Strategic male courtship effort varies in concert with adaptive shifts in female mating preferences

Very few studies have presented compelling evidence for adaptive shifts in female mating preferences. Additionally, there is almost no data on how males adjust courtship effort when female mating preferences change predictably. How should males respond? Should a currently more attractive male increase his courtship effort because he has, for now, a better chance of reproducing? Or should he maintain/lower his courtship effort and conserve energy because he already has an edge on the competition? We experimentally measured female mating preferences and male courtship effort (i.e., male mating preferences) in the fiddler crab Uca mjoebergi. Using robotic males, we documented a consistent shift in female preferences for male claw size across each of 6 biweekly mating periods: females tested at the beginning of a mating period preferred large males, whereas those tested at the end preferred small males. This is one of the fastest-known temporal changes in the mean mating preference of a population and supports our prediction of an adaptive response due to time constraints on larval development. Males adjusted their courtship effort across the mating period in concert with the observed daily shift in the mean female mating preference. Interestingly, changes in courtship effort depended on male size. We interpret this shift as males increasing their courtship effort to take advantage of their current attractiveness to females. To our knowledge, this is the first study of the interactions between shifts in female and male mating preferences.