Humans don't always remain faithful to just one mate for life, but many -- at least theoretically -- subscribe to the idea of monogamy.
Scientists are still asking: Where did this tendency to form lasting relationships with just one other person come from?
Two studies published this week address the question of why social monogamy evolved in some mammals. But they don't agree on the answer: One says dads stick around to protect their kids from being killed; the other says it's because of resource distribution and the female diet.
Why, or why not, monogamy
About 90% of bird species demonstrate social monogamy; this is true for less than 3% of mammals.
Living in pairs makes sense for birds because successful child rearing requires both parents to incubate and provide food for the babies, which hatch from eggs. On the other hand, in mammals, the fetus grows inside the mother, and she lactates to feed the baby -- activities in which males don't play a role.
"We would predict that males should have additional time and additional energy, which they might use to increase their fitness by mating with additional females," said Dieter Lukas, a postdoctoral researcher in the department of zoology at the University of Cambridge. "Why then do males in some species stick around rather than try to find additional females?"
Typically, male mammals mate with multiple females in a single breeding season, a system called polygyny. If a male animal's evolutionary goals are to survive and reproduce, monogamy represents a problem; staying with one partner limits his seed-spreading potential.
In primates in particular, however, about a quarter of species display social monogamy. The phenomenon appears to have developed about 16 million years ago, which is relatively late in the history of primates, according to a new study led by Christopher Opie at University College London.
This breeding structure is also seen in wolves, jackals, beavers, meerkats, spiders, shrimp and many other animals. So there must be an important evolutionary reason (or reasons) that males of some species stay with just one female partner until one of their deaths. But what?
Because of male infanticide?
Opie's study, published in Proceedings of the National Academy of Sciences, focused on primates, a subgroup in which humans fit, along with relatives such as chimpanzees and gorillas. It's important to note that the researchers used "social monogamy" in this context to mean "living in pairs."
Researchers analyzed the family tree of primates and used a statistical technique to gain insights from what we know about the genetics and behaviors of 230 primate species. They posited that there are three possible explanations for social monogamy in this group.
One is parental care -- that the father sticks around to help carry the children; for instance, some monogamous New World primates often give birth to twins.
Another idea is that females are spread out in the territories that they occupy, meaning males have a hard time claiming more than one female.
The explanation that these researchers favor, however, is that a male that lives with a female mate can protect their offspring from other males, that might want to kill these children.
Why? Mammals usually don't get pregnant while they are lactating, so a male that is not the father might increase his chances of mating with the mother if he kills the offspring while she is still weaning these children. Infanticide is not advantageous in seasonal breeders, however, since everyone will have to wait until the next breeding season for copulation.
Although parental care and dispersed female ranges are also traits of social monogamy, male infanticide is the one that appears to have preceded, historically, the shift to social monogamy, the researchers say.
"Our analyses suggest that socially monogamous species are much more likely to have low male infanticide rates," study authors wrote.
The risk of infanticide goes down because one or both parents can defend a child against another invading male, they argue. It also appears that species practicing social monogamy have a relatively shorter lactation period, which allows fertility to resume more quickly and reduces the incentive for other males to swoop in and kill offspring.
This may be an uncomfortable thought, but researchers suggest that male infanticide may have put pressure on our ancestors to stay in long-term couples. There is evidence that Australopithecines and early modern humans displayed monogamous behavior. In humans in particular, researchers have proposed that the transition to social monogamy depended on females choosing to stay faithful to males -- and that may be because they wanted to protect their children.
But Maren Huck, primate researcher at the University of Derby in the United Kingdom, doesn't entirely buy this story. She finds the classifications of key characteristics of various species in this study "questionable." Given that, she said in an e-mail, "It would be premature to confidently claim that infanticide was the key factor in the evolution of social monogamy in primates."
Because of territory and food resources?
The conclusion of Opie's group's study also diverges from what a different group of scientists concluded in a separate study in Science this week. The second study looked at the family trees relating to more than 2,500 mammals. This represents a much larger cross-section of animals than Opie's team's analysis, but this time, humans were not included.
"I'm far from convinced that humans are really monogamous," said Tim Clutton-Brock, professor of ecology and evolutionary biology at the University of Cambridge.
Lukas and Clutton-Brock view "social monogamy" somewhat differently: as a breeding male-female pair ranging together, with or without their offspring, associating with each other for at least one breeding season.
They suggest that in some mammalian species, females started finding higher-quality foods in areas that were farther apart from each other. As a result, the females would aggressively defend territories that contained this food, keeping other females out.
"As they then started to spread out further, that's when males seem to have changed their mating strategies," said Lukas, lead author of the study.
Other species that came to specialize in more abundant food, particularly in the grasslands, evolved to be polygynous, Clutton-Brock said.
The Cambridge team did a separate analysis of primates -- using more species than Opie's group did -- and did not find the same association between social monogamy and male infanticide as the other study. They haven't pinpointed the discrepancy, but the independent research groups may have classified species differently, or it may have to do with different sample sizes, researchers said.
Huck also had issues with some of the assumptions regarding various species in this study. It also appears, she said, that these results focus on mating monopolization -- breeding with only one mate -- rather than how some animals evolved to live in pairs.
"Unless we are clear about the classifications and what exactly we are talking about, we cannot be clear in what the results we get out of a complicated evolutionary model actually signify," she said.
What about humans?
Clutton-Brock cautioned against drawing any definite conclusions about humans from the study but said it is possible that the dietary and resource patterns his paper described could have something to do with the evolution of human breeding strategies, as well as a need for extended paternal care in our species.
Given that other great apes are polygynous and that human males and females differ so markedly in their average body size and longevity, Clutton-Brock says "the ancestral condition for humans is probably polygyny."
Clutton-Brock and Lukas said they were unaware of Opie's team's study until just a few days ago; otherwise they would have hooked up beforehand to compare notes.
It's not too late. The Cambridge scientists say they intend to connect with Opie's group to trace the origins of their differences, which may in turn put them closer to the roots of human togetherness.