How to apply an evolutionary hypothesis about gestation to your pregnancy

To get up to speed, click on Part 1 here and Part 2 here to learn about the paper I'm writing about below...Or read it for yourself in early view here at PNAS. 

There have been some very personal reactions to the press that came with our recent paper on the evolution of human gestation length.

And I don't mean this kind:

I mean the what about my short/ long/ weird pregnancy? kind.

Result of googling "weird pregnancy"

This research has always been wrapped up in questions about human variation and even draws upon observations of human variation in gestation length. So I'm not surprised it's causing people to reflect on their own experiences. And I'm also not totally surprised because I've been on the planet long enough to know that if you claim to know anything about pregnancy, you get all the stories.

But I didn't fully anticipate how strongly our work about humans as a species would be seen as work about "me." I guess we're only human.

So today's post is for all the people who read media about our paper and are dying to know what it's got to do with their own pregnancy.

Some things first.
1. I see the world through evolution goggles. Take that as close to literal as you can.

2. I have more scholarly experience with skeletonized (dead) and fossilized (extremely dead) humans than living ones.

3. I am not trained in medicine or health sciences.
4. I will not give medical advice.
5. I do not know what doctors are, or should be, telling pregnant women about eating and exercise.
6. It took me five years to write this paper from first notes to publication and I needed the help of brilliant experts to make it as strong as it is. I do not expect to fully appreciate its implications on the week it is published--not for human evolution, not for pregnant human mothers, not yet! If you have ideas... go on with your bad self and test them! I'll try to do the same.

Here we go, then...

How to apply an evolutionary hypothesis about gestation to your pregnancy

#1 thing to think about. 

Evolution is everything about you, but it is not all about you.

When reports of our research say "moms" we're not talking about you in particular. We're talking about "moms" in a general comparative evolutionary context, species-wide, primate-wide, mammal-wide.

#2 thing to think about. 

The EGG hypothesis explains species-level phenomena

Many evolutionary papers like ours are about understanding species level phenomena and comparing differences and similarities between species to better understand those phenomena, to explain whether patterns exist and, if they do, how or why.

So using the EGG hypothesis to explain why you gestated 9 days past your due date is a little bit like this: Try using the broad ecological and biological rules and patterns that explain variation in body size across mammals to explain why Fred the elephant is 9 cm taller than Frank the elephant. That's a challenge. That's what you're attempting to do if you read our paper (or reports on it) and think of yourself first rather than your species.

Here's another way to think about it. You might have seen our paper described as finding, "Metabolism, not the hips, limits gestation."  Metabolism might get you thinking of yourself but the hips hypothesis (obstetrical dilemma; OD) never did right? I could be so so wrong but nobody thinks that there's some way the fetus can sense when its head or shoulders are about to be too big to fit through the birth canal and then initiates labor so it can escape. Nobody thinks that the mom's body can detect when the baby is about to get too big to pass through her birth canal and initiates labor so it can escape. Nobody really thinks that these sorts of mechanisms exist in mothers do they? (It's possible but I don't know of any literature suggesting this.) So the hip constraint hypothesis (OD) was never about individuals, it's about our species over evolutionary history, with hips shaping our gestation length to be the right length for babies to escape in time. Generations over deep time... that's where your brain needs to be with this EGG idea too.

Sure, we need to consider individual human variation, like yours and mine. To formulate the EGG hypothesis we drew heavily upon Ellison's (2001; and in our paper) metabolic crossover (MC) hypothesis for the timing of human birth: Babies are born when they begin to starve in utero. This happens when the needs of the fetus surpass the mother’s ability to meet them or, in other words, cross over to become larger than what the mother can provide. Labor is then triggered and carried out by a complex biochemical process. Some of the evidence he provides includes:

Gestation length can be truncated according to metabolic parameters.

• Gestation is shorter in mothers with lower body fat composition and lower metabolic rates (Klein et al., 1989; Ellison, 2001)
• Mothers living at high altitude can give birth earlier than counterparts at low altitude (Lichty et al., 1957)

Gestation and fetal growth can be increased according to metabolic parameters.

• Fetal brain size pathology à longer gestation (Higgins, 1954)
• Increase maternal caloric intake à neonatal increase (Prentice et al., 1981)
• Increase maternal caloric intake à preterm births decrease (Prentice et al., 1981)

The MC is very much about individual within-species variation and it's possible that the MC explains all individual variation in gestation length among humans, however, that's uncertain right now. Since it's specific to the biochemical pathways of humans, the details of the MC don't apply to other species with different physiologies. However, the idea that human gestation is limited by mother's metabolism--the cornerstone of the MC-- is what EGG applies to human/hominin evolutionary history and to gestation in other primates and other mammals as well, since a mother's body size (a nice proxy for metabolism) predicts fetal size and gestation length across mammals.  This is really not news to a lot of researchers considering the body of research supporting it.

It's a useful method in evolutionary biology to look at variation within a species and use it to hypothesize why variation exists between species. That is what we have done with EGG. Mother's body sizes differ between species like say, humans and orangutans, and so do their metabolic traits! EGG suggests variation in metabolism between species explains variation in gestation length. It predicts that species do not exceed their species specific metabolic ceiling during pregnancy. It will be exciting to find out whether some species give birth well before they reach their metabolic capacity!

#3 thing to think about. 

Evolution is about common ancestry and change over time. "Ideals", "optimization," "standards," "greater value in this form, lesser value in that one"... these do not exist in nature except in our minds.

You worrying that you gestated too long or too little compared to the species average is a bit like you worrying that you're shorter or taller than average, have a larger or smaller head than average, have more saliva than average, or that you can't intentionally fart. Stop worrying about your normal variation. Variation exists because it works. There's safe wiggle room around most traits and sometimes there's even full-on spasmodic dancing room. We'd be extinct if there wasn't any room for variation in how to survive and reproduce. Celebrate your weirdness, your slightly long healthy gestation, your slightly short healthy gestation, your big healthy baby, your small healthy baby, your freckles, your asymmetrical face, your hairy knuckles, your lack of wisdom teeth, your pterodactyl toes. Who cares! If life's getting on with your weird ass, then you can certainly get on with life.

Further, it helps if you don't require EGG to be all about adaptation. It could be. But it's easier to think of it as just the way it is. Mothers can only gestate so long. Period. The mechanism that initiates labor based on those metabolic cues (MC)... totally adaptive! The process the EGG explains? Not really ... a limit's a limit! How could it surpass it? It would be physiologically  impossible. Adaptive ideas aren't necessary for EGG unless it's somehow adaptive to keep the fetus inside mother right up until that threshold. Which is possible. But it could also just be the only way to trigger labor. And so we're back to the EGG being just how it is.

***

So how should you apply this evolutionary hypothesis to your pregnancy?

It sheds light on why it's difficult to give birth. It sheds light on why babies seem so helpless compared to other primates.

But regarding your specific individual details that differ compared to other human mothers and their babies?  Please talk to your doctor who's your main brain on this. And read read read read read, if you're interested.

There are some pretty cool cultural and philosophical implications of our paper. I'll save those for tomorrow's post.


References
Ellison, P. 2001. On fertile ground: A natural history of human reproduction. Cambridge: Harvard University Press. [link to book on amazon]

Higgins LG. 1954. Prolonged pregnancy (partus serotinus). Lancet 2: 1154.

Klein, J, Stein Z and M Susser, (1989) Conception to Birth: Epidemiology of Prenatal Development. New York: Oxford University Press.

Lichty JA, Ting RY, Bruns PD, and E Dyar. 1957. Studies of babies born at high altitude. Part I. Relation of altitude to birth weight. American Journal of Diseases in Childhood 93: 666-669.

Prentice AM, Whitehead RG, Roberts SB, and AA Paul. 1981. Long-term energy balance in child-bearing Gambian women. American Journal of Clinical Nutrition 34: 2790-2799.