In evolutionary biology, there’s a framework that seeks to explain, in part, the “women do all the work” stereotype. In mammals (animals that have hair and produce milk for their young, like us), females “pay” the energetic costs of gestation (growing the baby) and lactation (making milk for the baby). There’s just no biological way around it: if that female is going to pass any genes along into the next generation, the baby is going to need her womb and her milk to survive. The consequence of this, biologists explain, is that females invest more time and energy into each offspring, even when it is outside the womb, than males do.
This makes sense if you think about it in cold, economic terms (and maybe start by thinking about it in mammals that aren’t humans — we’ll get to us later). In the natural world, the whole name of the game is to survive long enough to have as many offspring as possible. This is referred to as your genetic fitness. As a female mammal, doomed by biology to invest time and energy into growing an embryo into a fetus and then feeding that baby milk once it emerges into the world, you better pick a good mate to provide the other half of that offspring’s DNA, because you’re about to invest a lot of time and energy into making sure it grows up well.
On the other hand, for mammal males, there are little to no energetic costs associated with growing that embryo into a baby (as I said, humans are different — more on that later). If ‘winning’ in evolutionary terms means having as many offspring in the next generation as possible, it makes the most evolutionary sense for males to not spend any time on one baby, but rather try and make as many of them as they can.
These sex-specific strategies — quantity for males, quality for females — exist because the behaviors behind them have a genetic component, and copies of the genes driving those behaviors get passed along to the next generation. So, if Female A’s strategy is to have as many babies as fast as possible and invest very little in each, she will pass those behavioral genes on to each of those children; let’s imagine this strategy leads to 5 surviving offspring. If Female B’s strategy is to invest time and energy into each offspring, those genes will be passed on to hers; this might result in 10 surviving offspring. That means that there are twice as many individuals with the “invest time and energy” gene in the next generation, and the behavior will become more and more common. Natural selection at work!
Now, humans do something different, for the most part. Across societies and cultures, men typically plan a critical role in providing for and raising their offspring, investing lots of time and energy into them even (and especially) when they are out of the womb and growing. We will talk more in a later post about why male humans have a different evolutionary strategy than other mammals, but for now, I want to focus on that much earlier time, when the baby isn’t a baby yet, but rather is just barely an embryo. Even if I have a male partner who is going to help once this baby is a human in the outside world, as the one doing the gestating I can't help but wonder: am I really doing it all in those first 9 months?
Actually, not at all. It turns out that sperm does far more than just transport half of baby’s DNA to the other half waiting in the egg. To begin, sperm and the DNA that it carries is essential for just getting the whole process of growing a baby started. Studies with roots in male infertility treatment have shown that sperm with ‘broken’ DNA can inhibit fertilization, even if they meet an egg at the right moment or stop a fertilized egg from successfully implanting or developing (1).
Second, one the egg is fertilized and implanted, sperm bring critical components for taking that zygote into the embryo stage. They do this in two ways. First, sperm are the source of a critical cellular organ (called an organelle) called the centrosome. Centrosomes are the directors of cell division, making sure that each ‘daughter’ cell resulting from a dividing ‘parent’ cell has the right number of chromosomes within it. Eggs have non-functional centrosomes. The one that directs all the cellular divisions as your baby goes from one cell to billions? It’s from dad (2). Second, sperm are also responsible for triggering something called oocyte activation, which is exactly what it sounds like: fertilized eggs are great, but unless something entices those cells to divide and multiply, it’s never going to grow into a baby. That trigger for cell division comes from the sperm. In fact, it only takes about 4 hours in between when sperm meets egg and when DNA synthesis begins!
But what about later, when I’m bloated and huge and feeling like I could eat the whole world? Is that sperm still helping, or is he just there to kick-start this whole adventure? It turns out, when we look back at the suppressed immune response that is stopping my body from kicking the parasite-baby out, the sperm actually has quite a bit to do with that, too.
Remember those regulatory t-cells, the ones that are peacekeepers between fetus and mom? Research from the University of Adelaide has demonstrated that increased exposure to seminal fluids from your partner actually encourage these regulatory cells to multiply. In other words, the longer your body has been exposed to your partner’s semen before you make that baby, the more regulatory peacekeeping t-cells your body has to protect the growing fetus because seminal fluid encourages the production of these critical peacekeeping cells (4).
Part of this is similar to the effect of exposure on mild allergies. Have you ever known someone allergic to cats, but after they live in a house with one for a few months, the allergies decrease or disappear? Exposure is key to modulating and soothing our body’s immune responses, and the same seems to be true of the maternal immune response to baby. The more familiar the female’s body is with signals that come from the male, the better off the baby will be (5). In IVF patients, couples who have intercourse regularly during treatment have greater success rates, with a higher proportion of embryos viable at 6–8 weeks that those that abstained (6).
But of course, with great power comes great responsibility. All those recommendations that mom needs to cut back on alcohol, caffeine, smoking, and drugs before and during pregnancy, eat a healthy diet and keep a healthy weight, reduce stress and avoid infections? Well, turns out the non-gestating partner isn't totally off the hook. Study after study shows links between paternal health and offspring health. You can read about the recommendations everywhere from WebMD (7) to popular news outlets (8, 9) to medical journals (10), but the punchline is clear: two healthy parents before and during pregnancy really does make a difference.
References:
Loutradi, K. E., Tarlatzis, B. C., Goulis, D. G., Zepiridis, L., Pagou, T., Chatziioannou, E., … & Bontis, I. (2006). The effects of sperm quality on embryo development after intracytoplasmic sperm injection. Journal of assisted reproduction and genetics, 23(2), 69–74.
Sathananthan, A. H. (1997). Mitosis in the human embryo: the vital role of the sperm centrosome (centriole). Histology and histopathology, 12(3), 827–856.
Stice, S. L., & Robl, J. M. (1990). Activation of mammalian oocytes by a factor obtained from rabbit sperm. Molecular reproduction and development, 25(3), 272–280.
Bromfield, J. J., Rizo, J. A., & Ibrahim, L. A. (2018). Paternal priming of maternal tissues to optimise pregnancy success. Reproduction, Fertility and Development, 30(1), 50–55.
Robertson, S. A., Prins, J. R., Sharkey, D. J., & Moldenhauer, L. M. (2013). Seminal fluid and the generation of regulatory T cells for embryo implantation. American journal of reproductive immunology, 69(4), 315–330.
Tremellen, K. P., Valbuena, D., Landeras, J., Ballesteros, A., Martinez, J., Mendoza, S., … & Simón, C. (2000). The effect of intercourse on pregnancy rates during assisted human reproduction. Human Reproduction, 15(12), 2653–2658.
https://www.webmd.com/men/news/20160517/dad-role-baby-health#1
https://www.cnn.com/2018/04/16/health/dad-health-baby-preconception-study/index.html
Day, J., Savani, S., Krempley, B. D., Nguyen, M., & Kitlinska, J. B. (2016). Influence of paternal preconception exposures on their offspring: through epigenetics to phenotype. American journal of stem cells, 5(1), 11.
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