Courtship behavior as a war of attrition in a simultaneous hermaphrodite: Supplementary material

In outcrossing hermaphrodites with unilateral mating, where for each mating interaction one individual assumes the female role and the other the male role, each individual must take a sexual role opposite to that of its partner. In the polychaete worm <i>Ophryotrocha diadema</i>, the decision on sexual role is likely at stake during the day-long courtship. Here we describe, for the first time, courtship and pseudocopulation in this species, quantify their pre-copulatory behavior, and search for behavioral traits predicting the prospective sexual role (i.e., behavioral sexual dimorphism), by analyzing the courtship behavior of pairs of worms during the day preceding a mating event. We did not find any behavioral cue predicting the sexual role worms were to play; partners’ pre-copulatory behaviors were qualitatively and quantitatively symmetrical. We interpret this as the outcome of a war of attrition where partners share the preference for the same sexual role, and both hide their ‘willingness’ to play the less preferred one, until one individual reaches its cost threshold and accepts the less preferred sexual role.