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

Saturday, August 24, 2013

Warning! Colors! Part 3: Evolution of Coral Snakes

Warning! Colors!

Part 3:
Evolution of Coral Snakes
Coral snakes have maintained their respective populations in a variety of environments throughout the world. They live with different lighting, background and predation pressures. 
Yet there are consistent trademarks despite the regional or geographic variations.
Coral snakes are usually live in forests, but are also found in grasslands and deserts. Their distribution ranges throughout the tropical, subtropical and neotropical regions in the world. Like virtually all snakes, with the exception of cobras, (which are arrogant) they are secretive and have a docile disposition. They generally don't bite unless truly threatened such as by being stepped on.

The coral snake is really several species all belonging to the genus Micurus of the cobra family, the Elaphidae. They are generally small (maximum recorded size five feet) with a common length between eighteen inches and two feet. 

All coral snakes are diurnal (active in the day-time) predators, thus they have round eyes for the daylight. This condition is similar to the eyes of non-poisonous snakes. The coral's eyes are very different from the oval eyes of nocturnal (active in the night-time) snakes and most other poisonous snakes. However, coral snakes also have similar oil droplets in their eyes as do some birds. This oil droplet appears to aid their color vision by filtering out certain wave-lengths of light.

The head of all coral snakes is blunt, the scales smooth and glossy. The body shape is narrow, tapering to a sharp tail. The body is banded in triads (in threes), that may be narrow or wide. They have alternating strips of red and black always separated by yellow. This is typical and most species have this although there are variations in proportions or intensity of color. Some species have shown melanism (tending darker) and others albinism (tending lighter).

The triad red, yellow and black banding is the general rule. However, there are also exceptions to this banded body pattern in certain parts of the world. Instead of banding across the body it may be along the body length-wise, or in unusual cases it may have a different pattern. An example of this is the blue coral from Asia. The blue coral snake is noted for its conspicuously bright red head and tail. Although the blue color may not be a common color in the corals, this species is still aposematic.

Like all snakes, corals snakes are exclusively carnivore. They are extremely quick and nimble, preying specifically on other snakes. They may also take lizards and other reptiles. Prey are killed by venom only if the prey is dangerous to the snake. The venom is highly toxic though small in quantity. 

Coral snakes have two enlarged, tubular, maxillary teeth in a fixed anterior position in the upper jaw of the mouth. New fangs are continuously replacing older ones. These tubular fangs are hollow. Each one is individually connected to a duct that is in turn connected (only during discharge) to a venom gland.
This venom gland (in fact, a modified salivatory gland), is interiorly located just to the rear of the eye in the skull. It is surrounded by a tissue sheath and is activated by a set of muscles that are independent of the bite mechanism. By this structure, the coral may inject no venom at all, or may inject from only one fang or both. It has complete control over the amount of venom released and has rarely been observed to eject full content.
Whether the development of color in the species occurs first, after noxiousness, or simultaneously, the predator-prey model asserts that at least some conspicuously colored prey will be sacrificed in order for a predator to learn and make the association of the pleasant experience with that color and pattern of animal. The unpleasant experience itself must be just noxious enough to deter further attack and thus reinforce learning, but ultimately not kill the predator outright. Still, it must deter with a minimum number of trials so as to allow the prey density to be relevant.

The second point is one that is often draws the questions that most people have difficulty to grasp. If it has no previous experience of the prey's threat or danger, how does a predator know to avoid it? This point will also lead to other questions. How can it benefit the warning prey if the predator does not initially know the warning signal? Put another way, how can the trait be selected for and preserved, if the snake is killed or fatally injured during the predator's first learning experience? Then again, there is the similar but converse point. How can a predator learn about the prey's danger and avoid the prey, if it's dead after its first encounter? How can this be adaptive or help with fitness?

The third point address the fact that a hungry predator might still be tempted to "go for it". The warning signal must in fact be backed-up with something that is strong enough to illicit the avoidance reaction. The proverbial stick in the saying "Walk softly but carry a big stick." has to really be there and it must be a pretty big stick, even a deadly one!
The individual is marked in a clear manner.
It warns potential predators of harm or danger.
The predator must know the warning.
Or the predator must have some way of learning it.
The warning must be enough for deterence.
The predator should actually avoid a direct encounter.
Determinants to Conspicuousness
1. Amount and type of available light
The source of light, time of day, as well as clouds or other cover.
2. Transmissivity of the material and the light
Light absorption-color reflection and type of material or texture.
3. Background color or patterns in the visual field
Dark or light and simple or complex backgrounds. 
The loss of life that is risked or incurred by the individual for a predator's learning curve can be compensated for by inclusive fitness. An adaptation by inclusive fitness means that any inherited trait that increases the individual's genes in the population is fit by Darwin's definition. The trait then, will enhance the organism's chances of perpetuating its genes by the survival of its kin. It is not necessarily the individual's survival that bestows fitness and the survival of a species.

A trait may be adaptive by kin selection. This can be done through the number of kin and the genes in common with its surviving relatives. Hamilton’s rule, the central theorem of kin selection, states that an altruistic trait can increase in frequency if the cost of the trait to the actor in terms of individual fitness is offset by that individuals gain in inclusive fitness. Hamilton’s rule may be expressed as rb - c > 0, where r is the genetic relatedness of the actor to the recipient, b is the benefit to the recipient, and c is the cost in fitness to the individual. The trait is fit by increasing the individual's genetic contribution to future generations.

There is another effect that may come about in the process of educating a predator. If the genes expressing for the phenotype are carried even by non-related individuals then the trait may still be adaptive by a kind of green-beard selection. A green-beard is an expression to indicate a genetic marker, something that identifies another individual as having some similarity. In this type of selection it is the phenotypic similarity between the actor and the recipient, rather than the genetic relatedness by descent that is selection is acting on.

Therefore, despite the lack of assurance of survival to the individual the trait for conspicuousness and indeed the warning color will be adaptive under several scenarios.

The dominant colors seen for aposematic coloration are varying shades of red, yellow and black. The conspicuousness of color concerns the question of not only what color but also the use of color in pattern. In considering the visual aspects of certain colors and patterns in coral snakes, a general outline is useful. Accordingly, there are three principle elements in conspicuousness listed below:
The chief predators of coral snakes are diurnal birds, king snakes, and some mammals. It is presumed that, in fact, color vision mechanisms are adapted to the priorities of food location, mating and predator detection in these animals. Logically then, it can be seen that coloration will take advantage of these characteristics.

Oil droplets in the retina, specifically in the cones of some animals, have been long known to be effective cut-off filters. They are only found in such animals as; amphibians, reptiles, birds and marsupials. These oil droplets act as filters to short wave-length light and transmit the long wave-lengths of red and yellow. Of the bright colored oil droplets, red has only been found in turtles and birds.

It is widely known and accepted that plant flowers pollinated by birds, various fruits whose seeds are dispersed by birds as well as many noxious and presumed aposematic animals are most often colored combinations of red and/or black, yellow and/or black, or red, yellow and black. Incidentlaly, the black makes an excellent contrast and so enhances the other color next to it.

It is appears that natural selection favors individuals that recall only one or two important clues for that is the prevalent trait in a wide range of animals in the world. Animals learn to associate food with any visual clue that fosters storage or those where the association is forgotten slower than other clues.

It is found that many animals will never touch a potential food item if it is associated with a noxious experience, even when there is a delay in the negative experience and even if the noxiousness is from an external source. Each species is programmed to identify the good or forbidden food encountered in the future by its own set of rules. There is much research pointing to aposematic colors playing a large role.

 Despite the opportunities for escape and learning, over evolutionary time, there appears to be selection pressure for innate avoidance due to fatal encounters. An insect eating monkey when offered over 200 different species of insects will accept 83% of cryptically colored (camouflaged) species but only 16% of conspicuously colored ones. This was found to be so, even though many of the insects belonged to species that the monkey had previously never encountered. It seems that conspicuous colors of aposematic animals are very effective as a means of not being eaten.

Many studies show that some mammals have clearly negative responses to coral snake patterns. This avoidance reaction occurs even in those species that were never exposed to coral snakes in their lifetime, but it seems in the past their ancestors would have encountered coral snakes.
What is good or bad in getting food is important

It is remarkable that many young animals
are very alert to the feeding habits of others.
Imprinting, especially in birds is well documented.

Important in the motivation for learning:

Nutrient procurement
Safety and survival
Mating and reproduction

Elements of learning itself:

Perception and cognition
Storage of information
Recall of information

There is a definite avoidance of coral snakes.
There is ample evidence of predators learning to avoid dangerous snakes by witnessing an attack, and this empathic type of learning need not be restricted to social animals. Innate avoidance to the coral pattern may not be generalized to all birds however, those species from tropical zones that would have contact with them demonstrate a strong avoidance. There is data to show that when presented with painted wooden dowels, some birds would attack only the portion of the dowel painted with coral snake colors.
Thus, it would appear that coral snakes are indeed conspicuous to both potential prey and predators. In spite of any other functions, these colors are a strong signal and do attract attention. If they are so conspicuously colored, then a form of defense against the likelihood of increased predation would be selected for. This is of course the reverse of the general assumption that color came after the deterrent. However, the idea that color came first is supported by scientific conclusions that snake venom is a rather costly form of prey capture when in fact other snakes feeding on similar prey have not needed venom at all. Add to this the observation that venom is used only in submissing dangerous prey. It shows that venom has probably been developed in snakes primarily as a defense.
It would seem quite evident that the color patterns of coral snakes can certainly be recognized either by learning or as innate signals to stay away. Since these colors do in fact serve to warn potential predators of a true fatal threat or highly undesirable encounter, they certainly fit the criteria for being aposematic.
You might also like: / Warning! Colors! Part 1 / Warning! Colors! Part 2 /
© 2013 MU-Peter Shimon

1 comment:

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