Author: Brooke Wolford

Editors: Andrew McAllister, Molly Kozminsky, and Whit Froehlich

Image by Sierra Nishizaki

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 process of a sperm meeting an egg to create a cell that successfully implants in the uterine wall and subsequently creates a human is incredibly intricate. Similar to the world of dating, two have to meet, decide they like each other, and then invest time and energy to grow together as a couple. From finding a mate to the biological processes behind pregnancy, reproduction may seem downright impossible. Luckily mother nature has devised sneaky and fascinating ways to improve the chances of a successful pregnancy. Evolution favors those who pass their DNA on to as many offspring as possible, and natural selection has worked for years to optimize reproduction. If only Tinder were that good at getting you a date!


To explain one of the tricks of the evolutionary trade, we first need to have a quick sex ed refresher. Only 1-5% of semen is sperm, and the remainder is proteins, sugars, and chemical elements 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 emerges as an infant nine months later. In some organisms, like fruit flies, the female lays the fertilized egg which then develops outside of the mother. The fruit fly egg hatches into a larva (the fruit fly version of a caterpillar) and ultimately into a cute little fruit fly. Scientists have shown that semen influences pregnancy in female fruit flies and mammals in a variety of surprising ways.

Advantageous properties of semen in fruit flies

Fruit flies are great model organisms for studying reproduction and we have learned a lot from them. Female fruit flies have an egg-laying hormone which is responsible for stimulating egg laying for several days after mating. The semen of male fruit flies carries a chemical, Acp26Aa, that mimics the egg laying hormone and further stimulates egg laying by the female for one day after mating. When a male fruit fly without Acp26Aa mates with a female, the female lays fewer eggs than when she mates a male whose semen contains Acp26Aa. 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, decreases 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 friend’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.

Semen promotes healthy pregnancy in mammals

Moving from fruit flies to mammals, studies on fertility in mice and humans paint an even more intriguing picture of the role of semen in pregnancy. Mothers’ 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 cervical biopsies in women who had unprotected sex, protected sex, and no sex found a post-coital immune system response only in the unprotected sex group. Further research is necessary to determine whether 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 that a pregnancy results from copulation.

Research demonstrating semen’s effects on mammalian pregnancy has implications for clinical fertility treatment. A study demonstrated that exposure to semen during embryo transfer in in vitro fertilization (IVF), whether by a medical procedure or good ol’ fashioned intercourse, 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. Luckily, semen benefits mothers as well—the longer women have a sexual relationship with their partner, the less likely they are to develop pregnancy complications. This suggests that semen may specifically facilitate maternal health, but further research is needed to better understand this process.

While the process of creating human life is as complex as finding a mate, at least reproduction has semen on its side. In addition to delivering sperm to an egg, semen can change female fruit fly behavior and prevent the mammalian immune system from rejecting an embryo. These effects on the reproductive process have evolved over time 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!

About the author:

BC8U4701_croppedBrooke is a PhD student in the Department of Computational Medicine and Bioinformatics (DCMB). She studies the genetics of complex diseases such as type 2 diabetes and cardiovascular disease under the mentorship of Drs. Cristen Willer and Mike Boehnke. Originally from North Carolina, Brooke holds a Bachelor of Science in Quantitative Biology from the University of North Carolina at Chapel Hill. When not debugging her own code, you can find her teaching local high schoolers to code with DCMB’s Girls Who Code club. She also enjoys reading, running, and a good NETFLIX binge. Follow her on Twitter (@bnwolford) or find her on LinkedIn.

Read all posts by Brooke here.

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