A Surprising Stowaway in Semen

The “nerve” of ejaculate & induced ovulation.

Posted Aug 28, 2012

To quote a friend of mine who studies clinical psychology, the finding I am about to write about is one of those, “wow, we don’t know #$&% about #$&%” moments in science.

Ready? Here’s the money (or perhaps the money shot): A new research report in PNAS with a very clever title describes a surprise ingredient in the seminal fluid of all mammals examined thus far (including humans!). This ingredient enters the female body in male ejaculate and influences the release of an egg from an ovary.

Yes, you read that right: seminal fluid, increasingly recognized for its numerous functions beyond a simple transport device for spermatazoa, contains a hormone that can cause ovulation. This molecule, originally dubbed Ovulation Inducing Factor, turns out to be chemically identical to Nerve Growth Factor, a hormone that plays a key role in the mammalian ovulatory cycle.

Ovulation Inducing Factor acts when it enters a female’s system in male ejaculate and is absorbed into the bloodstream, hijacking the the hypothalamo-pituitary-gonadal axis (i.e. the body’s coordinated hormonal regulation system) and causing a hormonal chain reaction. This chain reaction is essentially the same one triggered every month in human females by Nerve Growth Factor, an identical molecule that binds to receptors on the ovaries, causing the release of Follicle Stimulating Hormone and resulting in the cyclical ripening and release of an egg from the ovary.

The interesting thing is, biologists have long known that there was something in the seminal fluid of certain mammals that caused ovulation—they knew this because in fact, there is a whole host of mammals that only ovulate after semen has been released during intercourse (e.g. the llamas studied in Ratto et al.1), but it turns out that the same induced ovulation effect occurred in cattle, a species of “spontaneous ovulators”. (Spontaneously ovulating species, like humans, release an egg in a cyclical process, regardless of whether intercourse occurs.) In other words, the “mystery” compound in semen that causes ovulation is not a mystery ingredient after all—it’s the exact same molecule—Nerve Growth Factor—that sparks ovulation endogenously.

baby llama

Look ma, I'm induced!

It’s also worth mentioning that this finding, should it turn out to be true for humans as well as ruminant mammals, may have major implications for the science of sexual selection and the evolutionary biology of human mating. Maybe it’s just me, but “ovulation-inducing seminal fluid” rings of multi-male competition (very much in line with the evolutionary biology of the human phallus), and thus sounds like a male reproductive strategy that would have evolved in a promiscuous species, rather than a monogamous pair-bonding species.  

I also wonder about the intricacies of the female body’s response to seminal Nerve Growth Factor in “spontaneous ovulating” species (like humans). Although I won’t go into the details of differential investment theory here, many mammals possess biological and psychological characteristics that highlight the competing reproductive strategies between males and females (for example, female humans have concealed ovulation, but male humans appear to be able to detect when women are ovulating anyway). It seems to me that if male-delivered Nerve Growth Factor were a major player in human fertility, it would be a lot easier for humans to conceive.

And last, but not least, I am looking forward to research investigating how this contributes to recent ideas in what I’m going to call “semen psychology”, which has been extensively (and very entertainingly) documented by Jesse Bering. Given the mood and preference shifts that accompany hormonal influences over the human female fertility cycle, perhaps an external dose of Nerve Growth Factor has unique effects on the female brain.

In other news, if I were Ratto et al., I would still be cracking puns about publishing a semen paper in PNAS