Cooperative breeding is a social system in which individuals contribute care to offspring that are not their own at the expense of their own reproduction [1]. When reproduction is monopolized by one or few of the adult group members and most adults do not reproduce, but help rear the breeder’s offspring, the majority of these cooperative breeding species exhibit high reproductive skew. However, there are plural breeding species with low reproductive skew, in which the subordinates commonly breed. This kind of cooperative breeding systems is particularly rare and includes communal breeding, in which more than one female lays eggs or gives birth to young [2]. Cooperative breeding is different from alloparenting, which is simply the act of caring completely for another conspecific's offspring but not at their own expense. Alloparents always have ulterior motives such as practicing to be better parents, reciprocation and genetic interest. Additionally, cooperative breeding is not altruism, which is the care for others (not necessarily offspring) with no expectation of any compensation or benefits, whether direct or indirect.
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Generally the helpers in cooperatively breeding birds and mammals are relatives of the dominant breeders who still remain in the group or are individuals whose own breeding attempts have been unsuccessful and have come back to their original group to help with kin.
Kin selection was previously thought to be one of the major contributing factors to the evolution of cooperative breeding. Kin selection is when individuals increase their fitness through indirect benefits by helping relatives[3]. Today, kin selection is not a compelling explanation for the evolution of cooperative animal societies for a number of reasons. Most permanent groups of social animals are composed of relatives, and it is unclear if the degree of relatedness is consistently higher in cooperative breeders than in other species that simply live in stable groups that don’t breed cooperatively[4]. There are many studies of cooperative birds and mammals that have shown that unrelated helpers can invest just as much as close relatives. Another reason why kin selection doesn’t fully explain the evolution of cooperative breeding is that the relative importance of indirect fitness benefits of helpers has probably been overestimated. Many times, the benefits received by the helpers and those they confer on kin have both been included, leading to a double accounting of kin-selected benefits[5].
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[6]. 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[7].
Research has found that there is not one theory that can explain the evolution of all cooperative breeding species. In addition to mutualism and reciprocity, group augmentation has been suggested as one way cooperative breeding could have evolved in some species. Group augmentation occurs when animals living in a group behave in a way that increases the group's size. This type of behavior would be selected for if larger groups increased the individual's chances of survival. This ultimately would select for individuals that help raise other animals' offspring[8].
There are many other possible explanations for cooperative breeding other than kin selection. Helping is not restricted to groups of closely related members, and it could also be a form of gaining parental experience, acquiring social status, “rent payment” for a shared habitat, or a gateway for future mating opportunities.[9] Another primary explanation for cooperative breeding is group augmentation. In many species, survival is greater in large groups due to the benefits of large group numbers. Some species may experience passive benefits (actions that indirectly increase the fitness of an animal) due to the presence of other group members, such as the dilution effect, while others may experience active benefits (actions that are directed specifically to increase an animal’s fitness) that depend on the help that new members might offer.[10]These benefits of cooperative breeding will occur in a population only if it is better to stay and help other individuals than to move somewhere else and attempt breeding.[11] In this way, helping could evolve through group augmentation if the larger group sizes result in automatic passive benefits to all group members. However without passive benefits, larger groups could still receive active benefits in the form of delayed reciprocity. Therefore it might be advantageous for a subordinate to provide aid to a dominant individual even if it is completely unrelated.
The primary disadvantage of cooperative breeding is the cost of expending energy towards raising the offspring of another individual. A study conducted by Cornwallis et al. showed a strong negative correlation between the evolution of promiscuity and the evolution of cooperative breeding, suggesting that cooperative breeding is more likely to evolve in populations where females mate with fewer males[12]. For species with multiple-mating systems, the time and energy spent raising another’s offspring would prove too costly for cooperative breeding to be selected for.
Approximately 3.2 percent of bird species are known to regularly engage in cooperative breeding, including a number of jay, magpie, and crow species[13]. The benefits of cooperative breeding in birds have been well-documented. One example is the azure-winged magpie (Cyanopica cyanus), in which studies found that the offspring’s cell-mediated immune response was positively correlated with increase in the number of helpers at the nest.[14] Studies on cooperative breeding in birds have also shown that high levels of cooperative breeding are strongly associated with low annual adult mortality and smaller clutch sizes, though it remains unclear whether cooperative breeding is a cause or consequence[15]. Research has suggested that cooperative breeding developed among bird species with low mortality rates as a consequence of “overcrowding” and thus fewer opportunities to claim territory and to breed. This hypothesis is known as the “life history hypothesis.”
Meerkats become reproductively active at one year of age and can have up to four litters per year. However, usually it is the alpha pair that reserves the right to mate and will usually kill any young that is not their own. While the alpha female is away from the group, females that have never reproduced lactate and hunt in order to feed the pups, as well as watch, protect, and defend them from predators. Although it was previously thought that a meerkat’s contribution to a pup’s diet depended on the degree of relatedness, it has been found that helpers vary in the number of food items they give to pups. This variation in food offering is due to variation in foraging success, sex, and age. Research has additionally found that the level of help is not correlated to the kinship of the litters they are rearing. [16]