Fear of child murder made our ancestors monogamous

Driving force for the pair bond was the protection for the offspring

In gorillas, the killing of offspring by rivals is often © public domain
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Man is a real exception, because he is one of the few mammals that live predominantly monogamous. But why? There have been three hypotheses to date - which is true, but was debatable. British researchers have now systematically reviewed all three for the first time. It turns out that the drive for the couple bond is child murder - or rather the avoidance of the same. Because the man stayed with the mother of his child together, he prevented rivals kill his offspring in turn to mate the female, according to the researchers in the journal "Proceedings of the National Academy of Sciences".

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Mammals are more flighty in relationships: only three percent of them live more or less monogamous, the remaining species typically have sex with several different partners during a mating season. Even with our closest relatives, the great apes, a lively partner change is the rule. One of the few exceptions is man. But why our ancestors developed a predominantly monogamous way of life millions of years ago has been controversial.

Three possible reasons - but which one is right?

Theoretically, there were three reasons for doing so, as explained by Christopher Opie from University College London and his colleagues. The first reason: Because the children of the pre-human and humans need especially long to adulthood, they must be intensively cared for years. This works better if two parents take care of the food and well-being of the offspring.

The second hypothesis is that if the females of one species do not live in groups but have their own territories, it becomes more difficult for the males to defend several females against competitors. It is therefore more rewarding for them to focus only on one partner. And the third possibility: In many mammals, the female is not ready to conceive again until she has stopped breast-feeding her baby. "For a male, it can therefore pay to kill the still-suckling boys of a competitor, so they can mate with their mother, " say the researchers. In the case of lions and gorillas, such infanticide is common when males take on the rival's harem. display

Search for clues in the primate tree

To find out which of these three hypotheses might apply, the researchers performed an extensive pedigree analysis on 230 primate species. In doing so, they compared the types of monogamy, child prostitution, the female's own areas or the boys' joint rearing and when the respective behavioral patterns developed first. A special software analyzed whether these behaviors formed together or one after the other, and above all, which of them emerged at the same time or just before monogamy.

The result: for all three candidates - rearing, territories and child care - a close connection to monogamy was found. "But the joint care for the offspring always developed only after this species had already passed to monogamy, " the researchers report. The same applies to the guarding of territorial females by the males. Both factors are therefore rather the result, but not the cause of monogamy.

Monogamy as protection for the offspring

The only factor consistent with the onset of a monogamous lifestyle, however, was a high rate of child rearing by rival males. If these primate species then changed their pattern of relationships in favor of fidelity, the infants also became less common.

According to the scientists, this indicates that the males of some primate species changed their mating behavior exactly at that time - in order to prevent the death of their offspring. "This is the first time that it has been clearly demonstrated that preventing childhood was the driver of monogamy, " says Opie. This realization ends the long debate on the origin of monogamy in primates.

Exceptional gorillas

Why, however, with all the advantages of this lifestyle, not all primates became monogamous is still unexplained. After all, especially our close relatives, the gorillas, still prefer the non-binding partner change despite an enormously high rate of child birth. The researchers speculate that the switch to monogamy may only be beneficial and possible where environmental conditions permit.

Especially gorillas are in their habitat strongly threatened by predators. Their chance of survival therefore increases when they join forces to form strong groups - groups that are strengthened, inter alia, by alternating pairings. (Proceedings of the National Academy of Sciences, 2013; doi: 10.1073 / pnas.1307903110)

(PNAS, 30.07.2013 - NPO)