Courtship behavior as a war of attrition in a simultaneous hermaphrodite

in Animal Biology
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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 Ophryotrocha diadema, 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.

Courtship behavior as a war of attrition in a simultaneous hermaphrodite

in Animal Biology



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    Schematic representation of pseudocopulation in Ophryotrocha diadema: (A) Worms stay immobile in Head-to-Tail contact; (B) the female-role worm (in black) lays eggs inside the jelly cocoon and then (C) leaves the cocoon by sliding over the male-role worm (in gray); (D) the male-role worm enters the cocoon and (likely) releases its sperm. The arrows indicate the direction of movement of the female-role worm (black arrow) and of the male-role worm (gray arrow).

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    Schematic representation of the behaviors described in this study and their mean frequency in 30 min (A: active worm; P: passive worm). For the long-lasting behaviors (i.e., Rubbing, Staying Side by Side, and Following), the mean duration and the mean percentage of the time dedicated to these behaviors during the observations are reported. For the contacts, the mean number of occurrences and their percentages of the total number of contacts are reported. The arrows indicate the direction of movement of the worms.

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    Factor loadings on the six PCs (Rotated Component Matrix). The factor score loadings (>0.650) of the behavioral variables on the PCs, where high loadings indicate that the variable was highly correlated with the PC (variables with lower loadings have been removed from the table). The PCs are sorted by the relative variance they explain. In bold the variables that load on the same PC and represent the same behavior, in its active and passive form. PC4 was the only significant predictor of sexual role.

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    The probability of the focal worm playing either the male or the female role at the next mating interaction as a function of the relative egg maturation level and the amount of Tail Contacts and time spent Staying Side by Side (as summarized by PC4). Focal worms that performed more Tail Contacts and spent more time Side by Side with their partners (i.e., higher PC4 values) were more likely to play the female role, especially if they had more mature eggs than their partners. When partners were equally matched for egg maturation, behavioral predictors (PC4) were relatively less effective.


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