The beneficial sexually transmitted microbe hypothesis of avian copulation

Several hypotheses have been proposed to explain why female birds either co pulate repeatedly with a single mate or copulate with multiple partners eve n though only a single copulation may be sufficient to fertilize an entire clutch. We hypothesize that females may directly benefit from high frequenc ies of copulation and multiple copulation partners if they receive a cloaca l inoculation of beneficial sexually transmitted microbes (STMs) that can e ither protect them against future encounters with pathogens and/or serve as therapy against present infections. Experiments in domestic animal product ion, wildlife rehabilitation, and clinical medicine indicate that inoculati ons of beneficial microbes derived from the indigenous microflora of hosts can lead to nutritional benefits, resistance to colonization by pathogens, the elimination of infection, and improved immune system functioning in rec ipients. Our hypothesis predicts greater copulatory rates when the probabil ity of the transmission of beneficial microbes exceeds that of pathogens an d when the positive effects of beneficial microbes on host fitness exceed t he negative effects of pathogens. Patterns of copulatory behavior in birds suggest the potential utility of our hypothesis. We discuss our hypothesis in the context of observed patterns of copulation in birds and propose some ways to directly test our hypothesis. Information on the probabilities of transmission during copulation of beneficial and pathogenic microbes and th eir relative potencies in birds are needed to directly test the predictions of our hypothesis.