The beneficial sexually transmitted microbe hypothesis of avian copulation

Several hypotheses have been proposed to explain why femalebirds either copulate repeatedly with a single mate or copulatewith multiple partners even though only a single copulationmay be sufficient to fertilize an entire clutch. We hypothesizethat females may directly benefit from high frequencies of copulationand multiple copulation partners if they receive a cloacal inoculationof beneficial sexually transmitted microbes (STMs) that caneither protect them against future encounters with pathogensand/or serve as therapy against present infections. Experimentsin domestic animal production, wildlife rehabilitation, andclinical medicine indicate that inoculations of beneficial microbesderived from the indigenous microflora of hosts can lead tonutritional benefits, resistance to colonization by pathogens,the elimination of infection, and improved immune system functioningin recipients. Our hypothesis predicts greater copulatory rateswhen the probability of the transmission of beneficial microbesexceeds that of pathogens and when the positive effects of beneficialmicrobes on host fitness exceed the negative effects of pathogens.Patterns of copulatory behavior in birds suggest the potentialutility of our hypothesis. We discuss our hypothesis in thecontext of observed patterns of copulation in birds and proposesome ways to directly test our hypothesis. Information on the probabilitiesof transmission during copulation of beneficial and pathogenic microbesand their relative potencies in birds are needed to directlytest the predictions of our hypothesis.