The coalitions formed by the lower ranking males have been interpreted as direct reciprocity because the cooperative subordinate males have presumably cooperated before. However, while the savanna baboon behavior can be viewed reciprocal altruism, the evidence is weak and their behavior often deviates from the framework of reciprocal altruism. Another explanation for male alliances is mutualism. Mutualism benefits both individuals cooperating, in that the immediate benefit associated with cooperating outweighs costs associated with cooperating. Lions provide an interesting case of mutualism. Male lions cannot afford not to cooperate with one another. Single male lions cannot successfully defend and maintain access to a females against invading coalitions of males, which can invade the pride and force the single male to retreat. Therefore, males cooperate to gain and defend access to females. Resident male alliances maintain access to females in that pride, and sire all offspring during their control of the pride. Solitary lions rarely gain access into coalitions; however, when they do they kill all resident cubs before beginning their tenure. Therefore, it is it of vital importance for the males’ reproductive success to be a member of a male coalition and defend females against intruding males. While it was originally believed that male lions formed groups of related individuals, it is now clear that they may also form alliances with unrelated individuals. Male lion alliances are extremely successful in preventing solitary males from invading their pride. Packer et al. (1991) found that while lions often form coalitions with nonrelatives, they do so only under specific circumstances. The degree of relatedness of coalition members is related to coalition size. Small coalitions are often composed of unrelated individuals while large coalitions are largely composed of related individuals. Although larger coalitions result in more offspring per capita than small coalitions, large coalitions are only composed of close relatives. In large groups only a select few males were successful in mating, resulting in a large discrepancy in reproductive fitness among coalition members. In small groups no male was dominant and individuals displayed similar reproductive fitness. Therefore, in lions, researchers propose that cooperation between non-relatives is hypothesized has evolved in cases where there is little variance in mating opportunities among coalition members, and males share equally in mating opportunities. Larger coalitions result in greater variability of mating opportunities, and therefore researchers suggest that kinship (indirect fitness benefit) is necessary for maintenance and success of lager male lion coalitions. In smaller groups male lions are thought to share a “mutual dependence”. Smaller groups are composed largely of unrelated individuals which eliminates the possibility of kin selection.