Monogamy evolved in response to the threat of babies being killed by rival men, according to an extensive study of our nearest primate relatives that explains why a marital system based on one man, one woman became the norm for rearing children in many human societies.
A majority of parents in the world are bringing up children in monogamous relationships but this is not a cultural accident but the result of deep-rooted behavioural traits that evolved over many millions of years, scientists said.
A study of some 230 different species of primates – monkeys and apes – has found that the risk of infanticide by rival males was the driving force that led to monogamy being established in some primates, including humans, scientists claimed.
Monogamy is rare in primates but in those species where it has evolved it was always preceded by a non-monogamous breeding system where there was a high risk of incoming males killing the infants of rival males in order to take over their rival’s females.
In those species where monogamy became established, there was a corresponding decrease in infanticide as males guarded and protected their females and their offspring, said Kit Opie, an anthropologist at University College London and lead author of the study published in the journal Proceedings of the National Academy of Sciences.
“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. This brings to a close the long-running debate about the origin of monogamy in primates,” Dr Opie said.
“We can tell that infanticide evolved first and monogamy evolved second, which was not the case with other types of behaviour that have been suggested as a cause of monogamy. It wasn’t just the case for instance that monogamy evolved because two parents were better than one,” he said.
Only about 3 per cent of mammal species are monogamous and it has been a puzzle as to why it evolved in some monkeys and apes but not others. Gibbons, for instance, are strictly monogamous but other great apes, such as polygynous gorillas and promiscuous chimpanzees, are not.
The scientists analysed the phylogenetic tree connecting all 230 species of primate and used a statistical algorithm to analyse any connections between the evolution of monogamy and the different behavioural traits that have been cited as possible explanations, such as bi-parental care, the guarding of solitary females from rival males and the risk of infanticide by rival males.
Mammals run a particularly high risk of infanticide because nursing females do not ovulate when lactating and so are effectively sterile for that period. It is in the genetic interests of rival males to therefore kill a female’s offspring to force her to begin ovulation, which happens for instance in lions.
Large-brained primates have the additional burden of rearing infants that are helpless for a relatively long time. Female chimps countered the risk of infanticide within a troupe by mating with multiple males so that no male could be sure about an infant’s paternity; whereas the solitary gibbon lives in monogamous pairs well away from other males and females.
“We know that human monogamy most probably evolved since the last common ancestor with chimps and, unlike gibbons, human monogamy evolved in a relationship between individuals who live in close-knit society of other individuals,” Dr Opie said.
Once a monogamous system is established it is rare for it to break down completely. In humans, monogamy gave the additional advantage of allowing children to be reared by more than one parent, enabling an extended childhood and brain development, said Susanne Shultz of Manchester University.
“What makes this study so exciting is that it allows us to peer back into our evolutionary past to understand the factors that were important in making us human. Once fathers decide to stick around and care for young, mothers can then change their reproductive decisions and have more brainy offspring,” Dr Shultz said.
Monogamy in humans, however, is not as strict as in some species. Many societies allow polygyny, with the number of wives based on a man’s wealth or status, and a few permit polyandry, such as in Tibet where it is possible for a woman to live with two husbands, usually brothers who co-own the same plot of land.
The closest living relative of humans that mate for life. Males and females are roughly the same size, indicating equal status. They spend a lot of time pair bonding by grooming one another and hanging out in trees. Like many seemingly monogamous species, however, further research has revealed that there is some promiscuity, mostly on the male’s part.
Monogamy is particularly common among birds, with about 90 per cent of species favouring it as a breeding system. The pair-bonds between swans can last many years, and in some instances for life. Although it may seem romantic, the biological reason has more to do with cutting down on the time needed to find a mate and establishing territories, which means more time and effort can be spent on the serious business of raising young.
Monogamy is rare in fish, but this species is famous for establishing life-long bonds. They travel and hunt in pairs and defend their territory against neighbouring couples. They may be cold-blooded animals, but they seem to have a warm vein running through their piscine hearts.
Posh n’ Becks
Humans are mostly monogamous, at least in spirit if not in the flesh. Some are better at it than most, helped in a few cases by the enormous marketing opportunities afforded by a strong pair-bond. Public displays of affection, jewellery and tattoos emphasise the one-man-one-woman coupledom.