A large number of lampreys gather in a hollow - a spawning bed made by removing stones from the riverbed - and lay eggs. (Photo by Kentaro Morita)

Researchers at Hokkaido University have discovered that the nonparasitic lamprey Lethenteron kessleri mates repeatedly without releasing eggs, in a behavior termed “sham mating,” suggesting the possibility that females choose their mates while engaged in promiscuous mating habits.