An important means for animals to find their mates is by smelling: they pick up whiffs of sex pheromones released by their conspecifics, which convey to them the senders' sex and reproductive status. What drive the release and detection of sex pheromones are the animals' internal physiological states, more specifically, the levels of certain neuropeptides and hormones.

Humans also give off complex scents and communicate chemically at the subconscious level. An earlier study conducted by the Human Olfaction Lab at the Institute of Psychology of the Chinese Academy of Sciences (CAS) showed that androstadienone, associated with men, subconsciously signals masculinity to straight women and gay men, whereas estratetraenol, from women, signals femininity to straight men.

Researchers there have since taken a further step to probe the neuroendocrine mechanisms underlying such human chemosensory communications of masculinity and femininity. They studied the effects of two structurally similar neuropeptides called oxytocin (known as the "cuddle hormone") and vasopressin. Both are produced by the hypothalamus, a brain area that controls the basic drives, and are heavily involved in a range of reproductive and social behaviors in animals and humans.

In a new study, Drs. CHEN Kepu, Ye Yuting, and their colleagues systematically tested 216 straight men and gay men over a total of 1056 sessions. The participants were nasally administered with different doses of oxytocin, vasopressin, or an antagonist for both oxytocin and vasopressin receptors called atosiban.

They then viewed point-light displays of human gaits and made forced-choice gender judgments under the exposure to androstadienone, estratetraenol, or a control solution that smelled indistinguishable from the androstadienone and estratetraenol solutions.

Basically, the researchers assessed how different intranasal drug manipulations, which essentially altered the levels of oxytocin and/or vasopressin in the brain, acted on the extent to which androstadienone or estratetraenol swayed one's gender perception.

Results showed that oxytocin modulates both the decoding of feminine information carried by estratetraenol in straight men and that of masculine information carried by androstadienone in gay men, in an inverted-U-shaped manner - too little or too much oxytocin would inhibit our subconscious decoding of these sexual chemosignals.

In addition, the effect of intranasal oxytocin also depends on one's social proficiency, which, according to previous research, partially manifests his/her endogenous oxytocin level. Vasopressin, on the other hand, plays no role regardless of its dose. Intranasal atosiban, the antagonist of both oxytocin and vasopressin receptors, blocks the chemical communications of masculinity and femininity.

"There is a causal link between neuroendocrine factors and subconscious chemical communications of sex-specific information in humans," said Dr. CHEN.

Fig. Oxytocin, but not vasopressin, regulates whether one can sniff out their preferred sex. (Image by YE Yuting)

The study was published in a paper titled "Oxytocin modulates human chemosensory decoding of sex in a dose-dependent manner" on Jan 13 on eLife.

It was funded by the National Natural Science Foundation of China, the Key Research Program of Frontier Sciences and the Strategic Priority Research Program of CAS, and the Beijing Municipal Science and Technology Commission.