Ultimate Causation of Aggressive and Forced Copulation in Birds: Female Resistance, the CODE Hypothesis, and Social Monogamy

SYNOPSIS. Except in ducks and geese (Anseriforms), aggressiveor forced copulation in birds is rare. The rarity of forcedcopulation in birds theoretically is dueto morphological andphysiological mechanisms of female resistance that place fertilizationmost often under female control. Traits theoretically associatedwith resistance by females include: digestive epithelium liningthe section of the cloaca receiving sperm and powerful doacalmusculature used to eject contents, including waste materialand sperm. These traits suggest that the Immediate FertilizationEnhancement Hypothesis may be an inadequate ultimate explanationfor forced copulation when it occurs. Ideas in Heinroth (1911)and Brownmiller (1975) suggested an alternative, the CODE Hypothesis,which says that aggressive copulation creates a dangerous environmentfor females. This, in turn, fosters male mating advantage viasocial monogamy, because selection sometimes favors femaleswho trade sexual and social access for protection from maleaggression. Thus, theoretically, "trades" of protection forcopulation favor the evolution of social monogamy even in specieswith little or no paternal care. Individual males may accrueselective advantages through direct benefits, kin-selected benefits,or reciprocal altruism. The CODE hypothesis for social monogamypredicts variation in extrapair paternity from preferred mates,variation in male reproductive success, and variation amongfemales' post-insemination resistance mechanisms as functionsof variation among females' vulnerabilities (ecological andintrinsic) to aggressive copulation. Observers will base intraspecifictests on variation among females in their vulnerabilities tomale aggression against them.