What Humans Can Learn from Empowered Females Primates That Keep Families Together – ryan

Heated disagreements about sexuality and gender are front and center in today’s culture wars. Evolutionary biologist Nathan H. Lents argues in his new book, The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships (Mariner Books) that we need only look to the animal world to see that there’s no one correct way to approach our sexual relationships. In fact, sexual diversity helps animal species thrive. In this excerpt from his book, Lents discusses two different mating styles that each contribute to strong parental bonds and shed light on the origins of the human family.

In just two generations, the sexual landscape has completely changed throughout most of the developed world, and so it is no surprise that many people find all of this terribly unsettling. What is surprising to me, however, is how little the biology and natural history of sex has factored into the public conversation. I assert that this moment of sexual turmoil is actually a rediscovery of the much more expansive relationship with sex that our ancestors once had and that other animals enjoy today.

In the modern world, much of how we express our sexuality, and how we form our sexual relationships, stems from cultural constructions, not innate biological wiring. Even a cursory glimpse of the sex lives of other animals demolishes any notion that sexual activity is narrowly purposed toward procreation. Biologists have discovered an ever-expanding list of reasons that animals have sex with each other. Animals use sex for bonding, social cohesion and alliance building. They use sex deceptively, competitively and financially. They even have sex for the same reason that we most often do it: just for the fun of it.

Moreover, in no other species but humans is sex rigidly restricted by broad constructions such as heterosexuality and sexual monogamy, nor was it so restricted for most of the history of our own species. As humans explore this supposedly new sexual territory, we can look to our animal cousins for guidance on how to manage this minefield of social upheaval.

For generations, anthropologists have argued whether humans are evolved for monogamy or some other mating system, such as polygyny, polyandry or promiscuity. But any exploration of monogamy must begin by separating it into two completely different concepts: social monogamy and sexual monogamy. Sexual monogamy is just what it sounds like: the restriction of sexual intercourse to within a bonded pair. Social monogamy, aka economic monogamydescribes the bonding itself, a strong, neurohormone-driven attachment between two adults that facilitates food and territory sharing, to the exclusion of others, for at least one breeding season, and generally purposed toward raising offspring. To help us understand the human experience of monogamy, it is helpful to look at how monogamy works in some other primates.

Going Nuclear

Coppery titi monkeys (Plecturochebus) are often considered the primate with a family structure closest to our own. One male and one female form a group with their children, a nuclear family, which can occasionally incorporate a third generation as well. It was long known that these monkeys form strong heterosexual pair-bonds and mate for life, but in 2020 it was discovered that these monkeys have the distinction of being the eighth mammal species discovered to be sexually exclusive, meaning they are both socially and sexually monogamous. (That’s eight out of 5,000 species, or just 0.15 percent.)

Males and females of this species look identical and behave very similarly. They partner very well, sharing as much of the child-rearing duties as possible. While females do the nursing, the fathers do most of the work carrying the children around until they can walk. They also work together to procure food and defend territory and family. The attachment between mated titis is very intense, and partners become distressed in each other’s absence. Both sexes are extremely jealous. In fact, immediately after it was discovered that they are sexually monogamous, they became the focus of research on the neurobiology of jealousy, since monogamy and jealousy go hand in hand.

The same study that demonstrated sexual monogamy also revealed a key reason why sexual monogamy is tenable for titis: mated pairs are not close relatives, which means this species already has a system in place to prevent inbreeding, the big danger that promiscuity is aimed to prevent. In titis, both males and females instinctively leave their home range when they mature into adulthood. This is called natal dispersaland with both sexes doing it, and the titi population being large enough, it appears to be sufficient to ensure a genetically robust population—another example of how both sexes are wired for the same behaviors in these monkeys.

Males and females on equal footing appears to be a key ingredient in the recipe for sexual monogamy. Of the eight sexually monogamous mammals, three are primates. In all three, the males and females are physically indistinguishable, behave similarly and equally contribute to parental care. This is not common. In most primates, females and males have different markings or different body sizes. Gorilla males are almost twice as large as females, for example. And behaviors are usually sexually dimorphic as well, given the realities of pregnancy, nursing and the long developmental period of primate younglings. The fact that the three sexually monogamous primates also have physically indistinguishable sexes can hardly be a coincidence. There are many primate species whose males and females look identical but that are not sexually monogamous, so this is clearly not enough, but it may be a necessary starting point for sexual monogamy to emerge. And this makes sense because monogamy is a potent equalizer between the sexes.

It Takes a Village

At the other extreme of sexual exclusivity are the Barbary macaques (Macaca Sylvanus), which, despite forming tight-knit social bonds, are totally promiscuous sexually. There is no shortage of fascinating facts about these monkeys. They are from the oldest branch of macaques, are the only macaque species found outside of Asia and are the only nonhuman primate to live north of the Sahara Desert. These resourceful midsize monkeys also have the unique distinction of being the only primate to have successfully colonized Europe in the recent past: stowaways on ships from Morocco to Gibraltar have established a stable population of more than 300 individuals.

But Barbary macaques are also unique among primates for their system of distributed alloparenting—when groups of animals contribute communally to the care of offspring, regardless of parentage. Both males and females contribute to the raising of all of the offspring in the troop. Male Barbary macaques are constantly carrying infants around, grooming and playing with them and helping them eat once they are weaned.

Barbary macaque troops are also matriarchal, with dominance determined by relatedness to the top-ranking female. Males form coalitions, or friendships, and are often invited into social interactions by one male handing another male an infant to care for! Although they engage in sex more often during estrus (“in heat”), females enjoy sex throughout the year, with both males and females. The males are also bisexual and promiscuous in this very sexual species, but heterosexual sexual access (mate choice) is female-driven and mostly follows the dominance hierarchy, meaning that females prefer males of high social status. Because of all the sex going on, paternity is a complete mystery. And the mothers only weakly favor their own genetic offspring.

Male and female Barbary macaque monkeys cooperate to equally parent their young despite being sexually promiscuous.

Want/Getty

The communal parenting of Barbary macaques is truly impressive, and it is also unexpected. Among mammals, whenever paternity is obscured, we generally see less paternal investment, not more. In the mind of a mammal, if they don’t know which child is theirs, they usually don’t bother investing in any. But in these macaques, it seems to be the opposite. Since any of the children could be theirs, they invest in them all. And what’s more, the communal parenting seems to be the social glue that holds the group together. Could this be a result of the matriarchal structure? It’s possible that it could be a contributing or necessary factor, but it’s definitely not enough, since there are many matriarchal primate species—including the bonobos and over 100 species of lemur—and only the Barbaries are this special.

While there are no mated pair-bonds in Barbary macaques, they do show strong social bonding and attachments. In fact, social partners, particularly males, can form short-lived dyads (groups of two) while caring for youngsters together, like little temporary foster families.

Due to their egalitarian nature, highly invested fatherhood and matriarchal structure, we might be tempted to expect little or no sexual dimorphism in Barbary macaques, like in the titi monkeys, but we would be wrong. This is because sexes that are indistinguishable seems to correlate with monogamy, and the Barbary macaques are the opposite of monogamous. They don’t form stable dyads, and everyone has sex with everyone else. In terms of dimorphism, males are about 15 percent larger than females, and females have genital swellings that make them easy to spot. They behave differently also, forming sex-segregated coalitions and a female-led dominance hierarchy for grooming and food sharing. Females care for the youngest infants, but then males take over most of the parenting duties as the infants wean and become more physically capable.

Happy Wife, Happy Life

Although the titi monkeys, with their total lack of sexual dimorphism and strong sexual and social monogamy, seem like the opposite end of the spectrum from Barbary macaques, which are sexually dimorphic and show no monogamy whatsoever, these two monkeys actually have two things in common that put them at odds with most other primates. First, they both show high paternal investment in offspring. And second, in both titis and Barbaries, females exhibit a high degree of autonomy and are the executors of mate choice. In many mammals, males are dominant, aggressive, sexually controlling and coercive, while females have little choice but to endure it. In these two species, however, the females are fully empowered.

At the risk of romanticizing, both coppery titis and Barbary macaques have both gender equality and social groups that live in relative harmony. With titis, the groups are strictly nuclear families, while in Barbaries, they are large blended families, but in both, the members work together cooperatively and mostly free of sexual conflict. This shows that there are many paths to a peaceful existence, and both monogamy and promiscuity can get you there. The common factor, at least in these two primates, is the empowerment of females, which we also see in some societies of the strangest primate of all, humans. Whether it’s coppery titis, Barbary macaques or human beings, when females are in charge, harmony and equity are the natural results. Something to ponder.

Book jacket of “The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships” by Nathan H. Lents.

Mariner Books