A sobering view of a Two-toed Sloth as it makes its way along utility lines on our way to Monteverde Preserve. This is what can happen to animals faced with disappearing habitat.
The Functionality and Evolution of Aposematic Coloration
In the realm of predator and prey, the preys are at a continual disadvantage, where they must always be on alert, in preparation for an attack. In many cases, natural selection has aided in alleviating this necessity for persistent alertness with mechanisms that provide for various degrees of defense. Common defenses include well developed claws, horns, and shells that will improve an organism’s survivorship. One defense which is difficult to oversee is aposematic coloration. Aposematic coloration is “a coloration consisting of distinct markings and colors that contrast with the natural background” (Leimar et al., 1986). Aposematic coloration is most commonly seen in the insect world, but also extends to amphibians and reptiles, among others. This paper intends to investigate the mechanism of aposematic coloration, analyzing its functionality, success and overall evolution. In conclusion, several organisms bearing such colorful traits are briefly discussed.
Color patterns in animals function for several reasons, including thermoregulation, communication and protection from predators (Endler, 1988). Aposematic coloration protects animals from predators by producing a “warning signal” from the combination of color patterns which they demonstrate. The “warning signal” identifies an animal as unpalatable, or distasteful to the inquisitive predator. Besides being unpalatable, certain animals may also internally produce a toxin, which will disrupt digestion and harm predators or make them unpalatable. It is believed that before warning coloration developed, a chemical defense system was first established in animals. Natural selection then favored a uniform color pattern for the species (Holloway et al. 1995).
Aposematic coloration must be easy to detect, recall and associate with a defense. This allows predators to identify the animals from a longer distance away and allots more time to make a decision whether to attack or not, based on passed experiences with the prey. These past experiences could be distastefulness, noxiousness or speed of the prey. It has also been proposed that some animals may naturally be tough, allowing them to survive after a predators attack. Experiments have shown that conspicuous prey takes less time to learn to avoid by predators, when compared to less conspicuous ones.
Common colors associated with aposematic coloration include: black, yellow, blue, red and orange. Color patterns can be directly linked to an animal’s potential for survivorship, as well as habitat preference. Black and dark colors result in faster predator learning and ultimately aversion. An increase in conspicuousness by increasing the mismatch of colors makes an organism reflect more effectively off it’s background and more recognizable by predators. Blues and yellow have the highest visual signals in shallow marine water, while reds and oranges are most effective in lake systems (Endler, 1988)
There are many questions as to how aposematic coloration evolved, because there are clear problems with why the mechanism was naturally selected for. In order for aposematic coloration to be effective, an undetermined amount of individuals must be killed, to educate the predator that their species must be averted, due to distastefulness or toxic danger. Normally, natural selection favors defenses with low risk that will increase a species production (Endler, 1988). There are several theories that attempt to explain the persistence of aposematic coloration, the foremost being gregariousness.
Gregariousness is a social characteristic where animals will meet in large groups. It is proposed that if animals of aposematic coloration and similar species form large groups, they will create a warning sign of increased signal proficiency (Gamberale and Tullberg, 1998). The signal will have greater surface area and be more noticeable to naēve and unconditioned predators. New predators will be attracted to the aposematic insects, will consume an undetermined amount of individuals and will then be conditioned for future aversion. Increased distastefulness will speed up the aversion process (Leimar et al., 1986) Aggregation is most common in insects and is believed to have evolved from the lack of profit in aposematic coloration. Within these groups, the risk of an individual being consumed is reduced, due to the large number of potential targets and larger warning coloration, which may be remembered by a predator from past experiences.
Leimar et al. (1986) describes “two aspects of predator behavior” that can allow aposematic coloration to become selected for. The first aspect is that aposematic prey provides faster avoidance learning than does cryptic prey. For this aspect to aid in the evolution of the coloration mechanism the predator must be exposed to the same coloration pattern. This can be achieved through repeated attacks on an aggregation of kin or having repeated experiences with the same prey individual that escapes and prevents its own death.
The second aspect is that of a peak shift in the generalization gradient of a predator’s behavior. The generalization gradient refers to the testing of an animal that has recently successfully learned discrimination of an aposematic colored individual. When the predator is “tested with a range of stimuli in the same stimulus dimension, they tend to generalize from the negative and positive stimuli to other nearby stimuli” (Gamberale and Tullberg 1996). For example, if a predator should come across a more conspicuous prey than previously experienced, the probability that it will be attacked is much lower based on the predator generalizing the stimuli with that of what it has negatively experienced in the past.
Aposematic coloration does not guarantee an individual’s survivorship. Any organism can serve as novel prey to a predator that has never been exposed to it. Once consumed, the predator may be conditioned to not eat the specific species in the future. Birds with low nitrogen levels have been known to eat aposematic colored insects for their protein, despite any dangers that they may incur. Certain birds are also not affected by the toxins produced by these insects, possibly having developed a tolerance from consuming toxic fruits in their area. There are a variety of fruits that are capable of producing secondary compounds within their pulp that are poisonous or digestion inhibitors. These toxins affect those birds that are not seed dispersers and will not benefit the plant directly (Herrera, 1985). Hererra (1985) noticed certain Spanish bird species consumed amounts of toxic fruit that were directly proportional to the amount of aposematic insects eaten.
Aposematic prey also suffers from strong antiapostatic selection (Lindstrom et al., 2001). Antiapostatic selection is the process by which predators prey more frequently on animals which are rare to them. Using wild great tits, scientists determined the effects of solitary vs. aggregated dispersion on aposematic prey survivorship. Lindstrom et al. (2001) found that “prey survived the best at the highest frequencies.” It would be safer for insects to congregate in groups, rather than alone with an inconspicuous background and a high visibility cost.
Experiments studying the evolution of aposematic coloration are still being preformed to determine how such an unprofitable system has continued to be selected for. An evident cost, the ultimate being loss of life, exists for those individuals displaying such conspicuous coloration. Methods such as aggregation and the physical toughness of an individual may balance out these costs. Be it poison dart frogs, coral snakes or monarch butterflies, these individuals and many others continue to flourish in the wild and attract not only the predator’s interest but also that of the human being.
Endler, J.A. 1988 Frequency-Dependent Predation, Crypsis and Aposematic Coloration.
Philosophical Transactions of the Royal Society of London. 319, 505-522.
Gamberale, G. and Birgitta Tullberg. 1996 Evidence for a Peak-Shift in Predator
Generalization among Aposematic Prey. Proceedings:Biological Sciences. 263,
Gamberale, G. and Birgitta Tullberg. 1998 Aposematism and Gregariousness: The
Combined Effect on Group Size and Coloration on Signal Repellence. Proceedings: Biological Sciences. 265, 889-894.
Herrera, Carl. 1985 Aposematic Insects as Six-Legged Fruits: Incidental Short-
Circuiting of Their Defense by Frugivorous Birds. The American Naturalist. 126, 286-293.
Holloway et al. 1995 A Quantitative Analysis of An Aposematic Colour Pattern and Its
Ecological Implications. Philosophical Transactions: Biological Sciences. 348,
Leimar et al. 1986 Evolutionary Stability of Aposematic Coloration and Prey
Unprofitability: A Theoretical Analysis. The American Naturalist. 128, 469-490.
Lindstrom, Leena et al. 2001 Strong Antiapostatic Selection against Novel Rare
Aposematic Prey. Proceedings of the National Academy of Science of the United
States of America. 98, 9181-9184.
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