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.
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.
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Primates such as chimpanzees and bonobo monkeys, pictured, do not conform to a mating system and regularly engage in frequent sex with multiple partners.
Emperor penguins usually mate for one year before moving on to a new partner.