Understanding the Trivers Willard Hypothesis Part 1

A two parter! Click here for Part 2!

Updated 10-3-22

Sometimes we get so caught up in the hows of swaying that we forget to think about the whys. But a deeper understanding of WHY swaying works is of vital importance to the success of everyone’s sway. This understanding of the mechanisms that underlie swaying can help you formulate a plan so you can work with your body and with Mother Nature, rather than against them. Plus, it can help you understand why atomic sagebrush advises the things that she does rather than slavishly sticking to the outdated, debunked, old-school sway tactics that other people swear have worked for them and may even have some pseudo-science-y gibberish behind them, and will make it easier for you to let go of things that simply don’t work in favor of concentrating on that which does.

What is the Trivers Willard Hypothesis?

For our purposes while swaying, the Trivers Willard Hypothesis hinges on the notion that animals have a very good reason, evolutionarily speaking, for being able to skew gender ratio in favor of the gender of offspring with the best odds of survival to adulthood/competing for mates/successfully reproducing, and that both behavioral and biological mechanisms have evolved (or, been designed into us by God) to ensure that this happens. For swaying purposes, we’ll set aside the behavioral aspect of Trivers Willard and focus on the biological – the stuff that is happening inside our bodies totally out of our control that causes us to conceive more of one gender than another – altho I may mention behavior along the way.

Trivers Willard is all about the genes – if a male offspring has a better chance at handing down their ancestors’ genes to the next generation, then a couple should have more boys (and be willing and able to invest in them more heavily after birth), and if a female offspring has a better chance at handing down her ancestors’ genes, then a couple should have more girls (and be willing and able to invest in them more heavily after birth).

What’s the difference between boys and girls, anyway? How could the gender of your baby matter to their future survival and their chances of handing down their genes?

Firstly, on average, boys need more calories from the moment of conception, throughout pregnancy, childhood (there ~may~ be a brief period in infancy and toddlerhood where boys do not need more calories, but the data is mixed even during that period) adolescence and into adulthood. Baby boys weigh on average 3.5 oz. more than baby girls do at birth and this difference extends into young adulthood with active older teens and young men requiring in excess of 1000 calories a day more than active females of the same age group.

Some people like to chalk this up to sexism; as in, boys eat more because parents expect them to be ‘growing boys’ or ‘more active than girls’ or whatever and also claim that moms of boys eat more during pregnancy because they know they’re pregnant with boys. The simple fact is that the average male is larger than the average female and needs more nutrients as a result. Tesearch has indicated that male fetuses actually send a signal to their mother’s body that makes them hungrier. So if you see this idea quoted as fact on the Internet, please disregard it because it’s motivated by political correctness and someone’s agenda, and not biological truth.

Even our breastmilk changes depending on whether or not we’re carrying a boy or a girl. Beyond the nutriens in our milk, many of us have found when we don’t ingest enough calories while nursing, our milk supply can suffer as a result and so since boys tend to need more calories, any period of time where we are not producing enough milk for our baby can really have an effect, boys more so than girls.

So? some may ask. So I don’t make enough milk for a couple days. What difference does it make really? My baby is not going to DIE because I don’t have quite enough milk for a day or two.

Keep in mind that the modern world in which we live, with a convenience store full on goodies on every corner, clean water, Nestle Good Start, vaccines, penicillin, and indoor heating, is something that has existed only a very short time in the grand scheme of things. Humans and their genetic ancestors have been around in some form or another for 85 million years, starting off as primates diverging from other mammals, and then becoming more and more human as the years progressed until we gradually became what we are today 50-100,000 years ago.

Most of our existence, the time during which all these funny quirky genes were developing, your baby absolutely could die if you didn’t have enough milk for a day or two. Infant mortality before the 20th century ranged from 30-50%…that’s not a typo, as many as HALF of all babies born did not survive the first year of life until only 100 years ago (and it may have been higher still in the environment where our primate ancestors dwelled.)

Aside from dehydration – still the number one killer of babies in the developing world, capable of killing within hours – and chronic starvation, even a temporary lack of food can depress the immune system and make a child less likely to withstand a virus or bacterial infection that would otherwise be harmless. As an example, rotavirus is a disease that is generally harmless to healthy children in the US, Europe, and Australia where babies have access to ample breastmilk or clean formula, but kills 450,000 children a year in the developing world.

Beyond sheer survival, a lack of nutrition in infancy can set a child up for a lifetime’s disadvantage. If one doen’t have enough food/nutrients even temporarily in childhood, there can be health effects that last a lifetime. Physical development and mental development can be stunted or delayed, the immune system is less able to fight off disease/parasites and so a child is chronically unhealthy. Human development is such a delicate thing, that a major nutritional deficit or life-threatening illness, when a child is 4 days old, 4 months old, or 4 years old can have repercussions that last a lifetime. Developmental delays don’t just go away with a couple of days of good eats. The brain and body can be forever altered by nutritional deficits or serious illness in infancy.

For males, this disadvantage grows huge as maturity approaches. In nature, primates tend to live in harems where one large, strong male is protector and several females live under his protection. Males compete physically for leadership of these harems and a weak male has very little chance of ever becoming the leader of a harem – strong young males band together and bide their time, either by taking the harem over or sneaking opportunities at willing females (sadly, sometimes unwilling females), but a weak male will never get that chance and is usually driven off by the others. If he is even able to survive (single males oftentimes don’t live very long), he’ll never get a chance to mate.

Females, on the other hand, are in perpetually high demand as mates. Even smaller, weaker, or more sickly females can typically find someone to mate with them – they don’t have to fight for the right to party (reproductively speaking). Thus, if a mama doesn’t have enough resources to guarantee a big strong son, she’s much better off having a daughter instead because a daughter has much higher chances of surviving to adulthood and successfully reproducing than a son does in that circumstance.

But atomic, humans aren’t deer, cows, or monkeys. Can this really even APPLY to us? We’ve been living in civilization for thousands of years, surely these factors no longer affect us.

Actually, there are some pretty compelling reasons to believe that humans may be the MOST susceptible to the effects of the Trivers Willard Hypothesis.

First of all, animals that have a lot of babies all at once, be it in the millions or even just 5-10 at a time (sponges, sea slugs, most amphibians/most fish/some reptiles, insects, and mammals and birds that have litters or clutches) have a lot less reason to alter the gender ratio than humans do. They can take a chance on sending out some boys and some girls and just take the gamble. It’s a numbers game. Odds are that someone will survive to reproduce even in less than ideal circumstances. Humans and most of our closest genetic ancestors have babies one or two at a time – Octomom notwithstanding – and so it is in our genes’ self-interest to stack the deck as much as possible in favor of our baby’s survival to adulthood/reproduction.

Secondly, most animals are able to raise a baby in a year’s time or even just a few months. Some animals like rabbits, breed, well, like rabbits, and can have 2-3 litters in a year. If you can have 30 kids in a year, if some are boys and some are girls and some live and some don’t, the odds are still with you that someone will manage to make it to the next generation. Whereas humans take a very long time and a huge investment in resources to raise ONE fragile little offspring to adulthood, so we have to make sure that one offspring has the best chance we can possibly provide, up to and including skewing the gender ratio to conceive the child with the best odds.

Thirdly, birth is an extremely dangerous proposition for humans. A combination of walking upright on two legs and the increase in head size that enabled us to become rapidly more intelligent than other animals, has created a situation where human birth is more dangerous for mother and child than it is for all other mammals. A baby human’s ginormous head full of clever brains, may not fit easily through the human pelvis, which altered over time to enable us to walk on two legs.

As a result, in order for us to be able give birth at all, our offspring have to come out weak, small, and helpless and require years of constant parental care before becoming independent. This severely limits the amount of offspring we can even have, especially in the case of women. If you have to expend a good 5-8 years raising a child after birth to the point at which leaving them alone for a heartbeat does not lead invariably to their demise, again, you have a lot of motivation to ensure that your effort does not go to waste on anything less than a child with the best chances of survival/reproduction.

Fourthly, humans have a funny quirk called hidden ovulation. We’re the only creatures on the face of the planet who come into “season” every month and conceive at any point and don’t KNOW we’re capable of conceiving (only very recently have we been able to tell this and even now it’s kinda dicey.) It’s very possible that we humans might get pregnant at times of year and circumstances that are not ideal, and in less than ideal circumstances, and so it is highly likely that maternal condition is MUCH more important to humans than it is to other animals.

You see, it’s fine for cows to give birth in March, they’ll have spring and summer and fall for their babies to get big and strong (not to mention their babies are born furry, cold-resistant, and able to walk already!). For humans, thanks to hidden ovulation, our bodies don’t know when we’re going to get pregnant; we can’t necessarily rely on coming into season at a favorable time of year. Our bodies have nothing to go on but our condition – and possibly a few environmental cues such as day length and ambient temperature that may affect our overall fertility. So it’s very likely that our maternal condition matters quite a lot in terms of fertility and also our offspring’s gender because that’s all our body has to go off of when judging odds of survival.

Finally, pair bonding. Unlike most other animals, humans tend to live in monogamous relationships that stay mated for some time and childrearing is done together, with both male and female being involved. Behaviorally, we are more similar to many birds than we are to some of our fellow mammals, even primates, and birds with similar behavior patterns to humans have been proven beyond a doubt capable of altering gender ratio quite significantly at times. The human male may bring more to the table (both literally and figuratively) than the average mammal/primate male who hangs around more to scare off aggressors and enjoy the favors of his harem (if he hangs around at all) than a human dad who provides a home, food, and care for his offspring. Women who are in committed monogamous relationships, especially in families headed by a male that is high in the social hierarchy, have more sons than is statistically expected.

Check out part two in the next blog post!

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