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There are many relationships that occur in nature that contribute to the over-all balance of the ecosystem. Whether they are positive or negative, an organism's survival depends on these relationships. Relationships like mutualism, commensalism, parasitism and predation occur with a dynamism that is constructed through many years of development and changes in the organisms that are driven by many factors in the environment. Many of the changes in an organism that share in a certain ecological relationship with another are also highly influenced by the changes that occur in the organisms that share this relationship with them. In a sense, the evolution of an organism as a member of an ecosystem is never solitary. As one organism changes, other organisms that are related to it also change accordingly to increase their chances of survival (Berryman, 1992).
Predation is one of the most dynamic ecological relationships as the organisms that are involved are constantly changing to eat or not be eaten. These organisms change morphologically, physiologically and behaviourally to effectively achieve advantage over their predator or prey. The survival of the predator depends on its ability to successfully capture its food, and the survival of the prey is dependent on its ability to evade the advances of its natural predator. Predator and prey are always trying to outdo each other in what many ecologists describe as a "biological arms race".
This paper aims to describe the dynamics and changes of predator-prey relationships that occur in nature by discussing the principles that are involved in these changes on the morphological and behavioural level. The paper will also cite a specific vertebrate predator-prey pair to illustrate these points.
II. Co-evolution: Changing together
Biologists over the years have observed that some species evolve in response to the changes in another. These changes can occur in different levels and in different intensities. Some species exhibit molecular changes such as the structures of macromolecules like proteins or lipids, while others show morphological and behavioural changes in the presence of other organisms. Some evolutionary changes that happen in a species can be a change that is most especially directed towards its adaptation in its relationship with another organism. This evolution that happens in response to the changes in the biotic factors of the ecosystem is called co-evolution. The habits or changes in one organism in an ecological relationship create a pressure that drives changes to another. Therefore, evolutionary changes in organisms due to changes in temperature, water and other abiotic factors are not co-evolution, even if changes happen simultaneously in organisms that are in a specific ecological relationship.
Easily observable co-evolution occurs in the level of two species interacting, but co-evolution can also be driven by a number of species interacting with each other. Co-evolutionary changes may affect interactions positively or negatively, depending on the type of relationship that drove it in the first place. For example, if co-evolution is to happen between two species of mutualistic organisms, an organism's evolution may be a response to the change that occurred in one of the interacting species to keep the mutualistic relationship running, which affects the relationship in a positive way. Co-evolutionary changes that happen in the prey which hampers the predator from successfully capturing the prey affects the predatory relationship negatively because it reduces the chances of the predatory relationship continuing.
However, it is important to remember that the changes that are pertained to are genetic changes -the ones that can be passed on from generation to generation. Therefore, physical disabilities of an individual species that are purely phenotypic in nature that affect an ecological relationship or are caused by that relationship. For example, if a hairy rodent that lost a patch of fur from a previous attack from an eagle that it managed to escape from gains advantage by making it invisible to other eagles is not a product of co-evolution. If, however, some genetic mutation makes a certain members of a rodent species lose a patch of hair so that it becomes safe from the eagle, its natural predator, this could be considered co-evolution (Yoshida, Jones, Ellner, Fussman, & Hairston, 2003).
It is also important to remember that the ecological relationship will remain even though co-evolution occurs. Changes in an organism might be driven by the other organism, but never will one outdo the other permanently. They will both change for the better so as both will always be almost on the same level of fitness.
III. Co-evolution in Predator-prey Relationships
Co-evolution that happens in an ecological relationship is most often illustrated in negative relationships like competition, parasitism and predation. In these relationships, co-evolution is most often seen as a race towards outdoing the other.
The predator depends on prey for food. Since their prey are motile organisms, many predators develop morphological characteristics that make them fit for this purpose. Predators have morphologies, physiologies and behaviours that make them effective in their niche (Smith & Remington, 1996). These adaptations developed overtime in the whole course of their evolutionary history.
Morphological characteristics like long legs, stronger muscles and higher lung capacity give predators an edge when it comes to chasing down exposed prey. Also, most have sharp teeth for tearing flesh and disabling prey and their large size makes it easy to overpower their relatively smaller prey. It is rare to see a predator that has a prey that is larger than they are. In such cases, some predator species have morphological and physiological adaptations that compensate for their lack in size.
Some predators exhibit different colorations and markings that enable them to blend into their surroundings. These markings allow the predators to become relatively invisible to the prey. These make their hunt relatively easier. They can be markings or colorations that allow them to blend with their surroundings (camouflage) and some also use these markings to mimic a relatively harmless object that is naturally found in the environment (mimicry). Furthermore, these colorations and markings can be a sign of another characteristic that some predators have to aid them in capturing prey. Most venomous animals that are predators bear distinctive markings that serve as warning signs of their poisonous nature.
But prey also develop certain characteristics that seem to counteract the advantage that predators present. Some prey have lighter builds; to compensate for their lack of size, most prey also run fast. Some have short fur or have hairless, slippery bodies so that they would not be easily gripped by strong claws or teeth (Hederstrom & Rosen, 2000). Like predators, they also have mechanisms to camouflage themselves in their habitat. Their markings and colours also serve for camouflage and mimicry. Some prey also have various venoms and toxins and other defense mechanisms like spines (i.e, porcupines) and odors (i.e., skunks) that deter attacks.
Some behavioural adaptations also occur in both prey and predator. These behavioural adaptations are supported by their morphology and physiology, and most arise because of these physical characteristics. Night time hunting is common for some predators as many species of prey are especially vulnerable at night. Predator eyes that are adapted for low light conditions allow this behaviour to persist. Some predators mimic the behaviours of an organism that its prey consider its food source or is harmless. In effect, they draw their prey to them because of these behaviours.
Some prey base some behaviours on the fact that most predators do not like to feed on carrion. Because of this, they learned to use a behaviour that involves "playing dead", or mimicking a dead animal. This makes the predator move away from its prey, thus allowing it to survive another day.
Co-evolution can be seen in the adaptations that both predator and prey undergo in their entire life history. Many species have been lead to survive and persist until today even if they have natural predators that existed throughout their natural history. The developments that occurred in them alongside their natural predators have allowed them to gain advantage and not die out. Similarly, predators have continued to persist even though their prey have developed characteristics that allowed them to resist the predator's advances. This is due to the fact that they responded to the pressure that the adaptations that their prey possesses and developed characteristics themselves that allowed them to continue being predators. This dynamism always occurs in a balance that is almost never disrupted. No species truly outdo another and no species are eliminated because of a "co-evolutionary advantage."
IV. Case study: Predator-species interaction of the Indian Mongoose and King cobra
Many think that cobras are not preyed upon by other animals because of their venom but mongooses like the Herpestes brachyrus are some of the animals who consider cobras as their prey. Although mongooses do not usually have a preference for cobra meat, they can and they would feed on these dangerous snakes if given a chance.
Mongoose is a common name for more than thirty species of the Herpestidae. They are small, rodent-like animals whose size range anywhere from 0.30-1.3 meters in length. Most mongooses are about the size of a domesticated cat. These organisms usually are found alone, but some species live in groups.
King cobras (Ophiophagus Hannah) are known as one of the most dangerous species of snakes in the world. Many of these snakes can easily be recognized by their hoods and trademark upright stance when threatened or hunting.
Cobras strike their prey and their attackers to release venom that can kill a small organism within minutes, or in the case of humans, for 30 minutes or less. These snakes possess potent venoms that are most usually neurotoxic. Cobra venom has an ability to block acetylcholine receptors. In effect, all muscles cease to move and become paralyzed. Soon, the heart and other vital organs cease functioning.
The mongoose seems not fit to become a predator of such a deadly organism, but its morphology, physiology and behaviours allow it to become one of the cobra's potent natural enemies.
The mongooses possess relatively thick coats which make it difficult for the fangs of a cobra to penetrate. They are very quick and agile creatures, which allow them to strike a cobra fatally with its sharp claws before it can even strike to defend itself. The mongoose targets the cobra's head first with its claws and then proceeds to bite it with its sharp teeth, disabling it and eventually, killing it.
The cobra's venom, its primary defense, is relatively useless against the mongoose. Mongooses possess a natural resistance to cobra venom as the acetylcholine in its body do not bind with the receptor blockers of cobra venom (Barchan, Kachalsky, & Neumann, 1992).
Fig. 1. A mongoose attacking a king cobra. The mongoose disables the cobra by targeting its head.
It is important to note, however, that mongooses cannot withstand high doses of the venom. Only its speed and thick fur saves it from a second, and possibly fatal, strike.
Cobras have a natural defense too when it comes to mongooses. Some snakes manage to escape mongooses because of their speed and slippery skin. Some learn behavior such as feigning death, leading the mongoose to leave them alone (Gehlbach, 1970).
Organisms have certain characteristics that allow them to gain advantage in their habitat and these characteristics may have arisen as a product of their relationships with other organisms. Co-evolution occurs when some genetic characteristics are developed as a result of a pressure caused by another organism that is closely related to it in niche or in another ecological relationship. These changes may be morphological, physiological or behavioural.
Co-evolution in the predator-prey relationship is often viewed as a race to outdo each other. But inasmuch as we see that both species seem to overcome advantages that are developed by the other, it is important to remember that the changes that they get happen in such a manner that one cannot absolutely outdo the other to the point of eliminating the other completely.
The mongoose and the cobra have a very unusual predatory relationship as the prey is considered as one of the most venomous animals in the world. However, these two organisms display the principles of morphological and physiological advantages when it comes to the predator-prey relationship.