In a similar scenario, the parasitic relationship between cuckoos and other birds, the development of resistance to a parasite leads to the evolution of the parasite. This polymorphism is known as coevolution. Nitecki uses grass as a simple example of this phenomenon(1983). Grass evolves a resistance to a strain of rust by making a single gene substitution, and the rust counters this step with it s own single gene substitution(Nitecki, 1983). He adds that many parasites are host specific, so they are keyed into their host and can adjust to the appropriate changes when necessary. This is why parasites are a continual problem, not just an irritant that is rendered extinct by one simply change in the host s evolution.
This helps explain why the cuckoo continues to successfully lay it s eggs in the nests of Meadow Pipits, Reed Warblers, Pied Wagtails, and Dunnocks(Brooke, 1991). According to Brooke(1991), the host birds usually are deceived by the cuckoo s egg and then raise the cuckoo chick instead of their own. By examining the cuckoo, it is easy to see how evolution has perfected the parasitic process. According to Brooke (1991), the cuckoo will watch it s prey as it builds its nest, wait until both parents are away from the nest, then enter the nest to remove one of the original eggs and lay it s own. Each species of cuckoo has evolved to specifically target one of the four possible birds. According to Brooke, (1991) the Great Reed Warbler-Cuckoo will lay an egg that is similar in size and color to the hosts, and the cuckoo has perfected the intrusion to a science, spending about 10 seconds in the nest of it s host.
The next step of parasitism comes once the cuckoo has hatched. The process that the chick goes through is described by Brooke (1991); the chick hatches before the rest of the clutch due to it s shorter incubation period and then pushes the other eggs out of the nest. The host family will not abandon the chick, while the exact reason is not known, there are several theories. According to Brooke (1991), the parents have nothing to compare the chick with or do not decide that it is too late to raise a new clutch and will raise their adopted chick.
Brooke describes some of the tests carried out in his research (1991) concerning the factors that influence the rejection rate of cuckoo eggs. Most birds will not reject eggs that are similar too their eggs, but larger eggs are have a higher rate of rejection. But if the host birds see the cuckoo in the nest, then the rate of rejection is much increased(Brooke, 1991), which explains why cuckoos have evolved such a fast predatory process.
Brooke shows an example of the evolutionary process at work when he examines the Dunnock s relationship with the cuckoo(1991). The Dunnock-Cuckoo has not developed an egg that mimics the Dunnock egg because Dunnocks accept eggs of any size and color. Brooke (1991) believes that the Dunnock is a new species of bird under parasitism, for only 2% of the Dunnocks are preyed upon in England. Therefore, Dunnocks have not yet developed any defenses against the cuckoo, so the cuckoo has no need to develop any traits to aid in parasitism. Brooke (1991) showed other examples of evolution by testing isolated species of hosts. These birds were not as discriminating, implying that they lacked the evolutionary advancements of detecting and rejecting parasitic eggs. The cuckoo and their hosts are clear examples of how both the predators and they prey affect the evolution of each other.
In some cases, predator/prey relations take place between members of the same species. Many animals exhibit group behavior; worker bees serve the queen bee and wolves follow an established ranking system. But when members of the same species endanger each other for individual protection, the member of the species that faces death is being used as prey by the member of the species surviving. Robert Heisohn describes this relationship in lions when territorial disputes occur. The leader lion will be 50-200 meters ahead of the laggards when approaching an invading lion(Heinsohn, 1995). The leader will face severe injury and even death while the laggards reduce their risk by staying behind(Heinsohn, 1995). Similar behavior has been observed in many species of birds. The hatchlings commit siblicide in order to maximize their own chances of survival as described by Hugh Drommond et al. (1990). Drommond et al. observed cases of siblicide in black eagles; one of the chicks is hatched usually 3 days before the other and therefore is significantly larger than it s sibling (1990). Drommond et al. observed the older eaglet deal 1569 pecks to it s younger sibling in 3 days, eventually killing the younger chick. This phenomena supports several key concepts in evolution. The older sibling is competing with others for resources(food and nesting space), so killing the weaker member promotes the survival of the older bird (Drommond et al., 1990). If resources are limited and both siblings cannot survive, the species will continue to survive due to the death of the younger sibling. However, Drommond et al.(1990) point out that there are several evolutionary losses that occur when a sibling dies; reproductive potential is lost as well as a degree of insurance(in case one of the offspring does not survive to maturity). Excuse the pun, but putting all of the eggs in one basket is a large risk.
Predators and their prey are part of a cycle; both are necessary components and they depend on each other for their existence. Any change made in one area will affect the other.
Overall, predator prey relations are very complex. By breaking the topic into the three topics of; symbiotic relationships, defense mechanisms, and the influence relationship between predators and prey. It is important to see how all three of these subjects tie in together. Parasitism is an example of a symbiotic relationship, parasites are predators living off of their prey, and parasites also effect the evolution of their hosts. Natural selection favors species that are resistant to parasites, so these organisms evolve. The organisms have a range of defense mechanisms available in order to protect themselves from predators. So, predators now face tougher prey, so they undergo evolution in order to stay successful. This completes the cycle and leads to a diverse and interesting world.
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