The problem is that in general, subordinate males cannot defend females from dominant males, and besides, females tend to prefer mating with dominant males for the genetic benefits, producing stronger, healthier offspring.
So what changed all of this was probably several transitions happening together around the same time. By about 2.5 million years ago, we had started to incorporate more meat into our diet. We know this from various lines of evidence, including — this is cool — stone tool cut marks on animal bones dated to 2.5 million years ago. That’s cool. I love this stuff!
And then by about 2 million years ago, brain size really started to increase, and with that came a lengthening of the juvenile period, so now kids became both really costly and costly for a long period of time. And this made male provisioning both possible and necessary.
Possible because it’s much easier to bring back calories, protein, fats, in the form of meat than trying to do that by transporting plant foods, and necessary because kids became so energetically costly that individual females would have had trouble providing resources for themselves and their offspring.
And when we look at modern hunter-gatherer societies, that’s what we see. These are aggregate data across several hunter-gatherer societies on net daily calories.
Are you bringing in more calories than you consume, or are you consuming more than you bring in? And the green bars are net daily caloric surplus, in other words, bringing in more than you consume. And the red bars are a deficit, so you’re consuming more than you bring in.
And you’ll notice that men from about 20 years of age to 60 years of age are operating at a daily caloric surplus. They bring in, generally through hunting, more calories than they can consume, and these calories are distributed. If it’s large game, it’s generally distributed equally to everybody in the village or camp.
Smaller items can be brought back to individual family, but this contrasts with what’s going on with women, in their reproductive years, they’re operating at a daily caloric deficit. Gestation, lactation, carrying babies, are extremely costly energetically and limit one’s ability to forage efficiently.
So male provisioning, both possible through hunting, and necessary. And this change had profound impacts on human mating and reproduction. In a sense, it tipped the balance for females.
So now it was sometimes worth mating with a subordinate male, even if he may not possess the best genes, if he provided resources. And this is baboon pornography. I probably should have warned you there’d be monkey porn. This is from PlayBaboon Magazine.
All right, I’m going to stop with the jokes. This is a female baboon in estrus, so her genitals are swollen, and this happens in a lot of primate species. Females’ appearance changes over the cycle, and becomes more attractive and this incites male competition for females during the fertile part of the cycle, with dominant males tending to monopolize copulations, closer to ovulation.
Well, we don’t look like this. And you knew that. But what you might not know is that women’s attractiveness does change over the cycle. My lab, and others, have shown that women’s faces, voices, even odors, are more attractive to men during the fertile part of the cycle.
But these changes are extremely subtle. And compared to other primates, the evidence indicates that we’ve evolved to suppress cues to ovulation. That in a sense, ovulation is concealed in humans.
But think about what impact this would have. This would mean that dominant males would not be able to monopolize copulations near ovulation. It would protect the pair bond from invasion by a dominant male. So that a male in a pair would have more confidence that he was the father of the offspring.
The couple is having sex throughout the cycle. And this is unique to human mating, we don’t see it in many other primates. We have sex throughout the cycle. And so this would essentially increase a male’s confidence in paternity, because a dominant or some other male wouldn’t be able to target the female and bully their way in at the fertile point in the cycle.
And this would have important implications for parental investment, in particular, males providing resources for their offspring. Because across species, when males provide resources for offspring, they target those resources toward their own biological offspring, and they avoid investing in the offspring of unrelated males.
And so the evolution of male care for offspring and investing in resources and offspring, pair bonding, and concealed ovulation, went very much hand-in-hand over our evolution.
We have also evolved a specialized psychology for forming long-term romantic relationships with the possibility of investing in offspring together. We fall in love. All around the world, people prefer mates who are kind and generous and capable and willing to care for mates and offspring.
In one of the largest cross-cultural studies of human mate preferences ever conducted, covering 33 countries shown in red here, the single most important mate choice criterion to both men and women, was mutual love and attraction.
But as you also know, people are not always perfectly faithful to their mates. And in particular, women sometimes face a tradeoff between good genes and investment. Women sometimes find themselves in relationships with men who may be caring providers, but may not possess the best quality genes for offspring making them strong and healthy.
And several features of women’s mating psychology seem to have evolved, in part, to resolve this trade off. And I mean, recruiting genes, if you will, from outside of the long-term relationship. For example, women have more sexual fantasies about men other than their long-term partner, during the fertile part of the cycle, and that’s particularly true if the long-term partner has physical signs of being lower in genetic quality, like he’s less physically attractive. I think that’s interesting.