Cooperation between males in reproductive contexts is an underappreciated and puzzling behaviour.
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We define male–male cooperation in reproduction and identify different types of behaviours.
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We list species where males engage in cooperative reproduction and describe taxonomic patterns.
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We outline potential causes, from proximate to ultimate, associated with this cooperation.
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Challenges and opportunities for the development of sexual selection theory are discussed.
Competition among males for access to reproductive opportunities is a central tenet of behavioural biology that has critical implications for studies of mating systems, sexual selection and the evolution of numerous phenotypic traits. Given the expectation that males should compete vigorously for access to females, it may at first seem paradoxical that males in some species cooperate to reproduce, often resulting in the apparent sacrifice of direct fitness by some members of these cooperative partnerships. Because this form of cooperation lies at the interface between natural, sexual and kin selection, studies of the adaptive consequences of male reproductive cooperation may yield important insights into how complex and sometimes conflicting selective pressures shape individual behaviour. Here, we define and review examples of reproductive cooperation among male animals. We take an integrative approach to reviewing the potential causes of male–male cooperation, including potential adaptive hypotheses, ecological correlates, phylogenetic patterns and physiological mechanisms. The impact of male reproductive cooperation on sexual selection theory is also discussed. We conclude by outlining several important directions for future research, including efforts to improve understanding of the ecological and demographic contexts in which male reproductive cooperation occurs. Collectively, such analyses promise to improve our understanding of multiple fundamental concepts in evolutionary biology.
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