In a previous post, ‘Why 2 sexes?’, I explained a hypothesis for why the sexes first diverged (the original asymmetry was that the females pass on the mitochondria to their offspring whereas the males don’t). In this post I want to hypothesise and speculate1 in more detail about how this initial asymmetry could naturally lead to the (statistical) differences between male and female behaviours and roles with which we are familiar with today, in the context of the species’ mating system.
Firstly, it is not such a leap from the initial difference to see that it is natural in most cases for the female to host the foetus as it develops and correspondingly for the male to inject into the female (since the female sex cells providing the mitochondria and everything else for the development of the foetus, except half of the genes, would naturally require them to be larger and less mobile).
At this point let’s abstract away most of the details of female and male, except for this characteristic of the female as the ‘hoster’ and male as the ‘injector’ and see what we can derive from this alone2.
The first observation that comes to mind, is that the number of offspring a female can have, as the ‘hoster’, is limited, whereas the number of offspring a male can have is potentially unlimited. Of course not every male can have unlimited offspring, because the total offspring of all males is constrained by being equal to the total offspring of all females, which is fixed.
So the picture we have is that each female has an approximately constant number of offspring, but the number of offspring each male has can be variable and is only subject to the constraint that the sum, over all males, of the number of offspring is fixed. We see already that males and females are potentially very different types of player in natural selection’s game.
There are two extremes of the way this could play out. On the one hand you could have complete monogamy where each male is paired with exactly one female and both have an equal fixed number of offspring. The other extreme is that one male fathers the whole of the next generation, and all other males end up childless. A species can fall anywhere between these extremes, on what I like to call the ‘Mating Spectrum’3. We humans are nearer the monogamous end, although we are quite flexible and have exhibited behaviour nearer the other end with things like Harems. Species on the other extreme, are called ‘tournament’ species where mating is like a tournament for the males. These include ‘lekking’ species where males gather together in competitive displays, such as in some birds of paradise. Also towards this end are species with polygynous mating systems, such as Gorillas, where mating groups consist of one dominant male and several females; and some of our closest primate relatives – the Chimps and Bonobos – with promiscuous mating systems where there are mixed mating groups and higher paternal skew due to higher female sexual choice.4
What is it that determines where a species falls on this spectrum5? To work it out, let’s start by considering the game from the perspective of the female player6. Remember that in natural selection’s game, the aim is to optimise your genes’ long-term survival value, which in turn involves optimising the quantity and quality of your offspring. Since for a female the quantity of offspring is approximately fixed, a female strategy predominantly revolves around trying to optimise the quality of their offspring. And the impact on quality of offspring from the choice of male mate(s) is made up of two factors – quality of genes and quality of paternal investment. And I think this is the crux – the importance of the quality of paternal investment is not fixed across species (or within a species across varying ecological/sociopolitical climates), and I think this variation is exactly what determines the variation along the spectrum. To be explicit, paternal investment having zero value corresponds to the tournament end of the spectrum7, and the larger the value of paternal investment relative to gene quality the closer to the monogamous end the species tends to.
It is interesting to note that this would mean complete pure monogamy is unattainable for any species, as this end corresponds to an infinite ratio of the value of paternal investment to gene quality in female’s choice of mate, which is never the case because gene quality is always a significant factor. And this is indeed the case; there is no species which is completely sexually monogamous8. There are many species which on the surface look monogamous like us; for example many species of bird pair for life, but when you inspect the nests of female birds of these species you can find that actually 40% of the eggs have DNA not corresponding to the primary partner9. It is not enough to cite male promiscuity as the only reason monogamy is unattainable because believe it or not, every time a male has sex with a female, a female has sex with a male10. If the females had no reason to cheat outside their primary partners then no cheating would happen, even if the males would seek it. There has to be a reason for females to want to cheat; for males it is for quantity, and females cheat for gene quality. So really essentially all outwardly appearing monogamous species, are monogamous with a not insignificant amount of infidelity11.
The fact that infidelity is inevitable, combines nicely with a second observation of our abstraction of females as ‘hosters’ and males as ‘injectors’, which is that males as ‘injectors’ can not be sure the offspring are theirs, unlike females who can. In a world where females had no reason to cheat this wouldn’t be a problem, but since they do, there is always an element of paternal uncertainty. This explains why males tend to invest less in their offspring than females do, as it involves an element of risk; A male risks raising a child which is not theirs if their female partner cheats, which is a significantly higher cost as compared to the cost of a male partner cheating (which is often just a slight diversion of paternal resources, assuming they don’t leave). Hence why female infidelity has traditionally been seen as worse, and relates to why promiscuity is generally found less attractive in women than men (when choosing a life partner).
An important caveat/refinement to what I said before, is that high value of paternal investment does not always lead to complete monogamy (with adultery), as that assumes all males are able to provide equally well. If for some reason there is large variability in how well males are able to provide, this could mean some males are able to cross the polygyny threshold, and support multiple wives. An example of this is uneven distribution of wealth, which has particularly been the case for humans since the agricultural revolution when it became possible to accumulate wealth; In extreme cases of wealth singularities this has sometimes manifested in harems. From a more abstract perspective you could also look at the paternal protection found in chimpanzees where there is a high risk of infanticide from other male chimpanzees, as a form of paternal investment (it’s more abstract because ironically it is induced by there not being a first order need for paternal investment in the first place). It is variable among males because male chimpanzees form alliances and hierarchies, that give males in a stronger alliance the ability to provide better protection. In contrast Bonobos, while similar to chimpanzees in that the mating is promiscuous, avoid all problems of infanticide and other male violence, and I think the reason is that Bonobo communities are small enough that everyone can have sex with everyone which together with concealed ovulation means it is harder to infer paternity. So in summary, paternal reproductive skew is not always due to the importance of gene quality taking over the importance of paternal investment; it can also be caused by variation in the quality of paternal investment. The difference between the two is that in the case of variable paternal investment the males hold the power and jostle for it (patriarchy) but where gene quality is the dominant factor the females hold the power through sexual choice (matriarchy12). Also the paternal skew is generally more concentrated in the latter case of gene quality being the dominant factor, as explained in footnote 7.
In the case however where paternal investment is important but not variable, such as with direct paternal care of offspring, monogamy is the natural arrangement. This is the case more so for humans than our closest primate relatives, because human babies are born at a more premature and vulnerable stage of development, which came about to support our larger brains13. So this could explain why humans have evolved a more natural predisposition to monogamy (for at least long enough to raise a child) than our closest primate relatives. However humans are incredibly flexible in what mating arrangements we biologically support14, and while I think we are naturally predisposed to monogamy (and minority polygyny), I imagine that pre agricultural revolution, when human communities were nomadic, human sexual arrangements could have been more flexible and not always based around marriage (whether monogamous or polygynous marriage), including arrangements closer to the promiscuous arrangements found in our closest primate relatives. But other than our flexibility, what is particularly unique about humans is that we are able to think abstractly about mating systems and design and agree on them as a community15, and with written word pass this down through many generations. Human societies that have attached a religious significance to monogamy may have been able to create more equal, economically productive, scalable and stable societies that were stronger against enemies16, and this may be why such religions have been so successful, even though the ideals are sometimes in tension with our less simple biology.
- A lot of my thinking around the subject of this post was originally inspired by the book ‘The Red Queen’, and I have since also drawn ideas from a number of sources on the internet. My focus in this post is on presenting a unified and consistent framework in which to understand these ideas and draw logical connections, and less on ensuring everything said is correct. ↩
- This model doesn’t apply to all species’ females and males but it certainly applies to all mammals, and to a lesser extent species where the female lays eggs. ↩
- To be precise, the parameter that defines this spectrum is paternal reproductive skew. (A good observable indicator of a species’ position on the spectrum is it’s degree of sexual dimorphism as this correlates closely). ↩
- I’ve lumped Chimps and Bonobos together here because they are both examples of promiscuous mating systems but they’re actually quite different, as described further on in the post. The Bonobos are interesting to me because before I came across Bonobos, I used to think the tournament side of the spectrum was always characterised by higher aggression among males, but the Bonobos provide an interesting counter-example of a species with high paternal reproductive skew and yet low male aggression within a community. ↩
- …at a particular moment in time, as remember human mating strategy is flexible so may adapt according to the current environment. ↩
- The male player is more complicated as the optimal male strategy depends largely on the position of the spectrum which is what we are trying to work out so that would send you into an unhelpful reasoning loop. ↩
- The reason this inevitably leads to a very small proportion of males doing all the mating (even though gene quality is not always distributed in such a singular way) is because there is a feedback loop at play here called sexual selection – a male’s attractiveness is by definition how attractive females find him, so if there is even the slightest tendency within a population to find a certain male attractive that will be amplified by the fact that, a male is more attractive the more females that find him attractive. Which leads to a kind of herd behaviour of all females choosing the same male. ↩
- …which is not to say that there aren’t individuals or couples which are monogamous and exclusive for life, just that no species is purely monogamous as a whole. ↩
- This has been found in a species of bird called the Indigo Bunting, according to Matt Ridley’s book ‘The Red Queen’ in chapter 7. ↩
- … but it does make sense to say males tend to be more opportunistic when it comes to sex. Which explains why homosexual male-male couples statistically tend to be more promiscuous than homosexual female-female couples. ↩
- As Matt Ridley puts it in ‘The Red Queen’, “We are designed for a system of monogamy plagued by adultery”. ↩
- I think matriarchy is actually the wrong word here, as the dominant factor can be female sexual choice without there being any social structure or hierarchy among the females; indeed hierarchy makes less sense among females as males are not inherently a divided resource. Some species do have some social structure among females together with significant female sexual choice, and can be classified as matriarchies, one example being the Bonobos. ↩
- This is similar to the way a lot of birds are monogamous (90%) probably due to the fact that a lot of the development happens outside of the womb, and could explain why conversely the rate of monogamy in mammals, where animals are born more developed, is low (3-5%). ↩
- Not necessarily in the same individuals – it is likely some individuals are more predisposed to certain arrangements than others, but all ‘alleles’ have been maintained among the human population as a whole. ↩
- This is not necessarily by conscious intelligent design – cultural myths can be selected by natural selection too. ↩
- Also less vulnerable to the spread of sexually transmitted disease in large societies. ↩
RSS - Posts