Brooke is a Ph.D. pre-candidate and graduate research assistant at the University of Michigan. She’s brilliant. (She didn’t put this in her bio, but trust me.)
If you’re a millennial who thinks dating in the age of Tinder is difficult, you may find parallels between your dating life and the complexities of reproduction.
The intricate process of a sperm meeting an egg to create a cell that successfully implants in the uterine wall and subsequently creates a human being is a surprisingly accurate metaphor for finding your life partner. In the world of dating, two people have to meet, decide they like each other, and then invest time and energy to grow together as a couple.
However, reproduction differs from dating in that mother nature has devised sneaky and fascinating ways to improve the chances of a successful pregnancy. Evolution wants you to pass your DNA on to as many offspring as possible and natural selection has worked for years to optimize reproduction. If only Tinder could improve your chances of getting a date that well!
To explain the tricks of the evolutionary trade, we first need to have a quick sex ed refresher. Semen contains sperm in addition to other chemicals and proteins necessary for successful fertilization of the egg and implantation of the embryo. In humans, this fertilized egg grows inside the mother’s uterus into an embryo, then a fetus, and finally an infant that poops and cries a lot. In some organisms, like fruit flies, the female lays the fertilized egg which then develops outside of the mother into a larva and ultimately a cute little fruit fly.
Fruit flies are great model organisms for studying reproduction and we have learned a lot from them. For instance, we now know that semen can influence female pregnancy in many ways. Female fruit flies have an egg laying hormone (ELH) which is responsible for stimulating egg laying for several days after mating. The semen of male fruit flies carries a chemical, Acp26Aa, that mimics ELH and further stimulates egg laying by the female for one day post-coitus. When a male fruit fly without Acp26Aa mates with a female, the female lays fewer eggs than when she mates with his Acp26Aa carrying friend. Thus, the Acp26Aa male has more offspring with the female — which is the name of the evolution game.
Another component of fruit fly semen, dubbed the sex peptide, changes female fruit flies’ behavior and bodily processes in several ways. The sex peptide was shown to decrease a female fruit fly’s receptiveness to mating with another male. This makes sense from an evolutionary point of view because males want to pass on their own DNA, not their bromate’s DNA. The sex peptide also increases appetite and reduces day-time sleep by 70% in females — if you eat more and sleep less you are more likely to successfully lay eggs.
Studies on fertility in mice and humans paint an even more intriguing picture of the role of semen in pregnancy. Maternal bodies have to make adjustments during pregnancy to prevent the embryo from being attacked by the immune system, which may recognize the embryo as a foreign object (similar to how our bodies may attack a transplanted organ). T regulatory (Treg) cells help the immune system recognize foreign versus self and are key in suppressing maternal immune attack against the embryo. In fact, Treg deficiency in mice causes implantation failure and in humans is linked to infertility and miscarriage. Semen activates these Treg cells, which primes the body for pregnancy by preparing the mother’s immune system to see the embryo as self and not foreign.
A study of cervix biopsies (ouch!) in women having had unprotected sex, protected sex, and no sex demonstrated a post-coital immune system response in the cervix specific to the unprotected sex group. Further research is necessary to prove this semen-induced immune system response is linked to better pregnancy outcomes. The evidence on the whole suggests evolution has created semen as a powerful elixir that works hard to ensure a successful outcome for the offspring.
Luckily, semen benefits mothers as well — the longer women have sexually cohabitated with their partner, the less likely they are to develop pregnancy complications such as preeclampsia. While correlation doesn’t equal causation and more research is necessary to prove that semen specifically facilitates maternal health, this trend is hard to ignore. Furthermore, this research has huge implications for clinical fertility treatment. A study demonstrated that exposure to semen, whether by a medical procedure or good ol’ fashioned intercourse, during the embryo transfer process of in vitro fertilization (IVF) increased the clinical pregnancy rate by 23%. In addition, a study performing IVF in mice found a greater rate of fetal loss and abnormality when embryos were transferred to females who were not exposed to semen via mating versus those that mated with a vasectomized male.
By now you might be thinking, “So you’re telling me that males influence females’ bodily processes with their semen?” Yes, ma’am I am, and while I would love to go all Betty Friedan and Gloria Steinem on the situation, the fact is that evolution has created these biological mechanisms because they accomplish the ultimate goal of species proliferation. Semen increases the chances of a successful pregnancy, and is beneficial to the health of both mothers and their offspring. I’d swipe right for evolution!
Shweta Ramdas, a fellow UM grad student, helped edit this column.