However difficult relationships may be, humans are fairly monogamous, especially parents. Now, scientists in Britain say they know why monogamy evolved among some primate species and not others -- infanticide.
"It’s the predominant way we live," said Christopher Opie, an anthropologist at University College London. "What we have now is an evolutionary pathway for the emergence of monogamy."
Around 90 percent of birds are monogamous, while only 3 percent of mammals are, because internal gestation and post-natal lactation is lengthy, and encourages males to seek multiple mates.
Opie and his team took a family tree of 230 primate species, including lemurs, monkeys, apes, and modern humans and noted their mating behaviors, infanticide rates and levels of paternal care.
Then, they simulated these species' evolution over 75 million years to the modern day to test three hypotheses about the purpose of monogamy.
One is that offspring with large brains are more demanding, and two parents are better than one. The second hypothesis is "mate guarding," which suggests males must protect their mates from rivals. The third proposes males protect their offspring from rival males.
Having run the simulation program through millions of iterations, researchers concluded that monogamy in a species was only commonly preceded by infanticide.
“You do not get monogamy unless you already have infanticide, and you do not get a switch to paternal care if you don’t already have monogamy,” concluded Opie, published in the journal Proceedings of the National Academy of Sciences.
In general, the bigger a species' brains became, the bigger their social groups became, and the longer their reproductive cycle became. Infanticide was a male strategy to make females fertile again.
The authors note that monogamy is only one strategy for dealing with infanticide, saying only that monogamy was always preceded by infanticide in the simulations.
Chimps, for example, mate with all males in the group to confuse paternity, discouraging infanticide.