Male reproductive alliances can best be understood within the context of traditional male–male competition, and as a specific case of cooperative competition. Such cooperative behavior, however, does not necessarily result in the equal sharing of resources among cooperating individuals. Cooperation often requires that individuals decrease their own fitness to increase the fitness of another, a behaviour referred to as reciprocal altruism. This behavior becomes even more striking when it occurs within the context of cooperative reproduction, where individuals decrease their own reproductive fitness to improve the reproductive fitness of another. In some species, males cooperate by forming alliances between related or non-related individuals to gain access to females and prevent other males from mating. Such alliances often result in the monopolization of mating opportunities by one dominant male. The resulting unequal sharing of mating opportunities contradicts the traditional male–male competition over access to females that natural selection implies, making male reproductive alliances an ideal case to study the costs and benefits associated with subordinate individual cooperation. The cost of cooperation (a decrease in fitness) makes it difficult to reconcile the principles of natural selection and cooperation unless there are specific circumstances that make cooperation favorable. Despite the apparent contradictory nature of cooperation, it does occur in a variety of species. Male reproductive alliances have been documented in bottlenose dolphins (Tursiops sp.), slender mongooses (Galerella sanguine), lions (Panthera leo), chimpanzees (Pan troglodytes), and other primates. However, such behavior may or may not have evolved within the context of reproduction. Alliances may improve an individual's fitness by either improving foraging capabilities or lessening the cost of defending territories. The reproductive tradeoffs for males participating in reproductive alliances depends on the extent to which mating is shared among alliance members, and the extent to which alliance membership incurs a reproductive fitness advantage over competing as a single male. Three mechanisms have been hypothesized to reconcile the principles of natural selection and cooperation: kin selection, direct reciprocity and mutualism . Separate cases have provided evidence supporting all three of the routes described. Male alliances have been hypothesized to have evolved within the context of kin selection in red howler monkeys, within the context of direct reciprocity in savanna baboons and within the context of mutualism in lions. Determining the evolutionary context of cooperative behavior can be difficult. Two things to consider regarding male alliances are whether the coalition comprises related or unrelated individuals and how stable the coalitions are. Male alliances involve complex interactions with many costs and benefits, making the study of such cooperative behavior both difficult and fascinating.
Kin selection and inclusive fitness When coalitions are composed of relatives, the contradictory nature of male reproductive alliances is easily resolved through inclusive fitness theory. The theory of inclusive fitness, proposed by Hamilton (1964) states that individuals can enhance their own reproductive fitness by securing the reproductive success of their relatives 10. For kin selection to increase the reproductive fitness of the altruist, according to Hamilton (1964) as cited in Nowak (2006) the coefficient of relatedness, between the donor and recipient of the altruistic act, must be greater than the cost-to-benefit ratio of the altruist act (r < c/b). In other words, the reproductive benefit gained by the recipient of the altruistic act times the coefficient of relatedness must be greater than the reproductive cost of the individual performing the altruistic act (rb > c). Therefore, in alliances composed of closely related individuals, where there is a large coefficient of relatedness, it is widely believed that inclusive fitness has been the principal driving force for the evolution of male reproductive cooperation. Pope (1990) demonstrated that it was advantageous for both dominant and subordinate male red howler monkeys to be members of a male alliance. While males formed coalitions of both related and unrelated individuals, related troops were more stable and lasted longer than unstable troops. Such findings indicate that despite the low reproductive success of subordinate males, given that dominant males secured all mating opportunities, it was still beneficial to be in a group as opposed to being alone, particularly if that coalition was composed of related individuals. The study suggested that for red howler monkey coalitions, kin selection was the primary mechanism involved in the formation of male alliances. While subordinate males decreased their direct fitness by cooperating with dominant males, they increased their inclusive fitness by cooperating with relatives. Male philopatry, or the behavior of remaining in natal groups, in many cases sets the stage for such alliances among relatives. Later Pope (2013) also discovered that the reproductive success of males within coalitions increases with increasing relatedness. That being said, while being a member of a male alliance has direct reproductive benefits for individuals, alliances come with a cost. Status shifts and mate guarding behavior often result in injury to the participants.
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