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Cooperative Breeding is a social system characterized by alloparental care: offspring receive care not only from their parents, but also from additional group members, often called helpers. Cooperative breeding encompasses a wide variety of group structures, from a breeding pair with helpers that are offspring from a previous season, to groups with multiple breeding males and females (polygynandry) and helpers that are the adult offspring of some but not all of the breeders in the group, to groups in which helpers sometimes achieve co-breeding status by producing their own offspring as part of the group's brood. Cooperative breeding occurs across taxonomic groups including birds, mammals, fish, and insects. Costs for helpers include a fitness reduction, increased territory defense, offspring guarding and an increased cost of growth. Benefits for helpers include a reduced chance of predation, increased foraging time, territory inheritance, increased environmental conditions and an inclusive fitness. Inclusive fitness is the sum of all direct and indirect fitness, where direct fitness is defined as the amount of fitness gained through producing offspring. Indirect fitness is defined as the amount of fitness gained through aiding related individuals offspring.〔Nicholas B. Davies, John R. Krebs, S. A. W. An Introduction to Behavioural Ecology.pdf. 522 (2012).〕 For the breeding pair, costs include increased mate guarding and suppression of subordinate mating. Breeders receive benefits as reductions in offspring care and territory maintenance. Their primary benefit is an increased reproductive rate and survival. Cooperative breeding causes the reproductive success of all sexually mature adults to be skewed towards one mating pair. This means the reproductive fitness of the group is held within a select few breeding members and helpers have little to no reproductive fitness.〔 With this system, breeders gain an increased reproductive, while helpers gain an increased inclusive fitness. ==Evolution of Cooperative Breeding== Many hypotheses have been presented to explain the evolution of cooperative breeding. The concept behind cooperative breeding is the forfeiting of an individual’s reproductive fitness to aid the reproductive success of others. This concept is hard to understand and the evolution of cooperative breeding is important, but difficult to explain. Most hypotheses aim to determine the reason helpers selectively reduce their fitness and take on an alloparental role. Kin selection hypothesis is the evolutionary strategy of aiding the reproductive success of related organisms, even at a cost to the own individual’s direct fitness. Hamilton’s rule (rB-C>0) explains kin selection will exist if the genetic relatedness (r) of the aided recipient to the aiding individual, times the benefit to the aid recipient (B) is greater than the cost to the aiding individual(C).〔 For example, Chestnut-crowned babbler (''Pomatostomus ruficeps'') has been found to have high rates of kin selection. Helpers are predominantly found aiding closely related broods over nonrelated broods.〔 Additional species such as ''Neolamprologus pulcher'' have shown that kin selection is a dominant driving force for cooperative breeding. Group augmentation presents a second hypothesis towards the evolution of cooperative breeding. This hypothesis suggests that increasing the size of the group, through the addition of helpers, aids in individual survival and may increase helper’s future breeding success. Group augmentation is favored if the grouping provides passive benefits for helpers in addition to inclusive fitness. By group augmenting, each individual member reduces their chances of becoming a victim of predation. Additionally, an increase in members reduces each helper’s duration as a sentinel (standing upon a high surface to survey for predators) or babysitting (guarding the offspring and den). The reduction in these guarding behaviors enables helpers to forage for longer periods. Lukas et al. proposed an evolutionary model for cooperative breeding, which linked the coevolution of polytocy, production of multiple offspring, and monotocy, production of single offspring, with the evolution of cooperative breeding. The model is based on the evolution of larger litters forcing the need for helpers to maintain the high reproductive costs, thus leading to cooperative breeding. Lukas et al. suggests polytocy may have encouraged the evolution of cooperative breeding. Their proposed model suggests the transition from monotocy to polytocy is favorable. Additionally, they found the transition from polytocy without cooperative breeding to polytocy with cooperative breeding is highly favorable. This suggests cooperative breeding evolved from noncooperative breeding monotocy to cooperative breeding polytocy.〔 Today, there is growing support for the theory that cooperative breeding evolved by means of some form of mutualism or reciprocity. Mutualism is a form of symbiosis that is beneficial to both involved organisms. Mutualism has many forms and can occur when the benefits are immediate or deferred, when individuals exchange beneficial behaviors in turn, or when a group of individuals contribute to a common good, where it may be advantageous for all group members to help raise young. When a group raises young together, it may be advantageous because it maintains or increases the size of the group.〔 The greatest amount of research has been invested in reciprocal exchanges of beneficial behavior through the iterated prisoner's dilemma. In this model, two partners can either cooperate and exchange beneficial behavior or they can defect and refuse to help the other individual. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cooperative breeding」の詳細全文を読む スポンサード リンク
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