Scientists asked men smell hundreds of different vulvar odors to test the “leaky-cue hypothesis”

A new study published in Evolution and Human Behavior suggests that modern women may not chemically signal fertility through vulvar body odor, a trait commonly observed in other primates. The findings indicate that men are unable to detect when a woman is in the fertile phase of her menstrual cycle based solely on the scent of the vulvar region. This research challenges the idea that humans have retained these specific evolutionary mating signals.

In the animal kingdom, particularly among non-human primates like lemurs, baboons, and chimpanzees, females often broadcast their reproductive status to males. This is frequently done through olfactory signals, specifically odors from the genital region, which change chemically during the fertile window. These scents serve as information for males, helping them identify when a female is capable of conceiving. Because humans share a deep evolutionary history with these primates, scientists have debated whether modern women retain these chemical signals.

A concept known as the “leaky-cue hypothesis” proposes that women might unintentionally emit subtle physiological signs of fertility. While previous research has investigated potential signals in armpit odor, voice pitch, or facial attractiveness, results have been inconsistent.

The specific scent of the vulvar region has remained largely unexplored using modern, rigorous methods, despite its biological potential as a source of chemical communication. To address this gap, a team led by Madita Zetzsche from the Behavioural Ecology Research Group at Leipzig University and the Max Planck Institute for Evolutionary Anthropology conducted a detailed investigation.

The researchers recruited 28 women to serve as odor donors. These participants were between the ages of 20 and 30, did not use hormonal contraception, and had regular menstrual cycles. To ensure the accuracy of the fertility data, the team did not rely on simple calendar counting. Instead, they used high-sensitivity urinary tests to detect luteinizing hormone and analyzed saliva samples to measure levels of estradiol and progesterone. This allowed the scientists to pinpoint the exact day of ovulation for each participant.

To prevent external factors from altering body odor, the donors adhered to a strict lifestyle protocol. They followed a vegetarian or vegan diet and avoided foods with strong scents, such as garlic, onion, and asparagus, as well as alcohol and tobacco. The women provided samples at ten specific points during their menstrual cycle. These points were clustered around the fertile window to capture any rapid changes in odor that might occur just before or during ovulation.

The study consisted of two distinct parts: a chemical analysis and a perceptual test. For the chemical analysis, the researchers collected 146 vulvar odor samples from a subset of 16 women. They used a specialized portable pump to draw air from the vulvar region into stainless steel tubes containing polymers designed to trap volatile compounds. These are the lightweight chemical molecules that evaporate into the air and create scent.

The team analyzed these samples using gas chromatography–mass spectrometry. This is a laboratory technique that separates a mixture into its individual chemical components and identifies them. The researchers looked for changes in the chemical profile that corresponded to the women’s conception risk and hormone levels. They specifically sought to determine if the abundance of certain chemical compounds rose or fell in a pattern that tracked the menstrual cycle.

The chemical analysis revealed no consistent evidence that the overall scent profile changed in a way that would allow fertility to be tracked across the menstrual cycle. While some specific statistical models suggested a potential link between the risk of conception and levels of certain substances—such as an increase in acetic acid and a decrease in a urea-related compound—these findings were not stable. When the researchers ran robustness checks, such as excluding samples from donors who had slightly violated dietary rules, the associations disappeared. The researchers concluded that there is likely a low retention of chemical fertility cues in the vulvar odor of modern women.

In the second part of the study, 139 men participated as odor raters. To collect the scent for this experiment, the female participants wore cotton pads in their underwear overnight for approximately 12 hours. These pads were then frozen to preserve the scent and later presented to the male participants in glass vials. The men, who were unaware of the women’s fertility status, sniffed the samples and rated them on three dimensions: attractiveness, pleasantness, and intensity.

The perceptual results aligned with the chemical findings. The statistical analysis showed that the men’s ratings were not influenced by the women’s fertility status. The men did not find the odor of women in their fertile window to be more attractive or pleasant than the odor collected during non-fertile days. Neither the risk of conception nor the levels of reproductive hormones predicted how the men perceived the scents.

These null results were consistent even when the researchers looked at the data in different ways, such as examining specific hormone levels or the temporal distance to ovulation. The study implies that if humans ever possessed the ability to signal fertility through vulvar scent, this trait has likely diminished significantly over evolutionary time.

The researchers suggest several reasons for why these cues might have been lost or suppressed in humans. Unlike most primates that walk on four legs, humans walk upright. This bipedalism moves the genital region away from the nose of other individuals, potentially reducing the role of genital odor in social communication. Additionally, human cultural practices, such as wearing clothing and maintaining high levels of hygiene, may have further obscured any remaining chemical signals.

It is also possible that social odors in humans have shifted to other parts of the body, such as the armpits, although evidence for axillary fertility cues remains mixed. The researchers noted that while they found no evidence of fertility signaling in this context, it remains possible that such cues require more intimate contact or sexual arousal to be detected, conditions that were not replicated in the laboratory.

Additionally, the strict dietary and behavioral controls, while necessary for scientific rigor, might not reflect real-world conditions where diet varies. The sample size for the chemical analysis was also relatively small, which can make it difficult to detect very subtle effects.

Future research could investigate whether these cues exist in more naturalistic settings or investigate the role of the vaginal microbiome, which differs significantly between humans and non-human primates. The high levels of Lactobacillus bacteria in humans create a more acidic environment, which might alter the chemical volatility of potential fertility signals.

The study, “Understanding olfactory fertility cues in humans: chemical analysis of women’s vulvar odour and perceptual detection of these cues by men,” was authored by Madita Zetzsche, Marlen Kücklich, Brigitte M. Weiß, Julia Stern, Andrea C. Marcillo Lara, Claudia Birkemeyer, Lars Penke, and Anja Widdig.