In my previous blog I introduced how the differences between Homo sapiens and other Homo species are described as ‘self-domestication’, with typical phenomena such as shorter ‘snout’, more bulbous forehead, decreasing brain size … Self-domestication is linked to selection against ‘reactive’ (or impulsive) aggression, typically among males. This reduction of aggression is necessary to make collaboration possible, so this self-domestication idea is an obvious candidate to explain our natural tendency to collaborate.
I investigated in my blog how Homo sapiens and wolves started to work together, resulting in the domestication of wolves into dogs. This powerful coalition was probably instrumental in Homo sapiens out-competing Homo neanderthalensis. In other words, our own species was able to collaborate with wolves and in doing so became the most successful hominid. At least, that is what I make of it.
But the story about the wolves does not explain how it all started, how Homo sapiens did become the most collaborative human species. We must look elsewhere for that.
What is the problem with male mammals?
The word mammals says it all, really. Nearly all mammals gestate their offspring until they are liveborn. After birth, mammals keep on feeding their offspring, giving them milk, sometimes for years on end, protecting them and teaching how to take care of themselves in a dangerous and difficult world.
Female mammals, that is. Females gestate and lactate. Males can contribute to rearing their offspring, but females carry the brunt of the investment in offspring. By doing so, they make themselves a very desirable resource for males. Males do not invest in offspring; they invest in females that take care of offspring. The males compete, fight with other males to win access to females.
There is worse. If a male lion takes over a pride of lionesses by chasing the previous male, he will kill the cubs sired by the other male. Makes evolutionary sense, too: killing the young lions will stop lactation and females will be ready to mate again faster and have offspring sired by the new male. Infanticide of this kind also occurs in some monkey species, relevant, as we humans are primates. Infanticide is a clear example of the tension between the two sexes, created by the asymmetric investment by females in their offspring.
Creating a collaborating group of male and female mammals is quite a challenge, evolutionary speaking. It did not happen all that often…
Male dominance is the common ‘solution’ in mammals. One male wins the fight for dominance, becomes the ‘alfa male’; and has priority or exclusive access to the females in the group. During the ‘reign’ of this male there is relative peace and no infanticide, as the offspring is his and he will protect them. The male may also chase other males away, so that more food is available for his ‘harem’. Lots of aggression and trouble, periodically, when power fights happen, but stability and quiet in between.
Male dominance is the cornerstone of chimpanzee society, close family of mankind.
When I still was a biology student, we all went to visit Frans de Waal’s research on a group of chimpanzees in Burger’s Zoo. He gave a lively and loud impersonation of a male chimpanzee throwing a tantrum in the Zoo’s restaurant that remains a fond memory. Plenty of male dominance among chimpanzees and Frans de Waal gained fame in his book on chimpanzee politics. It describes the complex social interactions that go into maintaining that dominance, such as setting up alliances and political games.
He later realized that the dominance was tempered by reconciliation gestures that helped to keep the group together. Male chimpanzees do need to collaborate to some extent during hunting and warfare with neighboring groups. But dominance is still the name of the game.
Our other cousins, the bonobos
Sometime after our visit to Burger Zoo, the University of Antwerp I was linked with decided to replicate de Waal’s research on bonobos, in Planckendael, part of Antwerp Zoo. I think everybody expected to find a similar male dominance and lifestyle as de Waal found among chimpanzees.
Not so. Bonobos have a completely different solution to the thorny issue of females as attractive resources males fight for. The solution: female dominance and sex.
Male bonobos do not generally fight and do not brutalize females. If they occasionally dare to use force, the females intervene as a group and severely punish the guilty male. There is some degree of male dominance, but males derive their power from the social status of their mother, not by violence.
And sex. Tension that arises in the group are resolved by sex. Any kind of combination, really. Females with females to strengthen their bond. Males with males, or males and females. Interestingly, bonobos do face to face copulation, as humans do but very few other animals. Sex as a tool to support interpersonal relations.
Free sex has other advantages, not just tension releasing. Plenty of sex makes the females a less scarce resource. No need to fight with competitors, no reason to try rape. Infanticide is genetically riskier as well: it can be your own child.
And what is most interesting: bonobos show signs of self-domestication. They were known as ‘dwarf chimpanzees’, but their bodies are quite similar in size to regular chimpanzees. They were called ‘dwarf’ because their skulls are smaller than those of chimpanzees. Smaller heads are typical for (self-)domestication. The difference between males and females is smaller and canine sizes, key factor in aggression, are more similar between the sexes than in chimpanzees.
This ‘self-domestication’ is not related to bonobos living close to humans. They live deep in the Congo tropical forest, south of the Congo river, not very close to humans. Self-domestication, the reduction of (male) aggression, is not related to the advantages of living closer to humans. It looks much more like ‘domestication’ of the males by the females, diminishing the advantages of aggression and stimulation collaboration instead.
The mechanism? Female collaboration and the use of sex as a bonding mechanism.
How about humans?
Remember, we are talking about self-domestication of Homo sapiens, as this is how we are different from other Homo species.
An overview of the different hypotheses is given by Richard Wrangham in his paper ‘Hypotheses for the Evolution of Reduced Reactive Aggression in the Context of Human Self-Domestication’. Sounds like the paper I was looking for, right, by a professor at Harvard too.
A major disappointment, really. His favorite hypothesis is that male aggression was selected against by males conspiring to kill males that were too dominant (based on the evolution of speech that allowed such conspiration). That sure happened in history - the murder of Julius Caesar comes to mind - but as a mechanism to breed gentler and more collaborative males, gang murder has some obvious down-sides.
It is not exactly like bonobos either, I think. The ‘make love not war’ attitude and living in promiscuous groups sounds great but does not quite resonate for humans and did not work out well in the late sixties either. The idea is more: finding a mate, building a home, and getting a couple of children.
Watching a 16-year-old girl separated from her boyfriend because of a COVID lockdown brings down the home truth: we are programmed to fall heavily in love, with massive doses of oxytocin rewarding us. And the pronounced detox pains we feel when lovesick.
Monogamy has advantages for the females, as the incentive for infanticide by males disappears and as the partner has an incentive to share the burden of child-rearing. Males still compete for females, but this competition largely stops once the pair has bonded. Sex rewards us with oxytocin and serves to sustain the bonding of the pair.
The differences in sexuality between chimpanzees, bonobos and humans are striking. One example: estrus, the cyclical period when women and females are fertile. In female chimpanzees, estrus is very evident, with pronounced, highly visible sexual swelling. Bonobo females have a longer period of estrus, a pseudo-estrus: they show a sexual swelling for a much longer period than they are fertile.
Dominant male chimpanzees need to stay with a female in estrus to make sure that her offspring are his and needs to chase away the other males. This is far more difficult for bonobo males, as the females are in estrus longer and a male cannot guard all females in the group. Sounds like an adaptation to help females maintain their dominance…
And among humans? Women do have estrus. The infamous ‘thermometer method’ of contraception is based on the slight temperature rise related to ovulation and fertility. The complexity and general unreliability of the method shows one thing, though: many women do not even know when they are fertile themselves, and men have even less information. Guarding females and chasing other men away during their fertility is just not feasible in humans.
So there we are: love, monogamy and a bond reinforced by enjoyable sex are the driving forces behind self-domestication. I think.
Let us face it guys, women domesticated us
Before you go to the pub to drink away this depressing thought with your mates, though, think of this. You would not have any mates but for the reduction of male – male competition. There would be no pubs either.
The reward of domestication is male – male collaboration instead of competition. Male chimpanzees hunt together, but it is all in all a clumsy activity. Human males have become far more effective social hunters. They can also share the meat with their partners and their offspring, so that the collaboration improves their evolutionary success. Females can specialize in gathering edible plants and collaboration in childcare; males exploit a different ecological niche, becoming effective hunters. Together they can provide for the metabolically expensive brains of their offspring and the long training period needed. Human tribal life is born.
What does it finally tell us?
One important lesson is that we have evolved as humans without hierarchy. Alfa males and harems, though common among social mammals, are not part of who we are, as biological species. I believe that this is important: we have evolved to be a relatively egalitarian species, with males and females each other’s equal. We fall in love, have enjoyable sex and are rewarded by massive doses of oxytocin that creates strong pair bonds. Collaborating males contribute to child rearing. Females collaborate as well.
Oxytocin is not only there when we make love, though. Trust sets oxytocin free. Good teamwork and meaningful work liberate oxytocin. But that is for the next blog.
We have evolved to become the collaborating primate. We recognized the opportunity for collaboration with wolves, another collaborating species. Creating an egalitarian society, with equal status for men and women.
Until we invented agriculture and the notion of property and hierarchy came into society again.