DAVIS, E.S.: Forced Copulation as an Evolutionary Epiphenomenon

To date, two general hypotheses regarding the evolutionary origin of forced copulation (FC) have been proposed. The first and most widely accepted was proposed by McKinney et al. (1983), who posited that FC serves as a secondary reproductive strategy for paired males. Although not essential to their argument, the implication is that FC behavior arose as a result of selection for this behavior. The second, proposed by Gowaty & Buschhaus (1999) is the CODE hypothesis for the origin of sexual coercion behavior in birds. They posit that males create a dangerous and coercive atmosphere for females such that a female’s only recourse is to solicit protection from a male in exchange for exclusive access to her for reproduction. Thus, they propose that sexual coercion arose as a mechanism to ensure female monogamy. It should be noted that these hypotheses are not mutually exclusive, as together they argue that FC has evolved as a polygynous male strategy, while at the same time helping to insure female monogamy. Studies presented here of mallards (Anas platyrhynchos) under semi-natural conditions lead me to propose a third hypothesis of the origin of FC behavior in waterfowl. That is, FC is an evolutionary epiphenomenon that initially arose because its underlying mechanism, high male testosterone (T) level, was sexually selected for another reason, namely mate guarding behavior. Note that this hypothesis posits that selection for mate guarding behavior precedes selection for FC behavior, in direct opposition to the CODE hypothesis. Here I will review the evidence for this hypothesis and its impact on the other two hypotheses.