Any list of the world’s as-yet-unsolved mysteries has to include these three questions: 1) Why are we here? 2) Why are yogurt containers sealed to pretty much guarantee spillage upon opening? and 3) How did monogamy ever develop?
New and divergent replies to the latter conundrum have been offered in two new studies, which, unbeknownst to each set of authors, came out in the same week. According to one, primates are monogamous so that nonrelated males don’t kill their babies. According to the other, that’s hooey; animals are monogamous because it was the only way they could guard their mates and thus their breeding rights.
Monogamy, as nearly any Ashley Madison subscriber likes to note, is not “natural.” That is, hardly any species practice it, except for birds (and, reportedly, cockroaches). Social monogamy, an arrangement in which two creatures mate and work together to meet their basic needs, is especially uncommon among the nonavian warm bloods; only about 5% of the 4,000 or so mammal species on earth hang around with just one mate. (These include wolves, beavers, naked mole rats and meerkats.) Since mating with only one female at a time tends to lower a male’s chances of producing as many offspring as possible, what good, evolutionarily speaking, can come of being monogamous? Why would mammals end up that way?
One reason, according to a report in the Proceedings of the National Academy of Sciences (PNAS), suggests that males stayed with one female to ensure their young were not killed by another male. It’s one thing to sire a litter of pups and another to ensure that they actually survive to reproduce to carry on their genetic lineage. Based on breeding and parenting behaviors collected from 230 different primate species over several generations, the researchers determined that males began balancing the need to spread their gene pool against the need to protect their young from being killed by other nonrelated males. The attacking males needed to kill the young so that they could breed with its mother, who would delay conception of another offspring if they were nursing. So the father hung around to ensure the safety of his genetic line and to help raise the young so that the mother could reproduce again sooner.
“This is the first time that the theories for the evolution of monogamy have been systematically tested, conclusively showing that infanticide is the driver of monogamy,” trumpeted Christopher Opie, a research fellow in the Anthropology Department of University College London, in a statement. “This brings to a close the long running debate about the origin of monogamy in primates.”
Well, not so fast. Another study, published in the journal Science, used a similar type of analysis, but across a much wider sample — about 2,500 mammals, or more than half the known species. Those authors, Dieter Lukas and Tim Clutton-Brock of Cambridge University, found no correlation at all between infanticide and monogamy. Their results suggest that monogamy works like real estate: it’s all about location and supply. “Monogamy develops where females live at low density,” says Lukas. Males cannot fend off rival suitors from more than one female at a time because they’re too spread out. Therefore, they cannot ensure their young are the ones the female is carrying, so they stick with one female. “It’s a consequence of resource defense.”
The Science study notes that in mammalian species that are monogamous, the females tend to be solitary and intolerant of other females. Unlike ungulates, who are rarely monogamous, these mammals’ nutritional needs are greater, and they therefore shoo off competitors for the food resources.
Both studies suggest that the third theory often advanced for the development of monogamy — in which males can assist in raising of the young — is much less likely. Rather than a cause of monogamy, such paternal assistance is probably a consequence of the mate-for-life scenario.
As for human monogamy, the PNAS study is much more comfortable extrapolating its results. Because humans have such big brains, their infants take a long time to nurture and are vulnerable for longer. Therefore human males had a compelling reason to hang around and protect their child-rearing female until breeding was done.
The Science study is more speculative. “We are cautious on making any definite statement. Humans are such unusual animals,” says Lukas. Adds Clutton-Brock: “I’m far from convinced that humans are indeed monogamous.”
So how important were kids in man’s move toward monogamy? It’s a fascinating fight, but ultimately whether or not monogamy is natural is less relevant than whether it’s desirable. When considering behavior, naturalness is not the most important issue. There’s nothing natural about reading, using toilet paper or skydiving, but they have their advantages. Monogamy, in humans, seems to be both an acquired taste and a learned skill. The question remains whether it’s worth the cost of learning it.