Cooperative breeding
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 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 the offspring of related individuals, that is, relatives are able to indirectly pass on their genes through increasing the fitness of related offspring. This is also called kin selection.
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 fitness, while helpers gain an increased inclusive fitness.
Evolution
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 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 explains that kin selection will exist if the genetic relatedness of the aided recipient to the aiding individual, times the benefit to the aid recipient is greater than the cost to the aiding individual. For example, the chestnut-crowned babbler 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 the 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 or babysitting. 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.
Environmental conditions
Environmental conditions govern whether offspring disperse from their natal group or remain as helpers. Food or territory availability can encourage individuals to disperse and establish new breeding territories, but unfavorable conditions promote offspring to remain at the natal territory and become helpers to obtain an inclusive fitness. Additionally, remaining at the natal territory enables offspring to possibly inherit the breeding role and/or territory of their parents.A final factor influencing cooperative breeding is sexual dispersal. Sexual dispersal is the movement of one sex, male or female, from the natal territory to establish new breeding grounds. This is highly regulated by the reproductive costs in producing a male versus a female offspring. Maternal investment within female offspring may be considerably higher than male offspring for one species, or vice versa for another. During unfavorable conditions the cheaper sex will be produced at higher ratios.
A second factor affecting the sexual dispersal is the difference in ability of each sex to establish a new breeding territory. Carrion crow were found to produce more female offspring in favorable environmental conditions. Female Corvus corone have been found to establish successful breeding territories at a higher rate than males. Male Corvus corone were produced at a higher rate under unfavorable conditions. Males were found to remain at the natal territory and become helpers. Thus, if environmental conditions favor the dispersal of a specific sex it is considered the dispersal sex. If environmental conditions are unfavorable females may produce the philopatric sex, therefore generating more helpers and increasing the occurrence of cooperative breeding.
Costs
Breeders
Breeder costs consist of prenatal care, postnatal care and maintenance of breeding status. Prenatal care is the amount of maternal investment during fetus gestation and postnatal care is the investment following birth. Examples of prenatal care are fetal, placentae, uterus and mammary tissue development. Postnatal examples are lactation, food provisions and guarding behavior.Dominant males and females exhibit suppressive behaviors towards subordinates to maintain their breeding status. These suppressive acts are dependent upon the sex ratio of helpers. Therefore, the costs will be altered depending upon the helpers. For example, if there are more male helpers as compared to females, then the dominant male will suppress subordinate males and experience a higher cost. The opposite is true for females. Breeders will even suppress subordinates from mating with other subordinates.
Helpers
The cost to helpers varies depending upon presence or absence of related offspring. The presence of offspring has been found to increase the helper's cost by the helper contributing to guard behaviors. Guarding behaviors, such as babysitting, can cause individuals to experience weight loss on an exponential scale depending upon the duration of the activity. Other activities, such as sentinel behavior and bipedal surveillance, cause helpers to have reduced foraging intervals inhibiting their weight gains. The reduced foraging behavior and increased weight loss reduces their chance to breed successfully, but increases their inclusive fitness by increasing the survival of related offspring.Helpers contribute depending upon the cost. The act of helping requires an allocation of energy towards actually performing the behavior. Prolonged allocation of energy may greatly impact a helper's growth. In banded mongoose juvenile male helpers contribute far less than females. This is due to a difference in the age of sexual maturity. Female banded mongooses reach sexual maturity at one year of age, but males reach sexual maturity at two years of age. The difference in age causes the prolonged energy allocation to be detrimental to a specific sex.
Male juvenile Mungos mungo may reduce helping behaviors until sexual maturity is reached. Similarly, if there is a lack of food due to environmental conditions, such as reduced rainfall, the degree of helper input may be reduced greatly within juveniles. Adults may maintain their full activity because they are sexually mature.
Additionally, the costs of being a helper can be more detrimental to one sex. For example, territorial defense costs are generally male dependent and lactation is female dependent. Meerkats have exhibited male territory defense strategies, where male helpers will fend off intruding males to prevent such intruders from mating with subordinates or dominant females. Additionally, subordinate female pregnant helpers are sometimes exiled from the group by a dominant female. This eviction causes the subordinate female to have an abortion, which frees up resources such as lactation and energy that can be used to help the dominant female and her pups.
Rarely, a female helper or breeder will defend the territory while males are present. This suggests specific helping costs, such as territory defense, is rooted to one sex.