Part 4. Humans are (Blank) -ogamous: Promiscuity & Physiology

This is the fourth part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the introduction here.



We left off with a list of eight traits in humans suggesting promiscuity in humans. Admittedly, the previous post was a little thick, as it dealt with imprinted genes and population genetics. The current one concerns human reproductive physiological and anatomical traits consistent with a multiple-partner mating structure, building on a couple of points addressed by Ryan and Jethá in their book. If you’re paying attention, that’s three posts concerning promiscuity and one (yet-to-be-written) on pair-bonding. Perhaps it seems I’m stacking the deck, but please reserve judgment. One reason more space is needed to make the case for the evolution of promiscuity is that the biology is less well known, and more effort is needed to bring it into the light. That single post on pair-bonding will be an important one, and quality matters just as much as quantity.

Continuing on with our list of traits hinting at promiscuity…

9. Sexual dimorphism in body size. This point remains somewhat contentious. In the majority of anthropoid species (monkeys, apes, and humans), males are the larger sex, with the degree of dimorphism ranging from slight to extreme (Plavcan 2001). This pattern correlates strongly with mating structure and male-male competition (Plavcan and van Schaik 1997). For monogamous species like gibbons, males and females tend to be roughly the same size. In species where females prefer larger males or where males compete for access to females, bigger males will leave behind more descendants. This is true for polygynous gorillas and dispersed, territorial orangutans, where males are physically about twice as large as females. A good non-primate example is elephant seals. On the other hand are horseshoe crabs, where smaller males cling to the backs of larger females and wait for the release of her eggs. This ‘reverse dimorphism’ is found in a few primates, but is slight and only in some prosimians such as lorises and lemurs.

Former sumo wrestler Konishiki and his wife Chie Iijima, an obviously cherry-picked example of extreme dimorphism. (From

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Part 2. Humans are (Blank) -ogamous: Promiscuity

This is the second part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the Introduction here.


I’ll be frank. True monogamy is rare. So rare that it is one of the most deviant behaviors in biology.” (Olivia Judson 2002: 153)


In their best-selling book, Sex at Dawn, Chris Ryan and Cacilda Jethá suggest that there is a good amount of direct and circumstantial evidence that extended monogamy does not come easily for humans, and that this derives at least in part from our fairly promiscuous evolutionary history. (To clarify, they use the term ‘promiscuous’ not in a judgmental way, but merely to convey having multiple sex partners). Their main premise is that rigid monogamy became common only after our ancestors made the shift from hunting and gathering to agriculture. With agriculture came an emphasis on fixed settlements, private property that could be inherited, genetic paternity, and female sexual fidelity. They argue that this stands in contrast to our hunting-gathering past, when sexual relationships were more open and not confined to an exclusive pair-bond.

Bonobo sex. From Lola ya Bonobo

Bonobo sex. From Lola ya Bonobo


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On Fatherhood: Proud Primate Papas

For Father’s Day, Scientific American compiled a series on the biology of fatherhood, including a list of 8 species where males are integral in raising offspring. Included were birds (rheas, emperor penguins), mammals (marmosets, red foxes, wolverines), fish (catfish, sea horses), and even insects (giant water bugs). For some of these species, male parental investment extends to carrying fertilized eggs until they hatch. In others, it entails postnatal protection and/or procurement of food for young offspring.

Two things stand out. First, although the list didn’t claim to be comprehensive, at just 8 examples, it seemed quite short (they couldn’t make it to ten?). In addition, there’s just something peculiar about highlighting species that exhibit good fathering skills to begin with. Consider how odd it would be to encounter a list of species where mothers care for offspring. In mammals, it seems almost tautological that parenting is associated with motherhood since they alone can gestate and lactate. However, there may be rare exceptions to this, such as lactation in male fruit bats (Kunz and Hosken, 2009), and possibly in Robert De Niro, though this issue remains unresolved. Are species with active fathers the exception to the rule? What good are males, then?

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On Optimism and ‘Human Nature’

In the last few days, I came across a couple of unrelated quotations on human nature and our internal tug-of-war between cooperation and conflict.

A 20 year-old Charles Darwin in an 1830 letter to his cousin, W.D. Fox:

It is quite curious, when thrown into contact with any set of men, how much they continue improving in ones good opinion, as one gets ackquainted (sic) with them. This was an argument used, in a religious point of view, by a very clever Clergyman in Shrews. to encourage sociability (he himself being very fond of society), for he said that the good always preponderates over the bad in every persons character, & he thought, the most social men were generally the most benevolent, & had the best opinion of human nature. I have heard my father mention this as a remarkably good observation, & I quite agree with him.

In “East of Eden,” John Steinbeck wrote:

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The Evolution of Childcare (& Washington, D.C.)

Our family took a road-trip to Washington D.C. this summer. After looking at the photos, I realized the subtitle of our stay could have been: how I carried my toddler son everywhere… in humid, 95 degree weather.

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The Sin of Certainty

Screen capture from wikileaks video, Iraq

Lane Wallace wrote a recent article in The Atlantic about bias in journalism, and how it influences the questions that one asks. As journalists are human beings, I think we can reasonably assume that they, like everyone else, have biases. What resonated with me was Wallace’s use of the phrase ‘the sin of certainty,’ coined by the science writer Jonah Lehrer. I think this phrase has great utility and should humble us when we consider how much we don’t know. As an example, chemists have just observed for the first time a new yet-to-be-named element (#117). It took until the late 1990s for physicists to recognize that perhaps up to 95% of the universe is actually comprised of dark matter and dark energy. These are far from trivial matters, yet they have eluded us until now.

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The Power of Love

Infant rhesus monkey with terrycloth ‘mother’

As social animals, we need to be around others. Virtually everything we do is social – trade, eating meals, watching sports in stadiums or movies in theaters, religious services, education, the internet, etc. Even war is a social activity. No human being on the planet is completely self-sufficient. Being social is more than utilitarian, however; it is also biologically and psychologically necessary. For example, one of the most severe forms of punishment in prisons is solitary confinement.

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