under which they occur, the sensory systems of the signaler and of the potential receiver, and the behavior of the signaling individual.
Conspicuous color patterns are used in several intra- and inter-specific communicative interactions. They include aposematic communication, anti-predator displays, territorial defense and aggression, to express motivational state, for courtship and/or mate preferences, in recognition of species, individuals, and sexes, to lure prey, or deflect predatory attacks, to emphasize an animals body weapons, or to advertise an animals location and/or body orientation. Color patterns may also have an affect on physiological processes. Examples include thermoregulation and water balance, providing protection against ectoparasites, ultra-violet light radiation, and in structural damage to body parts, such as preventing wear.
In some way or another, the coloration of a species aids in its survival. Three common methods of survival via coloration are camouflage, warning coloration, and mimicry. Other methods exist, but these three appear the most often and it is these three that this paper shall focus on.
Before we begin, I want to first point out that no two species are alike; therefore, no method used to ensure survival is better than another. Each species uses the method best formatted for their niche, and it would be ignorant of us, the human species, to judge one method as more complex and advantageous than another. To demonstrate survival of the fittest doesn't necessarily mean using the most advanced form of eluding extinction, it means using the best method while expending the least amount of energy.
The main function of camouflage is to provide concealment from predators and/or to pursue prey undetected. Camouflage is unique because it does not give any signal to the would-be predator or prey. Camouflage is a non-signal because a potential receiver does not perceive a camouflaged individual as being different from the environmental background. (Mallet and Joron, 1999) It works on the idea of "out of sight, out of mind". Prey that successfully mimics its surroundings will survive to propagate its camouflaging markings (demonstrating natural selection). (McKee et al., 1997) In a few cases, mimicry is used in conjunction with camouflage. Mimicry only works as camouflage if the prey remains quite still, since relative motion allows the predator to distinguish object from background. (McKee et al., 1997) On a personal note, my two younger siblings and I watch in astonishment at how our "pet" tree frog attacks the cricket we drop in its cage. Even though the cricket may not blend in with a selected background, the frog is still unable to see the cricket until it moves. It is fascinating to watch the stand-off that proceeds as the cricket remains deadly still and the tree frog is waiting for just a slight movement so it can locate exactly where the cricket is placed. Its equally amazing to see how long the cricket can remain absolutely still (up to 30 minutes!!!). The same type of situation occurs in the wild. Animals will remain still for as long as it takes because they know that any movement could be their last (more on vision and recognition later).
Often times, a fine line can be drawn between mimicry and warning coloration. Warning color and mimicry have been discussed from three different points of view. There is the traditional insect "natural history" angle, which makes simplistic assumptions about both predator behavior and prey evolution. There is the evolutionary dynamics angle, which virtually ignores predator behavior and individual prey/predator interactions. And finally there is the predator behavior "receiver psychology" angle, which tends to be simplistic about evolutionary dynamics. Mimicry provides a model system for the shifting balance theory as well as functioning as a barrier to isolate species. (Mallet and Joron, 1999)
Mimicry has diversified at every taxonomic level: warning color has evolved from cryptic patterns, there are mimetic polymorphisms within species, there are multiple color patterns in different geographic races of the same species, mimetic differences between sister species, and multiple mimicry rings within local communities. These contrasting patterns can be explained by the "number dependent" selection model first modeled by Fritz Muller in 1879. It stated: Purifying selection against any warning-colored morph is very strong when that morph is rare, but becomes weak in a broad basin of intermediate frequencies, allowing opportunities for polymorphisms and genetic drift. Today's "receiver psychology" model predicts that classical Mullerian mimicry could be much rarer than previously believed, and that "quasi-Batesian mimicry", a new type of mimicry intermediate between Mullerian and Batesian, could be common (although untested, it seems unlikely). (Mallet and Joron, 1999)
Put simply, Mullerian mimicry is mimicry between unpalatable species. It operates on the theory that if a constant number of individuals per unit time must be sacrificed to teach predators a local color pattern, the fraction dying in each species will be reduced if they share a color pattern, leading to an advantage in mimicry. (Mallet and Joron, 1999)
The other form of mimicry, Batesian mimicry, credits its existence to Bates when he noticed two features of butterflies in the Amazon. He noticed that color patterns of unrelated species were often closely similar locally and these "mimetic" patterns changed radically every few hundred miles. He deduced that a tasteful or palatable species will mimic an unpalatable species to fool the predator into not eating them. Bates theory is generally accepted and is widely seen, especially in butterflies and insects.
Warning color, or "aposematism", was first developed as an evolutionary hypothesis by Wallace in response to a query from Darwin, four years after Bates' publication on mimicry. The primary goal of warning color is to increase the efficiency with which predators learn to avoid unpalatable prey. Often times though, being uniquely marked can lead to a disadvantage. A warning-color variant within a cryptic but unpalatable prey will suffer a two-fold disadvantage because first, they are more conspicuous to predators and second, it does not gain form warning color because predators, not having learned to avoid the pattern, may attack it at a higher rate than the cryptic morph. (Mallet and Joron, 1999) This creates an initial spread, even though, once evolved, warning color is beneficial because by definition it reduces the number of prey eaten during predator learning. In exactly the same way, a novel warning pattern is disfavored within an already warning-colored species, essentially because of intra-specific mimicry. This selection against rarity makes it easy to understand why warning-colored races are normally fixed and sharply separated by narrow overlap zones from other races, but in turn makes it hard to understand how geographic races diversified in the first place.
If it weren't for unpalatability, mimicry would never be possible. Unpalatability is defined as any trait that acts on predators as a punishment, and that causes learning leading to a reduction in attacks. (Mallet and Joron, 1999) Being unpalatable has both its advantages and disadvantages. Individuals incur costs in synthesis or processing of distasteful chemistry. Individuals are also likely to incur damage, while other members of the population mostly benefit from predator learning. Also, benefits are shared among kin, and many believe that kin selection is responsible for the evolution of unpalatability.
Recent research has found that being unpalatable may not be that costly after all. Although expensive to process distasteful secondary compounds, in some cases the same biochemical machinery is required to exploit available food. Other advantages include the fact that most time predators taste test their food, and in addition the production of toxic chemicals adds to the resiliency of the individuals exterior, causing limited harm.
The primary objective of any species is to stay alive. Evolution has graced individual species with the ability to conform to the added pressures of surviving. Those who use their means effectively will live to safely proliferate. Those who do not, add to the long list of individuals that contribute to finding the best means of survival. In essence, evolution is a complex system of trial and error. Each time a new barrier arises, the characteristics of a species will change until that barrier can be safely hurdled, often times taking several generations.
As mentioned previously, no one method of survival is better than the other. A species creates methods of survival that enable it to meet the demanding needs of its environment. I hope I have demonstrated though that one survival characteristic is found in all species, coloration. From there, it is up to evolution and the individual species to determine what means of coloration will be most effective in their goal for survival.
Endler, J. A. (1992) Frequency-dependent predation, crypsis, and aposematic coloration. Philos. Trans. R. Soc. London Ser. B 319: 459-72.
Mallet, J. and Mathieu, J. (1999) Evolution of diversity in Warning Color and Mimicry: Polymorphisms, Shifting Balance, and Speciation Annu. Rev. Ecol. Syst. 30: 201-33.
McKee, S. P., Watamaniuk, S. N. J., Harris, J. M., Smallman, H. S., Taylor, D. G. (1997) Is Stereopsis Effective in Breaking Camouflage for Moving Targets? Vision Res. 37: 2047-2055.
Ortolani, A. (1999) Spots, stripes, tail tips and dark eyes: Predicting the function of carnivore colour patterns using the comparative method. Biological Journal of the Linnean Society 67: 433-476.
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