Mating behaviour in a hermaphroditic flatworm with reciprocal insemination: Do they assess their mates during copulation?

In the freshwater planarian Dugesia polychma (Tricladida, Paludicola), both animals fulfil the male and female role simultaneously in any given copulation. This study presents the first detailed account of the copulatory behaviour, timing and frequency in this species. We also describe an experimental set-up that enables continuous, undisturbed observation of large numbers of animals. D. polychroa pairs copulate repeatedly in the lab (up to eight times in 5 d). Animals that were kept in isolation for longer, were more likely to copulate. The mating behaviour lacks a behaviourally recognizable precopulatory courtship sequence, suggesting that precopulatory assessment is absent or only marginally important. Copulation duration ranged from a few minutes to 2.5 h and showed a distinct bimodal distribution. Two copulation types were recognized: short (<35 min) and long (≤ 35 min). Histological analysis showed that sperm transfer is rare during short copulations, but after long copulations all animals had received sperm and sperm transfer was reciprocal in all pairs where both partners could be investigated. Hence, only long copulations are 'true' copulations with sperm transfer. Pairs with short copulations (40% of all pairs) had fewer long copulations, but a higher overall copulatory activity. Copulation rate is constant over 5 d for both copulation types. It does, however, fluctuate with the time of day: most (long) copulations take place at night. Pairs that copulated more often, also produced more cocoons. We argue that in copula assessment during the first 30 min of the copulation determines whether copulations are sometimes interrupted before sperm are transferred and that this explains the occurrence of short and long copulations.