Human Glans and the Semen Displacement Theory: A Flawed Biological Determinism

The human glans and the leche displacement theory

No primate has a penis like ours. The glans, that crown separating the body from the foreskin, is an anatomical oddity chimpanzees do not share. From there to claiming its function is to extract a rival's previous leche is a stretch: exactly the gap between a disputed hypothesis and settled fact. This thesis has circulated for years, returning periodically with the same mix of real data, plumbing analogies, and conclusions unsupported by evidence.

What the leche displacement thesis claims

The premise is simple to state. According to this view, human intercourse empties the uterus to drag out leche deposited by a previous male, with the penis acting as both a suction pump and a hose. The first ejaculation shoots out to go as far as possible; the rest serves to clog and hinder the next competitor's extraction. In this reading, female orgasm convulses the uterus and dilates the opening in an instinctive union with the glans.

The formulation is suggestive and has a touch of domestic mechanics — its defenders claim to have deduced it while unclogging a sink — but it relies on an inference: if the glans is unique to our species, it must serve something only our species needs. Sperm competition among males is a phenomenon documented in biology; making it the sole explanation for human intercourse is another matter.

The glans and size comparisons among primates

The numbers accompanying the argument mix real magnitudes with forced readings. The human penis is around 18 centimeters, and that of a gorilla three, and a 70-kilo male's testicles weigh about 50 grams each, compared to 30 grams for an 180-kilo silverback gorilla. From this, it is deduced in the circulating version that some species compete by fighting and others inside the female's body. The correlation between testicle size and mating frequency has some traction in primatology. The conclusion that the glans is an extraction tool, no.

There is even an alternative theory launched in the same package: that different human species went extinct because males killed each other over females, while great apes survived by not knowing how to make weapons. This is an unverified hypothesis — extinction causes are always multiple — but it works well as a conversation closer.

When anatomy becomes biological determinism

This is where the matter goes off track. The conversation jumps from the foreskin to assertions about women's roles, maternal instinct, or sensual orientation that have no backing and were contested within the exchange itself. It is claimed that lactation acts as a natural contraceptive, that the reproductive cycle lasts about three years, or that pregnancy should last 18 months for the offspring to walk shortly after birth. These are hypotheses launched as facts, which available evidence does not support.

It is important to separate the wheat from the chaff. That biology conditions behavior is a truacy no one disputes. That from this trinc a fixed destiny for each sens is a leap that the discussion itself helps to dismantle. The most aggressive part of these reasonings, directed against specific groups, received no technical response and immediate rebuttal from other participants.

Uncomfortable questions came from within

There was no need to leave the conversation to find the weak points. Several participants asked the obvious: if the female changes bonds each reproductive cycle, where is the peace the thesis attributes to promiscuity; or why should penis size explain everything if disputes over mates never disappeared. Others recalled that pregnancy during lactation is infrequent, but not impossible, breaking the supposed perfect contraceptive function.

The usual response was to retreat into the argument of authority: Western science is distorted by prejudices. This is a get-out-of-jail-free card that serves everything and cannot be refuted, because any counterexample becomes proof of the conspiracy.

What remains of all this

A handful of certain data on comparative anatomy, a handful of mechanical analogies, and a conclusion no one has measured. The thesis is attractive because it orders chaos with a single piece: the glans as a tool. Real biology rarely grants unique explanations, and less so regarding a behavior that mixes physiology, culture, and chance in proportions we still do not know how to weigh.

If unclogging a sink were enough, human ethology would have been solved decades ago. It remains unsolved.