Cooperative breeding



It takes a village to raise a child.  

African proverb



Humans survive and flourish by cooperating with each other.  We are sometimes called the “hypercooperative” species.  Along with some ants and other highly successful eusocial insects, our runaway success in colonising almost every niche on the planet is due mainly to our intense prosociality and cooperation.  

Almost unique among primates, and unique among the great apes, human beings are a cooperatively breeding species.  While the babies of great apes spend almost 100% of their life before weaning (moving on from mother’s milk) attached to the mother, and interacting only with the mother, human babies are typically looked after by many other helpers as well.  These are known as allomothers (“other mothers”) and may include grandparents, fathers, brothers, sisters, cousins, unrelated mothers and other group members.  

Other cooperatively breeding species include elephants, wolves, bottlenose dolphins, crows, and some tamarins and marmosets.  


Enhanced empathy ...

We believe that cooperative breeding led to one of the crucial, basic, necessary abilities for human cooperation: unlike the other great apes, humans seek out and enjoy sharing in the mental states of others for prosocial purposes.  Put simply, people who are cooperating in a sophisticated way need to have a sophisticated understanding of each other’s mental states, and a willingness to share these mental states.  

The capacity to enter into the inner worlds of others, empathy, is thought to be formed when a baby bonds with its caregiver and is rewarded with maternal care and affection for sharing in her mental state.  In apes, this caregiver is the mother exclusively.

Human babies are typically cared for by allomothers much of the time.  The hypothesis is that, since the baby relies on many other people to care for it, it needs to monitor and assess the intentions of those people towards it.  It needs to know what many people think, at least towards itself.  A human baby will try and elicit care from people around it by seeking out faces, vocalising and looking “cute”.  

Therefore it is in the interests of a human baby to want to enter into the inner worlds of others for prosocial purposes, in a way that is not necessary for great apes.  

It is thought also that cooperative breeding led to a general reduction of aggression within human groups, as in conjunction with pair-bonding, adult males now knew who their children and other relatives were with good certainty.  See also:  evolution of generalised monogamy; self-domestication of the human race.  

In other great ape species, adults often kill the offspring of others, either (in the case of females) to assert dominance over another female, or (in the case of males) to ensure that another male’s young will not survive, or to make a female ready for mating again, or (in chimpanzees) when a female with young from another group is encountered during a border patrol.  


... added to the existing, advanced social and cognitive skills of great apes

Humans are a member of the great ape family, along with chimpanzees, bonobos, gorillas and orang utans.  It is thought that the human lineage diverged from that of our nearest relatives, chimpanzees and bonobos, around 6 million years ago.  

Since the other great ape species all possess approximately the same skills of intelligence, it seems reasonable to assume that the last common ancestor of humans and chimps/bonobos, six million years ago, also possessed similar skills.  


Skills of physical cognition (physical intelligence) in non-human great apes

Great apes are some of the most advanced species in making and using tools.  They are flexible in this regard, able to create novel solutions to new problems, and to plan ahead by saving particular tools for future tasks.  They have a simple understanding of the physical logic of cause and effect.  

Like many animals, when getting ready to act, they can play out various courses of action and possible outcomes in their minds before choosing what seems like the best one.  But they are especially sophisticated in that they can monitor and evaluate their own thinking and decision-making: for example, taking into account what they know and don’t know about a situation.  


Skills of social cognition in non-human great apes

Again, the socio-cognitive skills of great apes are advanced compared with other species.  They can read the mental states, intentions, goals, and perceptions of others, and understand how these generate actions.  They understand that others see things, know things and work things out.  They know that others have a different perspective from their own.  These constitute a “theory of mind”, an understanding of others’ inner worlds.  They enter into these inner worlds mainly for their own ends rather than cooperatively like humans do.  

Their communication is imperative: i.e. is made up of commands, telling each other to do things: “do this”, “I want that”.  


Social life of non-human great apes: mostly competitive, a little cooperative

In forming intentions and goals, using tools, eating food, and living their lives in general, great apes act alone, individually, for their own benefit almost entirely, even though they are in a group.  However, they form friendships and alliances for competitive purposes and keep track of who in the group is affiliated to whom.  Males will get together to defend the group against threats from outside, usually marauding males from other groups.  Great apes are unique among primates in showing consoling behaviour to others: soothing another after a fight or other misfortune.  


Prosocial cooperation in humans: thinking and acting together

The hypothesis is that the eagerness of humans to engage in the mental states of others made possible the “socialisation” of the existing cognitive skills of our great ape ancestors, and so these became available to be used for cooperative purposes (e.g. foraging together) as well as competitive.  In other words, it made it possible for humans to think, and therefore act, jointly as well as individually.  


Food sharing and tolerance


Would any of you offer his son a stone when he asks for bread, or a snake when he asks for a fish?  If you, bad as you are, know how to give good things to your children, how much more will your heavenly Father give good things to those who ask Him!

Jesus – Matthew 7:9–11


Our great ape cousins are very reluctant to share their food, even with their own young, and a chimpanzee mother will only grudgingly give shells and husks to its infant in response to begging.  Young, weaned great apes are capable of foraging fruit, insects etc. for themselves.  

Humans, on the other hand, very readily share their most preferred food with their children and with friends and strangers alike.  Human children are not capable of obtaining and processing the food they eat once they are weaned (i.e. no longer exist on milk) and they require adults to do this for them.  

When early humans first started living on the African savannah, around 2 million years ago, their previous diet of largely fruit and other vegetation would have been harder to find because of the grassland environment and because of competition from ground-dwelling monkeys such as baboons.  The food that would have been available: animal carcases for scavenging, and underground tubers, would have required adults to obtain and process.  In scavenging large carcases, adults would have had to band together to scare away other carnivore species competing for the meat; and underground tubers need to be dug up and cooked.  

Sharing valued food between adults and youngsters is clearly a fundamental prosocial action that chimpanzees almost never do, that requires tolerance, sharing and giving.  It is a further form of cooperation that results from the human need to cooperate in order to obtain food.  It is a way of levelling out differences in the abundance of food between people, that results in each individual being fed more properly more of the time.  It also works as a way to motivate all those concerned, as they see that their efforts will be properly rewarded.  Cooperative breeding would likely have provided a further motivating factor to share food among the group.  

In experiments by Michael Tomasello and his team at the Max Planck Institute for Evolutionary Anthropology, it has been found that pairs of chimpanzees who are more tolerant with each other around food are also more willing to collaborate together and then share the spoils of the collaboration.  

In most carnivore species, for example, vultures or carnivorous mammals, meat is not shared equally but is allocated according to dominance: those with the best fighting ability get the most meat, and others get what they can by harassing the dominant individuals.  Humans, on the other hand, are notable for the way they share their food equally among themselves.  In the context of early humans scavenging large carcases together, the hypothesis is that people collaborated to defend the carcase from other carnivores, and that individuals who tried to hog it for themselves would have been driven away by the others.  Those who did best were those who could peacefully tolerate others feeding beside them and who did not try to dominate the food.  

Evolution of cooperative breeding in humans:  Homo erectus

There is circumstantial evidence that cooperative breeding first arose in our ancestor species, Homo erectus or Homo ergaster, in Africa around 1.8 million years ago.  

It is thought that cooperative breeding evolves in conditions where it is necessary for adults to provide a lot of help to young who could not survive without help, and where it is necessary to live together in a group.  

In the harsh savannah environment in which Homo erectus lived, adults would have had to obtain and process the food that the youngsters needed.  Surviving in this environment required people to cooperate together, i.e. in a group.  

Homo erectus was the first hominin species to migrate from Africa, and it is thought that cooperative breeding makes it much easier to colonise new habitats because it greatly increases the chances of survival of a species in unfamiliar environments, by providing a lot of extra help to mothers and children, and because it is thought to allow a greater brain size and therefore greater intelligence.  We believe also that teaching would have been a major feature of this cooperative, collective child-rearing culture, allowing knowledge to spread and accumulate.  

Female Homo erectus were larger overall and with bigger brains than in previous species, indicating that they probably had more children.  

Pair bonding, cooperative breeding, brain size, and cooperation

The brain size of Homo erectus was the first to exceed the normal range in great apes in primate evolutionary history, and cooperative breeding is thought to allow for bigger brain size because it provides greater energy inputs (food and physical helping) to the mother and growing child, and this allows the mother to have more than one infant at a time.  Otherwise the species is constrained by the “grey ceiling” where a mother only has the chance to grow a limited number of infants in one lifetime, below which the species will die out.  The result of an expensive big brain is a longer bodily growth time (leading to a longer life span).  Fossil teeth of Homo erectus in East Africa are found to have grown more slowly than in comparable great ape species.

While cooperative breeding allows for a greater brain size, it is not, in itself, an evolutionary pressure that creates a requirement for a greater brain size.  

Among monkeys and apes, brain size is related to group size:  those living in bigger groups tend to have bigger brains.  Analysis of the data points to the quality (or “bondedness”) of relationships being the driving factor selecting for bigger brains, and a big group will give more opportunity for more closely bonded relationships.  Close bonds between group members are often strategic: for the purpose of creating coalitions which can compete with other coalitions or individuals for political power.  


Polygyny OR promiscuity in particular great ape species

The reasons for the particular mating setup in a particular species – whether polygynous (one male monopolising multiple females for the purpose of mating), or promiscuous multi-male multi-female, with no permanent mating pair bonds – are still not well understood.

In “Primeval Kinship – how pair-bonding gave birth to human society”, Bernard Chapais describes the mating setups of, at least, baboons, and great apes including humans.  The common factor is that a male will try and monopolise, or guard (control for his own mating purposes, and protect from other males) as many females as he can get away with, and this number is limited only by circumstances.  From this it follows that the mating setup of a species is determined by the living circumstances of that species.  


The social arrangement in chimpanzees and bonobos is a multi-male multi-female group with promiscuous mating and no pair-bonds between parents.  When conditions are harsh, the prevailing model in primates tends to be polygamy, where one male dominates several females.  Judging from the difference in size between males and females, it is thought that the ancestors of the Homo line, the australopithicines, practiced polygamy.  


Monogamy, polygyny, AND promiscuity in humans


... all types of marital unions could be cultural variants ultimately derived from the same ancestral mating pattern.  What, then, was this stem pattern like?  

In all likelihood its essence lay in the single major feature that transcends all types of marital arrangements: their selective and relatively stable character.  Regardless of the exact nature of marital unions, men and women form enduring pair-bonds, a term that encompasses more than strictly dyadic, monogamous unions.  

... Pair bonding is, essentially and simply, the opposite of promiscuity – short-term mating relations with several partners.  It includes all types of enduring reproductive relations between particular males and females.  Pair-bonding thus refers to a specific property of mating arrangements: their duration.  


... In no human society is sexual promiscuity the sole or the main form of mating arrangement, as it is in other primate societies categorized as sexually promiscuous, such as chimpanzees and macaques.  Promiscuity consistently accompanies various forms of stable breeding bonds.  As a matter of fact, much of the sexual promiscuity in humans takes place premaritally, and the permissive character of premarital sex only serves to emphasize the social importance of marital unions.  Moreover, sexual promiscuity by married individuals is universally disapproved of.  Thus not only are stable breeding bonds consistently present across human societies, but they prevail over promiscuity.  For that matter, procreation in humans is expected to take place in the context of marital bonds.  All human societies differentiate between legitimate and illegitimate children, a distinction that further illustrates the centrality of pair-bonding in the human mating system.  Logically, therefore, the stem ancestral pattern out of which emerged all known human mating arrangements, whether polygynous, polyandrous, or even homosexual, is the enduring breeding association itself.


Compared to humans, therefore, other primates have relatively rigid mating systems.  The remarkable flexibility of human mating systems is obviously a matter of cultural variation.  We have an unusual capacity to adjust mating arrangements in relation to subsistence patterns, forms of descent, and so forth.  But however culturally variable, human mating arrangements always build upon the same core feature: the pair-bond.  


Bernard Chapais – “Primeval Kinship – how pair-bonding gave birth to human society”


Homo erectus is thought to be the first human species to practice monogamous pair-bonding in the context of raising children: long-lasting bonds between (usually) a single man and a single woman.  The reason for this assumption is that for the first time in the human family tree, male and female Homo erectus were nearly the same size, which indicates reduced sexual selection for large males, which indicates reduced competition between males. A plausible reason for this reduction in competition was the invention of stone tools, which can be used as weapons, thereby reducing the difference in fighting ability between males.  


Pair-bonds, cooperative breeding, and “expensive babies”  

The human pair-bond can be thought of as another form of cooperative breeding.  Evidence supports the ideas that


... the costs of raising human children are disproportionate in a comparative perspective, and the father does contribute to reducing these costs.  [Chapais]


Pair-bonds, the family, and reduction in human male aggression compared with other primates

It is thought that parental pair-bonding increased the cooperative motivations of early human species because males would then recognise the offspring they had helped to raise, their own brothers and sisters, and their mating partner, and would then be motivated to reduce their overall aggression.  In general, it would have strengthened family bonds both within groups and between groups (through intermarrying).  

As part of the overall logic of human interdependence, cooperative breeding could perhaps also decrease the overall aggression by males, as they were required to help look after the offspring of their friends: the adults upon whom they depended to obtain food.  


Self-domestication and the evolution of cooperation

This evolutionary process of ancient human “self-domestication” would have been a necessary foundation for the evolution of human cooperation, since this relies on a reduction in competitive aggression.