Gestation length, tacos, and why can’t my husband gain the weight instead

Pregnancy feels like it lasts an eternity. I remember counting not just the days, but actually the hours. Like “ok, I’m four months, one week and two days, but in four more hours I’ll be four months, one week and three days, so that’s progress.” Maybe I just didn’t have enough to think about but despite work, other children, being exhausted, I could never keep my mind from cycling over and over back to the slow progress of gestating this offspring.

So, how do we compare, in gestational period, to other mammals? And why nine months? Should I celebrate my kinship with other mammals, and/or primates, or are human women exceptional in some way? I found a random webpage, of mammalian gestation times; I’ve confirmed some of the primate ones (in Harvey and Clutton-Brock, 1985) but not all the others so I’m on tentative ground here. Gestation clearly relates somewhat to species size…elephants are more than double human gestation time, so I guess that puts my bitching and moaning into perspective. According to that random webpage above, humans (266-ish) are most similar to orangutans (260) gorillas (257) and kinda close to chimpanzees (227). We’re actually really similar to bison (270, WTF) and cows (284) so that seems weird because even if I sometimes felt like a bison during pregnancy, I’m pretty sure they are, on average, larger. The real female warriors here are the elephants, the Asian elephants putting us all to shame with a gestation time of 645 days. So we are certainly not outliers among primates, and probably not necessarily amongst mammals. (I’m still interested in the finding of the similarity between us and bison. I think I’m on to something there. Maybe my investigation will be about the hidden wisdom of bison maternal instinct and what we can learn).

So we are not special in being pregnant for a long time. The fact that we share close gestation periods with chimpanzees and gorillas indicates that primates have had this 9-ish month gestation period for 5-10 million years.  So why does it feel so hard? To put it in simplistic terms, why couldn’t my husband gain some of that weight?

I have no idea what anything on this poster says but I want to see this movie.

To get down to the bottom of, why do women have the babies, and how are our bodies adapted to do so (and I won’t get anywhere near the bottom, will actually dip in the surface waters only) it’s hard to escape that initial union between two packets of genes, the maternal packet (egg) and the paternal packet (sperm). We are a biology-centric household. My husband is a physician/microbiologist, and I am a biology teacher. So we take pride in answering our kids’ nature-related questions, to the best of our abilities. For my second and third pregnancies, we enjoyed talking to my daughter, and then my son also, about the baby developing inside, how big it was, which organs it was developing this month, and so on. This was a good strategy until my oldest reached a certain age of more developed reasoning. Then she started asking, “how does the baby get into mommy’s tummy?” We talked about how there has to be a mom and a dad, and the baby started as one cell from each, and when the parents decided to have a baby those two cells joined to form the first baby cell. After a pause, she said, “Yeah, but how do the cells get together? How does the daddy cell get into the mommy?” This one so far has us stumped, so we’ve switched our tactic to changing the subject.

Nonetheless, the question of cells is an interesting one in light of parental contributions to offspring, and what we do and do not share with other primates, mammals, and animals. We also talk with our kids about “blood relatives” which is very hard for them to understand. We’ve told them that though we, their parents, are related to each of them by blood, mom and dad are not related to eachother. This stumps them, and it’s hard to understand without starting to explain DNA, those cellular instructions that are in their cells and are equally from both their parents.

And this one always brings up what, to me, is such an interesting paradox, in that their share of genes or DNA is equally my husband’s and mine; as far as we are two carriers of DNA experiencing a competitive world, our genetic legacy is equally reflected in our three children. And yet, starting with the first cells that made our offspring, our contributions are very unequal. The maternal contribution, in terms of strict energetics, is so much greater than the male contribution, a pattern that is mirrored not just in primates and mammals, but most of the animal kingdom. I remember holding each of my newborns and marveling that, aside from that first cell contributed by my husband, every single building block of every single cell, every tiny piece of lipid that made up the cell membranes, all the protein building blocks of that baby’s cells; each element, each building block of each molecule, started and was routed through my body. All those tacos I ate were somehow transformed into these beautiful babies.

This isn’t meant to be a “girls rule and boys drool” exercise. It’s just another question I’m following through: what is the contribution of the different sexes in the production/rearing of offspring, and when inequalities exist, how can that be explained in evolutionary or cultural terms?

Let’s list the fundamental inequalities. Perhaps in your high school health class you watched footage of sperm swimming up the vaginal canal, a million miniature cells traveling rapidly to reach the egg. The egg is like a sun, it dwarfs the sperm, and typically one sperm only will get through and the others are shut out (this may be the first step of competition; the genes of the sperm themselves are competing against each other.) But assuming that every biological product represents an energy cost, the larger female egg represents a greater energy investment, right from the beginning. This is certainly not unique in humans, in fact our eggs are tiny relative to our body size when compared with, say, birds. (Look at the chicken eggs we eat; those are some freaky sized eggs compared to the size of a hen, and we’ve bred them to lay them daily!)  The eggs of birds represent a huge investment on the part of the female.

How has evolution selected females to invest so much more in their reproductive cells? I think at first glance this isn’t as unequal as it looks. Females produce a large cell, but males produce many reproductive cells, all of which compete with each other to fertilize that one egg. All but one sperm are genetic losers. (Male primates will often mate without resulting fertilization, right? Hence evolutionary pressure on males protecting/fighting for fertile females) Sperm and egg seem to have been selected to follow different strategies: egg as a large, viable gamete, sperm as a “shot in the dark” strategy. So, OK, females produce a few large, viable cells, and males produce many cells, few of which will ever result in an actual offspring.

Then comes pregnancy itself, and now we’re talking placental mammals. Between the time of the two cells coming together, every bit of energy resources required for the zygote, the fertilized egg, to divide, and divide again, and so on until it  becomes a seven-ish pound baby…all that energy invested comes directly from the mother. That’s not to say that she might not be getting help in those energy needs from a partner, but the mother is directly on the hook to make that baby from nothing but the food going into her mouth.

So, how much extra food is it? And why is it that it’s so dang hard to eat during pregnancy, when it seems most important to get good nutrition? Between the nausea, the stomach constriction and the reflux, eating is difficult. For me, I’d vacillate between periods of nausea and sudden hunger so extreme that I would have eaten an actual baby if someone had wrapped it in a tortilla and slapped some hot sauce on it. I’d eat with embarrassing enthusiasm, only to get about half way through and realize that I couldn’t get any more down without it coming back up.

So how do human females do it? How do primates do it? What do chimpanzees do when they don’t have a convenient taco bar around?


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