Abstract
The influence of kinship on animal cooperation is often unclear. Cooperating Asiatic lion coalitions are linearly hierarchical; male partners appropriate resources disproportionately. To investigate how kinship affect coalitionary dynamics, we combined microsatellite based genetic inferences with long-term genealogical records to measure relatedness between coalition partners of free-ranging lions in Gir, India. Large coalitions had higher likelihood of having sibling partners, while pairs were primarily unrelated. Fitness computations incorporating genetic relatedness revealed that low-ranking males in large coalitions were typically related to the dominant males and had fitness indices higher than single males, contrary to the previous understanding of this system based on indices derived from behavioural metrics alone. This demonstrates the indirect benefits to (related) males in large coalitions. Dominant males were found to ‘lose less’ if they lost mating opportunities to related partners versus unrelated males. From observations on territorial conflicts we show that while unrelated males cooperate, kin-selected benefits are ultimately essential for the maintenance of large coalitions. Although large coalitions maximised fitness as a group, demographic parameters limited their prevalence by restricting kin availability. Such demographic and behavioural constraints condition two-male coalitions to be the most attainable compromise for Gir lions.
Highlights
Male coalitions have been reported from several s pecies[13], but have been studied in great detail in lions Panthera leo[15,16,17,18,19,20]
India which is primarily a woodland habitat with small modal prey and small female groups[20]). Such a linear hierarchy has been proposed to have resulted in an optimal coalition size of two male lions in the Gir system, below and beyond which the costs of coalescing are higher than the benefits for loners and subordinates[20]. This computation of optimal coalition size was based on fitness indices developed through behavioural metrics alone, and did not consider the potential role of genetic relatedness between partners in deciding the payoffs of cooperation
From studies across species and systems we know that relatedness often shapes the evolution and functioning of coalitions because of two primary reasons: (1) groups fare better than solos, and (2) resource competition often make certain group members rely primarily on indirect fitness benefits to maintain threshold inclusive fitness levels, explained through Hamilton’s p rinciple[1]
Summary
Male coalitions have been reported from several s pecies[13], but have been studied in great detail in lions Panthera leo[15,16,17,18,19,20]. Such a linear hierarchy has been proposed to have resulted in an optimal coalition size of two male lions in the Gir system, below and beyond which the costs of coalescing (or not) are higher than the benefits for loners and subordinates[20] This computation of optimal coalition size was based on fitness indices developed through behavioural metrics alone, and did not consider the potential role of genetic relatedness between partners in deciding the payoffs of cooperation. A flip side to this prediction would be that unrelated males would generally coalesce when the apparent reproductive skew is not as pronounced between partners as observed within large coalitions We test these predictions through genetic analyses of relatedness between male partners in coalitions of varying sizes of free-ranging lions inhabiting the Gir forests of Gujarat, India. We provide a novel examination of how relatedness between dominants and subordinates might even aid in amending such losses for the dominants, further propagating cooperative behaviour
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