The flatworms have a full set of female and male equipment, but it turns out that they all want to be the impregnator and penetrator. The only way of resolving who gets to play the male is by duel. Flatworms go sharp-tipped penis to penis; the loser is the first to be impaled, receiving sperm by hypodermic injection from its fencing partner. The mating ritual of Pseudoceros bifurcus was discovered by German researchers. Australian scientists watched duels among 17 pairs of the worms.
At Groningen University, in the Netherlands, researchers have been studying oystercatchers, normally monogamous, although 3 per cent of females share a male. Half of these love triangles involve ''highly aggressive'' relations between the females. But in the other half they share one nest and the male harmoniously, defend their territory against other oystercatchers and copulate regularly with each other and the male.