Pecking Orders In vertebrate organisms, intraspecific competition occurs between males as a group and between females as a group. Rarely is there male-versusfemale competition, except in species having high social bonding-primates, for example. Competition begins when individuals are young. During play fighting, individuals nip or peck at each other while exhibiting threat displays. Dominant individuals exert their authority, while weaker individuals submit. The net result is a very ordered ranking of individuals from top to bottom, called a dominance hierarchy or pecking order. The top individual can threaten and force into submission any individual below it. The number two individual can threaten anyone except number one, and so on. The lowest-ranked individual can threaten no one and must submit to everyone. The lowest individual will have the least food, worst territory, and fewest (if any) mates. The number one individual will have the most food, best territory, and most mates. The pecking order changes over time because of continued group competition that is shown by challenges, aging, and accidents. Pecking orders are evident in hens. A very dominant individual will peck other hens many times but will rarely be pecked. A less dominant individual will peck less but be pecked more. A correct ranking can be obtained easily by counting the pecking rate for each hen. In The Netherlands, male black grouse contend with one another in an area called a "lek," which may be occupied by as many as twenty males. The males establish their territories by pecking, wing-beating, and threat displays. The most dominant males occupy small territories (several hundred meters) at the center of the lek, where the food supply is greatest. Less dominant males occupy larger territories with less food reserves to the exterior of the lek. Established territories are maintained at measurable distances by crowing and flutter-jumping, with the home territory owner nearly always winning. Females, which nest in an adjoining meadow, are attracted to dominant males in the heavily contested small central territories. A baboon troop can range in size from ten to two hundred members, but usually averages about forty. Larger, dominant males and their many female mates move centrally within the troop. Less dominant males, with fewer females, lie toward the outside of the troop.Weak individuals at the troop periphery are more susceptible to predator attacks. Dominant males exert their authority by threat displays, such as the baring of the teeth or charging; weaker males submit by presenting their hindquarters. Conflicts are usually peacefully resolved. Female lions maintain an organized pride with a single ruling male. Young males are expelled and wander alone in the wilderness. Upon reaching adulthood, males attempt to take over a pride in order to gain access to females. If a male is successful in capturing a pride and expelling his rival, he will often kill the cubs of the pride, simultaneously eliminating his rival's descendants and stimulating the females to enter estrus for mating.
Phylogeny Phylogeny traces the history of life on earth through the study of how animals and plants have developed over time and how they are related to one another. It is similar to taxonomy, the science of classifying organisms based on their structure and functions. Taxonomists create family trees of living and extinct species in order to discover the origins and lines of descent of various forms of life. Very few family trees are complete to their fossil origins, however, because of gaps in the fossil record. The first system of classification was devised by the eighteenth century Swedish scientist Carolus Linnaeus. Linnaeus classified life-forms based on their appearance; the more they resembled each other in size, shape, and form, he believed, the more closely they were related. The theory of evolution developed by nineteenth century naturalist Charles Darwin was based on accumulated changes over time through natural selection and led to a new way of looking at the history of life and to the development of phylogeny as a method of classification. The new system classified the living world by similarities in ancestry rather than appearance. Life histories of species were derived fromthe study of comparative anatomy (the search for common features in different species), embryology (the study of the development of life from the egg to birth), and biochemistry (the study of invisible chemical characteristics of cells that link species that can look very dissimilar). Modern technology allows scientists to measure differences in deoxyribonucleic acid (DNA) molecules among species. DNA carries the genetic material of an organism and plays a central role in heredity. The degree of difference between two species helps scientists determine how much modern species have changed from their ancestors and to estimate when important events in evolution occurred. If a sufficient fossil record exists, biochemists can determine the timing of a major change that led to the development of a new characteristic such as a longer neck in giraffes or a different size eyeball in a bat.
