Monogamy, an asset for our species
Recent work is fueling the debate on the emergence of monogamy in humans. This sustainable lifestyle as a couple would have facilitated the development of a bigger brain.
Mammals are not large monogamists. Less than 10% of their species live exclusively in pairs. Monogamy is a little more common in primates, of which 15 to 29% of species favor this mode of existence. However, very few of them experience monogamy in the sense that humans understand it: an exclusive sexual partnership between two individuals.
Obviously, the monogamy of humans is far from perfect. They have extramarital affairs, divorce, and in some cultures, are married to more than one person at the same time. Polygyny (a man married to several women) is also present in most societies. However, even where it is permitted, it concerns only a minority of individuals. In general, social organization is based on the assumption that most individuals will form lasting couples and maintain sexually exclusive bonds.
Monogamy would be a heritage of the evolutionary course of our species and would have constituted a crucial stage in the development process of our ancestors. The couple has even become one of the pillars of human social systems and one of the keys to the success of our evolution. “It is the couple bond that has probably given rise to human social organization,” says Bernard Chapais, anthropologist at the University of Montreal.
If monogamy is not the exclusivity of man, it has indeed made possible the emergence of a strictly human characteristic: the constitution of vast and complex social networks. The young of other primates establish family ties only through their mother. Humans do it from both parents, which widens the family circle with each generation. Their network extends to include other families and even grows beyond community groups. According to B. Chapais, monogamy and these links forged between groups constitute “two of the essential characteristics of human society”.
For decades, scientists have struggled to understand the origins and consequences of human monogamy. When did we start forming lifelong couples? Why did this mode of existence have an advantage? And has this contributed to the evolutionary success of our species? These fundamental questions remain unanswered to this day and are hotly debated. But in the light of recent work, we are starting to unravel the mystery.
Ardipithecus, already a couple?
According to Owen Lovejoy, an anthropologist at Kent State University in the United States, the study of hominid fossils indicates that monogamy predates Ardipithecus ramidus. This species has been known since the discovery, in the Awash River region of Ethiopia, of a part of a female skeleton dating back 4.4 million years, and nicknamed “Ardi”. Were such ancient hominins already monogamous? According to O. Lovejoy, it is possible. What are his arguments?
The great ape lineage and ours separated over seven million years ago. Our predecessors would have adopted three new behaviors: they would have started to carry food in their arms, now released thanks to bipedalism, to form permanent couples and to hide the external signs of female ovulation. These innovations would have, with evolution, allowed the emergence of hominins — the human line after separation from that of chimpanzees.
According to this hypothesis of O. Lovejoy, the ancestral mode of reproduction based on sexual promiscuity gradually gave way to monogamy when, rather than fighting among themselves, the lower-ranking male hominins began to search for food to offer to the females. with which they wanted to mate. They preferred, rather than aggressive suitors, the males they could rely on for food. Subsequently, the outward manifestations of the females’ fertility periods disappeared, as they would have attracted other males while their regular mating partner was away to search for food.
When did the transition to monogamy happen? To support his script, O. Lovejoy draws attention to Ar’s teeth. ramidus. The size of its canines, in the male and in the female, does not present great differences. In the case of primates, fossils or modern, the canines of males are larger than those of females, because these teeth serve as weapons in their combats for reproduction. Observe a gorilla’s canines, then compare them to ours. In humans of both sexes, these teeth are relatively small, a characteristic feature of all hominins, including the earliest specimens of Ardipithecus. According to O. Lovejoy, Ar. Ramidus and the other monogamous hominins no longer required such weapons, hence the reduction in canine size over the course of evolution.
A certain correlation also exists between the reproductive behavior of primates and sexual dimorphism (the morphological difference between males and females of the same species). The more dimorphic a species of primate, the more likely it is for males to fight with each other for favors from a female. For example, male gorillas, polygynous, are twice as stout as females. In contrast, gibbons, mostly monogamous, have almost identical weights in both sexes. When it comes to dimorphism, humans are closer to gibbons: men weigh 20% more than women.
However, the interpretation of fossils has its limits. Paleoanthropologist Michael Plavcan of the University of Arkansas calls for caution: between fossilized bones and the social behavior of hominins, there is a step that cannot always be crossed.
Consider the case of Australopithecus afarensis, the species to which “Lucy” belonged and which lived between 3.9 and 3 million years ago. Like Ardipithecus, Au. afarensis had small canines, but its skeleton shows a level of dimorphism intermediate between that of chimpanzees and that of modern gorillas. Body size dimorphism suggests that males of Au. afarensis fought for females, while a lack of dimorphism in the canines indicates this was not the case, says Plavcan. What can we conclude from these contradictory observations?
Many other anthropologists also dispute O. Lovejoy’s scenario. According to them, factors other than foraging may have favored monogamy, and perhaps much later. In 2013, B. Chapais published arguments showing that the characteristics specific to the family and to the social structure of humans (monogamy, kinship ties through both parents, expansion of the circle of social relations) appeared in successive stages.
According to B. Chapais, male and female hominins had initially, like chimpanzees, several sexual partners and unstable relationships, of short duration. Then began a process of transition to polygyny with stable sexual ties, which we find in gorillas for example. However, maintaining several partners mobilizes a lot of energy. You have to fight against other males and watch the females. Monogamy may have been the strategy to reduce the effort required by polygyny.
B. Chapais refuses to speculate on the moment when this turning point took place and on the species concerned. Other researchers suggest that this happened between 2 and 1.5 million years ago, long after the emergence of the genus Homo, and simultaneously with the physical changes that manifested in Homo erectus. Compared to its predecessors, H. erectus had a much larger body, with proportions closer to those of modern man. He was about twice the size of Lucy and her ilk. Likewise, H. erectus appears to exhibit less sexual dimorphism than Australopithecines and early Homo. He attained a degree of dimorphism similar to that of modern men.
Homo erectus, probably monogamous
These clues suggest that H. erectus had a much less rivalry-based lifestyle than that of its ancestors. Since male and female primates of similar body size tend to be monogamous, H. erectus would also translate a more exclusive sexual behavior.
If researchers struggle to agree on when humans became monogamous, they are unlikely to agree on the reasons for such upheaval. In 2013, two teams, independently of each other, published statistical studies based on previously published scientific work, in order to determine the behaviors that could have led to monogamy in mammals in general and primates in particular. . The two studies aimed to find, among three hypotheses — the territorial spacing between females, the desire to avoid infanticide and paternal care — the one best able to explain the emergence of monogamy. The researchers came to different conclusions …
According to the first hypothesis, monogamy emerged when females began to need richer, but scarce, food resources (such as carcasses or ripe fruit, rich in protein). The females occupied larger territories in order to have access to more food, thus putting more distance between them. For the male, it has become increasingly difficult to maintain several females, to find new ones and to isolate them from other males. Establishing with a single female would therefore have made life easier for the male. The male also reduced the risk of being injured while roaming his territory and he could more easily ensure that his mate’s pups were his.
Zoologists Dieter Lukas and Tim Clutton-Brock of the University of Cambridge in the UK have found evidence for this hypothesis in a statistical analysis of 2,545 species of mammals. They built a phylogenetic tree to establish the kinship relationships between species. They then indicated the mammalian way of life, depending on whether the females are solitary (with a male sharing the territory of several females), living in pairs or in groups. Then the researchers found that, during evolution, 61 transitions to monogamy occurred, most often in carnivores or primates, suggesting that a species tends to be monogamous when females have a diet of rich and rare foods. These results support the idea that a solitary lifestyle of females, increasingly distant from each other, prompted males to mate with only one of them and to stay close to them. ‘she.
D. Lukas recognizes that while this idea applies well to some mammals, it probably does not apply to humans. Indeed, the social character of the latter seems hardly compatible with a hypothesis based on the low territorial density of females. Our ancestors may have already developed too social a character for females to scatter in the savannah like other mammals. However, the hypothesis could apply to humans if monogamy was already present in hominins before our tendency to live in groups developed.
The second hypothesis on the origin of monogamy invokes the threat that weighed on the little ones. If a rival challenged or supplanted a dominant male within a group, the usurper could kill offspring that were not his own. As the mothers stopped breastfeeding, they began to ovulate again, giving the marauder the opportunity to ensure his own offspring. To prevent these infanticides, the female would have chosen a male ally capable of defending her and her young. This protection was all the more difficult to ensure as the male had several females to protect.
The anthropologist Kit Opie, of University College in London, brings elements to support this thesis on infanticide. With his colleagues, he performed numerical simulations of the evolutionary history of 230 species of primates. Then he used statistical analysis to determine which factor seemed to cause monogamy. He found a significant correlation between monogamy in primates and the different factors imagined. However, only an increase in the threat from infanticide systematically preceded the onset of monogamy in several primate lines.
The biology and behavior of modern primates support this conclusion that the risk of infanticide favors monogamy. The threat to the offspring mainly concerns primates: they have large brains that take a long time to develop and make them dependent and vulnerable long after birth. However, infanticide has been observed in more than 50 species of primates, almost all of them non-monogamous. Hence a question that remains unanswered: if the infanticide hypothesis is correct, why are there not more monogamous primates? The question is all the more relevant given that, in gorillas for example, the share of infanticide in infant mortality represents 34%.
The third hypothesis relating to the emergence of monogamy concerns the discharge by males of their father’s duty. When a cub begins to demand too much energy and food from its mother who raises it alone, the father, if he lives with them and provides food and other care, increases the chances of survival of the cub and promotes closer ties with the mother.
Anthropologist Lee Gettler of Notre Dame University in the United States gives an example. According to him, the mere fact that the father carries his young would promote monogamy. Indeed, mothers must provide a lot of food for their young. However, for primates or hunter-gatherer humans, transporting a baby involves an expenditure of energy comparable to that required by breastfeeding. The fact that the males became available to carry the young may have liberated the females to meet their own energy needs by going for food. Thus, they feed better and begin to reproduce earlier in the season. They also live longer on average. Monogamy will thus be selected.
Paternal care reinforces monogamy
The douroucoulis of Azara, apes from South America, are an example that may help to understand how paternal care would reinforce monogamy. These monkeys live in small family groups made up of the father, mother, baby and one or two young. The mother carries the baby on her hip just after birth, but the father assumes almost all of the care given to the baby from the age of two weeks: he grooms him, plays with him, feeds him and carries him. The two adults literally stay in contact with their tails. The simple fact that the male remains so close to the female and her young would strengthen the emotional bonds.
A genetic study published this year by four researchers, including Eduardo Fernandez-Duque of Yale University, provides evidence that douroucouli is monogamous. DNA taken from several groups revealed that out of 17 pairs, all but one female and male were most likely the biological parents of their respective offspring. Couples in douroucoulis last an average of nine years, and those who stay with the same partner reproduce more frequently, a decisive advantage for this type of relationship.
What does the statistical study by K. Opie and colleagues and that of D. Lukas and T. Clutton-Brock tell us about the hypothesis relating to the care provided by the father to his offspring? They indicate that paternal concern is the least determining factor among the three hypotheses to explain the emergence of monogamy. However, says D. Lukas, “paternal care can still explain why a species remains monogamous. “
The question of the factors of the emergence of monogamy is therefore still open. Another question is the benefit that monogamy would have brought in the evolution of man.
Sarah Hrdy of the University of California at Davis explored the energy required to raise a human: a baby consumes some 13 million calories from birth to maturity. For a single mother, the task is heavy. Monogamy with the presence of the father made it possible to increase the intake of food, but this remains insufficient. The decisive step, which we observe in many human societies, was taken when the mother was able to count on her relatives (her family and that of her spouse, or other members of her entourage) to help her feed and care for the child. “Human mothers let others carry their babies from birth,” notes S. Hrdy, “it’s surprising and totally contrary to what monkeys do. In fact, none of them resorts to the help of their relatives to take care of their young.
H. erectus had a much larger body and brain than its ancestors. According to S. Hrdy, if he began to take the human path of late development and prolonged dependence, family cooperation may have been the key to providing the energy necessary for the care of the little ones.
Social assistance, a key element
Without this cooperation, conclude Karin Isler and Carel van Schaik, both at the University of Zurich, the first Homo would not have been able to cross the limit of 700 cubic centimeters that characterizes the brain of apes. To meet the energy cost required by a larger brain, an animal must reduce its birth rate or growth rate, or both. However, humans have achieved shorter weaning periods and better reproductive capacity than animals with brains as large as 1,100 to 1,700 cubic centimeters. K. Isler and C. van Schaik attribute this performance to social and family support which would have enabled H. erectus to have babies more often while providing them with the energy necessary for their brain development.
Cooperation, on the one hand in the form of the monogamous couple and, on the other hand, from the nuclear family or tribe, allowed humans to develop successfully, while our ancestors became extinct. Cooperation may well be the greatest skill we have learned over the past two million years — a skill that has enabled our very young species to survive environmental changes and stresses, and which may well be life-defining for our lives. to come up.
