By Colin Barras. Humans, meanwhile, show a variety of mating behaviours but often form monogamous couples. Michael Jensen-Seaman and Scott Hergenrother at Duquesne University in Pennsylvania think that it is the chimps — not humans — that have experimented with new sexual behaviours since our lineages diverged. But did male chimps inherit their mating plugs from the last common ancestor they shared with us or did they evolve it later? They found that the enzyme is four times as abundant in human semen as it is in chimp semen.