Humans are now mostly monogamous, but this has been the norm for just the past 1,000 years.



Scientists at University College London believe monogamy emerged so males could protect their infants from other males in ancestral groups who may kill them in order to mate with their mothers. Shutterstock

Scientists in Canada recently suggested that a rise in sexually transmitted infections, as social groups became larger among early humans, would have put pressure on staying monogamous in terms of mating behavior. Monkey Business Images/Shutterstock

Primates such as chimpanzees and bonobo monkeys, pictured, do not conform to a mating system and regularly engage in frequent sex with multiple partners. DPA/LANDOV

Emperor penguins usually mate for one year before moving on to a new partner. Thorsten Milse/Robert Harding/Barcroft Media/Landov

Male elephant seals, or "beach masters," protect harems of more than 100 females from other males thinking of moving into their territories. Justin Hofman/Barcroft Media/Landov

Swans -- symbols of love and fidelity -- are not monogamous. WILLIAM WEST/AFP/Getty Images

Love birds mate and "love" for as long the other mate stays alive. If one dies, the other develops a bond with another individual. EMPICS/Landov

Male lightning bugs entice mates by lighting up the night sky. The Asahi Shimbun via Getty Images

Queen bees mate with a very small number of male bees, drones, to produce many eggs. Sean Gallup/Getty Images