"Nesting assistance": a new hypothesis for the evolution of polyandry and a test in an African foam-nesting treefrog

Why polyandry evolves remains a compelling evolutionary question. While numerous material benefits of polyandry have been demonstrated, there has been limited attempt to link the fitness of polyandrous females to nesting behavior. Here, we introduce the "nesting-assistance" hypothesis as a new explanation for the evolution of polyandry. We theorize that in species where females are primarily responsible for nest construction, and males display some form of nesting assistance, that mating with multiple males leads to the production of more viable nests and improved offspring survival or performance. We provide an initial test of the "nesting-assistance" hypothesis in the African gray foam-nest treefrog (Chiromantis xerampelina), a species where multiple males commonly join females to construct arboreal foam nests. Field observations revealed that individuals involved in polyandrous matings synchronize the timing of bouts of leg churning, a conspicuous reproductive behavior essential for foam-nest construction. Moreover, matings involving more males produced bigger nests that were more resistant to desiccation. Paternity analysis revealed that males involved in polyandrous matings gained a share of paternity, suggesting a direct fitness benefit from nesting assistance. Our findings support the "nesting assistance" hypothesis and provide evidence for a new yet potentially widespread benefit of polyandry.