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

SYNOPSIS. Except in ducks and geese (Anseriforms), aggressive or forced copulation in birds is rare. The rarity of forced copulation in birds theoretically is due to morphological and physiological mechanisms of female resistance that place fertilization most often under female control. Traits theoretically associated with resistance by females include: digestive epithelium lining the section of the cloaca receiving sperm and powerful doacal musculature used to eject contents, including waste material and sperm. These traits suggest that the Immediate Fertilization Enhancement Hypothesis may be an inadequate ultimate explanation for 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 environment for females. This, in turn, fosters male mating advantage via social monogamy, because selection sometimes favors females who trade sexual and social access for protection from male aggression. Thus, theoretically, "trades" of protection for copulation favor the evolution of social monogamy even in species with little or no paternal care. Individual males may accrue selective advantages through direct benefits, kin-selected benefits, or reciprocal altruism. The CODE hypothesis for social monogamy predicts variation in extrapair paternity from preferred mates, variation in male reproductive success, and variation among females' post-insemination resistance mechanisms as functions of variation among females' vulnerabilities (ecological and intrinsic) to aggressive copulation. Observers will base intraspecific tests on variation among females in their vulnerabilities to male aggression against them.