Hermaphrodite mating behaviour is poorly understood, particularly in species with internal fertilization and mutual penis insertion. We collected detailed recordings on the frequency, timing and outcome of the mating sequence of the simultaneous hermaphroditic flatworm Dugesia gonocephala (Tricladida, Paludicola). Mating in this species differs from that in other planarians in that receptive partners show a stereotypic pre-copulatory behaviour which results in copulation in only 17% (n = 342) of all attempts. It includes an unusual phase during which one partner glides on top of the other and first the lower, then both individuals, spread out and flatten. Genital intromission is not possible in this position. All observed copulations (n = 57) were preceded by this behaviour and most mating sequences were interrupted at this stage. Pairs were seen starting new mating sequences up to 11 times. Which animal lay on top varied randomly within a series of attempts. Cocoon production was a good predictor of mating activity, and copulations mainly occurred during the night. Copulations lasted between 24 min and 9.3 h (mean = 4.7 ± 2.3 h) and penises were mutually inserted. In the field, 99% (n = 70) of all matings involved reciprocal spermatophore exchange, compared with only 50% in the laboratory (n = 26). The likelihood of reciprocal spermatophore transfer increased with increasing duration of copulation. Duration was independent of the events that took place during courtship. We discuss the possibility that partners interrupt a copulation when there is no guarantee of reciprocal spermatophore transfer.
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