"Zen teaches nothing; it merely enables us to wake up and become aware. It does not teach, it points." ~D.T. Suzuki

Monday, August 19, 2013

Warning! Colors! Part 2: Evolutionary Explanations



Part 2:
Evolutionary Explanations
"Nature, if I may personify the natural preservation or survival of the fittest,
cares nothing for appearances, except in so far as they are useful to any being."
Charles Darwin
Selection for Being Conspicuous

The first to suggest that the conspicuous color of some animals might have evolved through natural selection as a warning to predators was Alfred Russel Wallace. This was in reply to a letter from Charles Darwin in 1866. Darwin's enthusiastic response to Wallace prompted Wallace to request data that could be used to test the hypothesis at a meeting of the Entomological Society of London. It was this that lead John Jenner Weir to conduct experiments for 2 years with brightly colored caterpillars and birds that prey on them. His results provided the first experimental evidence for warning coloration in animals. The term aposematism itself was introduced by Edward Bagnall Poulton, a friend of Wallace, in The Colours of Animals (1890).

The significance of aposematic coloration has numerous and broad implications in ecology. For example in visual perception, communication and learning in predator/prey interactions, the mechanisms and environmental conditions in natural selection, insight into the convergence of traits, and the paradox of a fatal prey model in mimicry, just to mention a few.

As a side note, mimicry is arguably the oldest Darwinian theory not attributable to Darwin. It was Henry Walter Bates, another friend of Wallace, who gave the first scientific account of mimicry in animals.
Under certain circumstances conspicuous coloration and pattern may in fact be cryptic. A predator not tuned for colors that are easy to distinguish or a background where these colors blend or even a disruptive pattern can serve to camouflage an individual.

There are pigments that due to their structure show remarkable color change in different light intensity or angle.
In various lighting conditions the radiance and reflective transmission of color can be dramatically decreased. And this can be coupled with the loss of contrast on a similar background or even by interference. These pigments have a remarkable color change in different light intensity or angle, due to their structure.

As well, bars or bands at certain spatial frequencies are in fact cryptic. An example of this are a zebra's stripes. At twilight they actually make a zebra well hidden even at close range.
In other situations, the combination of quick movement and banded patterns may appear an achromatic grey blur to animals due to a well known optical effect in perception.

The basic premise of aposematic coloration is that the usefulness or adaptive quality of these phenotypes is that they function in some way for predator avoidance of the individual.

Predator-prey interactions will for the sake of predator avoidance, select for evolutionary compromises based on the strategy of an investment made to make predation difficult, since it is unlikely to be made impossible. Such an individual is marked in a manner which as a first line of defense warns potential predators of it's second line of defense, namely the genuine risk of harm to the predator. The risk of loss of life by standing out is compensated by the advantages of these traits such as increasing the chances for that individual's survival, but also its' offspring or its' relatives.

Whether color occurs first, after, or simultaneously with being noxious being this scenario asserts that some conspicuous phenotypes will be sacrificed in order for a predator to learn and make an association of the unpleasant experience with the color pattern. The experience itself must be noxious enough to deter further attack and reinforce the learning but with a minimum number of trails so as to allow the density of prey to be relevant.

This will have selective pressures on the predator to have a high motivation for learning certain color patterns well (and quickly). In some cases, as in long established or fatal prey interactions, an innate reaction of avoidance would be expected as is often seen. In this case, the prey is expected to be selected for a striking and unambiguous signal that would be clearly identifying the sender. The result would tend to a stabilizing selection for the color pattern and lower the variation in the trait used by the receiver's optical system. The effect of this would be greatly enhanced if the signal is fixed and "always on", so that attacks by predators will be discouraged regardless of whether the prey is aware of the predator.

Important in this, is the relationship between the advertised color and its visual perception by the predator as well as the convergence of aposematic patterns and their relation to the environment such as free exposure, diurnal activity and the organism's behaviors.

Important in being conspicuous are 3 the factors of available light, the background and the transmissivity. These factors in the conspicuousness of animals reveal a tendency for increased motion, that is in particular directional, repetitive and usually of sudden on set. In the patterns there is generally geometric regularity and surface contrast especially with colors outlined in contrasting hues of red and yellow, as well as spacial repetition.
Conspicuous coloration doesn't necessarily need to start out being aposematic. It is possible that if other functions of these colors provide do advantages over crypsis or dull colors, then selection may have operated on several factors for this phenotype. A consequence of this would be the development of defenses in response to the increased predation brought on by being conspicuous. In coral snakes and its' related elaphids, it seems that this scenario may apply in so far as the development of defenses since the colors and patterns of coral snakes, cobras and other snakes do conver other advantages in their ecology.
Coloration may also be used for species identification, especially for mating. This can be extemely important since an encounter with a similar colored aggressive mimic predator could be fatal. In the case of coral snakes, there are species of king snakes that range in coral snake regions. King snakes are not venomous, they are constrictors and as their name may suggest, they prey on other snakes. King snakes have the interesting adaptation of being immune to most types of snake toxin, they are also quick and strong and can even take on (and regularly eat) rattlers. Although they are less immune to elaphid venom they are also extremely good mimics of coral snakes.I've owned  a California King snake as a pet.
I got him as a hatchling and raised for over 10 years.

Thermoregulation is another possible function of color. Black can exaggerate heat gain. There are many short and long term benefits to this. Let's say for example... a reptilian predator that hunts early in the morning. A black color would be an advantage if this allowed the predator to warm its' muscles a little sooner and get jump on its' prey.

In poikilotherms, developmental processes are also temperature dependent. An increased metabolic rate would always foster growth but this is especially useful for predators that rest for digestion that would be helped by passive heat absorption. Even only slightly warmer animals in a population of poikilotherms can increase the rate of other metabolic processes and thus confer higher fecundity.

Other advantages to black coloration is seen in diurnal desert reptiles. Here there is the added benefit of protection from ultraviolet rays that penetrate tissue and cause dimerization in DNA. These induced mutations as well as other structural damage can be mitigated by melanin. Melanin is noted for maintaining the rigidness of structures in which it is deposited. There is also evidence that melanin pigments reduce water loss and thus give some protection against dessication.

But black and melanin pigments aren't the whole story. (I'm resisting the urge to pun orange is the new black...) Carotenoids are responsible for red and yellow pigments. They may also serve as protective coloration and have been suggested to function against excessive radiation as well as visual signals.
You might also like: / Warning! Colors! Part 1 / Warning! Colors! Part 3 /
© 2013 MU-Peter Shimon

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