Courtship Behaviour of Nasonia Vitripennis (Hym.: Pteromalidae): Observations and Experiments On Male Readiness To Assume Copulatory Behaviour

Abstract

A male of the pteromalid wasp Nasonia vitripennis usually induces sexual receptivity in a conspecific virgin female upon their first encounter; following a sequence of courtship she will open her genital orifice, and, at the same time, draw the antennae tightly to her head. The antennal movement acts as a signal for the male who is positioned on the female's dorsum, far to the front, with his fore tarsi placed on the female's head. As soon as the male perceives the antennal movement by the female he backs up and establishes genital contact. Nodding movements with the head are a conspicuous part of the courtship display of a male Nasonia. Head noddings occur in clusters; series of nodding movements are separated by pauses. Females will signal sexual receptivity only at first head nods of series. This coincidence is probably due to the periodic release - at the onset of a head series - of a pheromone by the courting male. The female's receptivity signal can be simulated by pushing down the antennae (of an unreceptive female) with an extremely fine watercolour brush. A pseudo-signal produced at an appropriate moment (a first head nod) scored a high percent of backing-up responses by courting males; pseudo-signals produced at other times released fewer backing-up reactions, or none at all. From these experiments we conclude that during courtship a male's tendency to back up is not constant but rather fluctuates; periods of maximal responsiveness occur shortly before the onset of a head series and with first and second nods. Since the organisation of male courtship is rigid, the timing of maximal responsiveness can be predicted accurately, once the interval between first and second head series is known. Unreceptive females are courted for limited periods; males will dismount without making an attempt to copulate. Dismounting always took place at a time that the male's tendency to back up could be supposed to be on a low level, to judge from the experiments with simulated signals. Comparative work provides some arguments for the idea that periodic readiness on the part of the male must be understood as an advanced situation, a continuous readiness being the ancestral condition. Some speculation is offered about functional aspects of periodic responsiveness.