2.3 Sexual conflict and forced copulations lead to the co-evolution  (Page 5/7)

Proximate reasons for mate choice

Testosterone was positively associated with forced copulations in mallards, greater time spent near females, increased mate guarding, and mating success (Davis 2002). Females choose mates in the autumn that they can accurately predict will have higher levels of testosterone come breeding season, and the females that had mates with higher testosterone also had fewer lost feathers due to forced copulation, presumably because the mate was a better defender (Davis 2002). This may provide direct benefits that complement the good genes hypothesis (same as mate-choice hypothesis for indirect benefits). However, research also shows that increased testosterone increases the FEPCs that drakes perform, so it may just be that the females that chose the most aggressive drakes do not have to deal with FEPCs (Davis 2002).

How successful is it?

As a reproductive tactic, the success of the insemination matters greatly to unpaired males and slightly to paired ones. The action itself would not be worthwhile if it failed each time. The data for this are mixed. Brennan claims that successful forced copulations are rare and successfully blocked by the female hens’ elaborate vaginas (2007). However, other studies show successful forced copulation rates of up to 50% (McKinney and Evarts 1998). Depending on the study, success rates vary; however, in general, it seems that forced copulations are successful enough to remain a viable reproductive tactic.

Why have unconditional resistance?

However, as mentioned above, the hens have an uncanny desire it seems to remain faithful to their chosen mates (Adler 2010). The hen’s struggles often attract groups of other drakes who will follow after her, see [link] (Goodburn 1984). The hen’s resistance is unusual as the cost of it is so high. In their excitement, males continually pile themselves on top of hens and grasp at their neck feathers to better position themselves for entry. This, along with her resistance, frequently causes injuries ranging from lost feathers to scratches, ruptured organs, and even drowning (Adler 2010). Many times it may be better to avoid this damage by accepting the forced extra-pair copulation (FEPC) as other species do. There are several hypotheses for this. The resistance may be related to maintenance of the pair-bond to assure that the mate will continue to guard the hen since he needs assurance of his reproductive success. It may be the good genes hypothesis , to ensure the best chicks survive as stated above. However, if this were the sole reason, then hens should engage in some extra-pair copulations (EPCs) with dominant males that have better genes which it is never seen doing.

Instead, Adler suggests that in spite of the high costs of resisting, the hen is filtering out the weak drakes that attempt FEPCs in favor of the fittest individuals (2010). She does clarify however, that this is to make the best of a bad situation, not to have a net benefit. In other words, this behavior is has the byproduct of selecting for forced copulation and complex phalluses to stay in the pool. Thus, the resistance provides indirect mate choice making it more likely that the male with the most adaptive genitals, that can bypass the vaginal labyrinth, would succeed. To clarify, the hen is not calculating the fitness profits and deficits from her actions, but gradual evolution has resulted in these behaviors being selected and working together optimally. This furthers the co-evolution of the two genitalia in a constant arms race as the fit males would be able to reproduce with greater success and the females should also evolve more elaborate vaginas that only the fittest could inseminate forcibly (Adler 2010).