Two new papers both involving a great deal of speculation and some statistical analysis and some mathematical modelling. The first (which includes Robin Dunbar of the Dunbar Number as an author) concludes that the risk of infanticide among large brain primates led to the fathers “hanging around” to protect their offspring leading to monogamy. The other study of mammals concludes that monogamy “represents a mating strategy that has developed where males are unable to defend access to multiple females”.
1. Christopher Opie, Quentin D. Atkinson, Robin I. M. Dunbar and Susanne Shultz, Male infanticide leads to social monogamy in primates, Published online before print July 29, 2013, doi:10.1073/pnas.1307903110 PNAS July 29, 2013
Abstract: Although common in birds, social monogamy, or pair-living, is rare among mammals because internal gestation and lactation in mammals makes it advantageous for males to seek additional mating opportunities. A number of hypotheses have been proposed to explain the evolution of social monogamy among mammals: as a male mate-guarding strategy, because of the benefits of biparental care, or as a defense against infanticidal males. However, comparative analyses have been unable to resolve the root causes of monogamy. Primates are unusual among mammals because monogamy has evolved independently in all of the major clades. Here we combine trait data across 230 primate species with a Bayesian likelihood framework to test for correlated evolution between monogamy and a range of traits to evaluate the competing hypotheses. We find evidence of correlated evolution between social monogamy and both female ranging patterns and biparental care, but the most compelling explanation for the appearance of monogamy is male infanticide. It is only the presence of infanticide that reliably increases the probability of a shift to social monogamy, whereas monogamy allows the secondary adoption of paternal care and is associated with a shift to discrete ranges. The origin of social monogamy in primates is best explained by long lactation periods caused by altriciality, making primate infants particularly vulnerable to infanticidal males. We show that biparental care shortens relative lactation length, thereby reducing infanticide risk and increasing reproductive rates. These phylogenetic analyses support a key role for infanticide in the social evolution of primates, and potentially, humans.
Abstract: The evolution of social monogamy has intrigued biologists for over a century. Here, we show that the ancestral condition for all mammalian groups is of solitary individuals and that social monogamy is derived almost exclusively from this social system. The evolution of social monogamy does not appear to have been associated with a high risk of male infanticide, and paternal care is a consequence rather than a cause of social monogamy. Social monogamy has evolved in nonhuman mammals where breeding females are intolerant of each other and female density is low, suggesting that it represents a mating strategy that has developed where males are unable to defend access to multiple females.
Science Magazine carries an article about these papers today and do I discern a somewhat protective attitude to the second paper (published by Science) in the form of a greater degree of skepticism about the first? Both papers are openly available:
I’ll let the specialists argue it out but I can see no reason why all primates or all mammals need to have had the same triggers or the same evolutionary paths for similar social behaviour. And the social behaviour represented by the form of monogamy practiced by some birds (swans or vultures) and that practiced by some angel fish and that practiced by gibbons are not all strictly comparable. Among humans we have not (yet?) got to the stage where society at large takes over responsibility for taking care of children which remains with the parents. And in the bringing up of children the “nuclear familiy” – representing at least a serial monogamy – still provides the most effective and efficient method available. Infanticide is no longer as great a threat as it was but some of that threat has moved into the world of abortions.
ScienceMag: …. Among primates, including perhaps humans, monogamy evolved because it protected infants from being killed by rival males. ……… social monogamy, has repeatedly evolved among animals, although in widely varying proportions among different groups. Thus, about 90% of bird species are socially monogamous, probably because incubating eggs and feeding hatchlings is a full-time job that requires both parents. But in mammals, females carry the babies inside their bodies and are solely responsible for providing milk to young infants—and only about 5% of species are socially monogamous. That leaves most mammalian males free to run around and impregnate other females. Primates, however, seem to be a special case: About 27% of primate species are socially monogamous; and recent studies by Christopher Opie, an anthropologist at University College London, and his colleagues have concluded that social monogamy arose relatively late in primate evolution, only about 16 million years ago. (The earliest primates date back to about 55 million years.).
…… Although humans aren’t completely monogamous, “the emergence of pair-bonding in humans was a major evolutionary transition, which dramatically altered the evolutionary trajectory of our species,” says Sergey Gavrilets, an evolutionary biologist at the University of Tennessee, Knoxville. Many researchers think that we could not have evolved our large brains without joint parental care during the extended period of helplessness required for infant brains to grow to their full size. “Understanding the forces that drove that transition can help us better understand the causes of human uniqueness,” Gavrilets adds.
Opie and his colleagues set about testing the three leading hypotheses in primates using a powerful method called Bayesian statistics. The team used previously published genetic and behavioral data from 230 primate species, representing nearly all known species such as Old and New World monkeys, lemurs, and apes, employing strict criteria to ensure that the data were reliable.
As the team reports online today in the Proceedings of the National Academy of Sciences, there was a strong correlation in time between all three hypotheses—parental care, female range, and infanticide by males—and the rise of social monogamy in the roughly 60 primate species that live in pairs. However, among the three explanations, only infanticide actually preceded social monogamy in time and thus could be a driving evolutionary force, the team concludes; the other two behaviors occurred afterward and were the consequences of social monogamy and not the causes. “Our analyses clearly show that infanticide is the trigger for monogamy in primates,” and likely was the trigger in humans, too, Opie says.
Nevertheless, the reaction to the study has been mixed. ….
Indeed, a paper to be published this week in Science looks at monogamy across all mammals and comes to a very different conclusion. Zoologists Tim Clutton-Brock and Dieter Lukas of the University of Cambridge in the United Kingdom analyzed monogamy among 2545 nonhuman mammal species. In contrast to Opie’s conclusion in primates, they find in this larger sample that social monogamy arose among species where females were widely spaced and males could not monopolize several of them at once; infanticide did not seem to be a driver for monogamy among all mammals. Opie counters that wide spacing among females doesn’t apply to highly social, group-living primates, so that humans, and perhaps all primates, may be unusual among mammals. If so, he says, looking at mammals across the board might mask the special features of primate evolution.