In female primates, sexual activity is modulated by the balance between progestins and estrogens and typically peaks around ovulation, when estrogens peak and the female is fertile. Yet, in many species, sexual activity also occurs outside the fertile phase and during pregnancy, raising questions about the hormonal mechanisms underlying non-reproductive sexual behavior. Bonobos (Pan paniscus) provide a valuable model for addressing this question, as females frequently engage in sexual interactions for social bonding, rather than reproduction. In zoo-housed populations, females often receive contraceptives with various hormonal compositions, including combined estrogen-progestin contraceptive and progestin-only formulations. This variation offers an interesting framework to investigate how ovarian hormones influence sociosexual behavior. We analyzed behavioral observations from 40 adult females across seven zoos and found that individuals receiving a progestin-only contraceptive exhibited significantly lower frequencies of sociosexual behavior compared to females receiving combined contraceptives or no contraception. Longitudinal data from 5 females further supported this pattern: sociosexual behavior substantially increased in females switching from progestin-only to combined contraception, nearly tripling their rates. Together, these findings show that the balance between estrogens and progestins modulates non-reproductive sexual activity in female primates, with important implications for understanding the impact of contraceptive use on social dynamics in managed populations.
Oxytocin plays a key role in social and reproductive behaviors, yet its involvement in the formation of odor-guided sexual motivation remains unclear. In the present study, we examined whether central administration of oxytocin, combined with exposure to estrous female-derived chemosensory cues, is sufficient to establish preference for volatile estrous female odor in sexually naive male rats. Animals received intracerebroventricular oxytocin or vehicle, followed by exposure to estrous female-soiled bedding or clean bedding. Preference for airborne estrous female versus male odor cues was subsequently assessed using a three-compartment apparatus, and sexual motivation was evaluated by non-contact erections. Oxytocin administration paired with estrous female bedding induced a significant preference for volatile estrous female odor, whereas neither vehicle paired with estrous female bedding nor oxytocin paired with clean bedding induced such a preference. Non-contact erections were observed in response to volatile estrous female odor, but not male odor, only in animals receiving oxytocin paired with estrous female bedding, although pairwise group differences in responses to estrous female odor did not reach statistical significance after correction for multiple comparisons. Moreover, the odor discrimination index, reflecting relative preference for estrous female over male odor, remained significantly higher in oxytocin-treated than in vehicle-treated animals on Day 7, indicating that the acquired odor preference remained detectable one week after treatment. These findings indicate that oxytocin does not directly establish preference but instead enables a condition in which female-derived chemosensory cues acquire behavioral significance, allowing volatile estrous female odor to subsequently elicit approach behavior and potentially contribute to sexual motivation.
Hormone-mediated maternal effects can shape offspring development in variable environmental conditions resulting in adaptive behavioral phenotypes. Numerous hormones can vary with environmental conditions and lead to different adaptive offspring responses. However, most of what we know about hormone-mediated maternal effects comes from studies on individual hormones even though the endocrine response to the environment is multifaceted and complex. Physiological stress responses in mothers may lead to greater offspring exposure to glucocorticoids or influence the allocation of other classes of maternal steroids (e.g. androgens, estrogens, progestogens) and any of these hormones may induce adaptive responses in offspring. Examining how the multivariate suite of yolk hormones responds to ecologically relevant environmental variation in natural systems is critical for assessing their role as potential cues of environmental conditions. Female western bluebirds (Sialia mexicana) in challenging environmental conditions have increased circulating glucocorticoids and produce better adapted offspring that are more aggressive and are more likely to disperse. We used this well-characterized adaptive maternal effect to test the functional importance of multiple classes of steroids using liquid chromatography–mass spectrometry (LC-MS-MS). We found that in more challenging environments – fewer nest sites, smaller territories – the multivariate yolk steroid profile shifted, with the strongest change in glucocorticoid metabolites, suggesting that maternal glucocorticoid processing varies with ecological context. We also found that estrone and etiocholanolone, but not other androgens, covaried with maternal environment. Overall, our results advance our understanding of the mechanisms underlying adaptive maternal effects in nature and highlight novel steroid pathways that may influence offspring phenotypes.
Background The biological relevance of the human vomeronasal organ (VNO) remains unresolved, and it is often considered a vestigial structure lacking sensory function. However, the VNO exhibits marked interindividual anatomical variability that may reflect underlying reproductive or neuroendocrine biology rather than chemosensory signaling. Methods We conducted a cross-sectional study of 102 adults (77 females, 25 males) recruited from a clinical setting between 2024 and 2025. High-resolution nasal endoscopy was used to assess VNO size, vascularity, and pit morphology, which were combined into a composite vomeronasal organ score (VNOS). Participants completed blinded, structured questionnaires assessing sexual behavior, sexual drive, and reproductive history. Associations between VNOS and outcomes were evaluated using Poisson, negative binomial, and ordinary least squares regression models adjusted for age and, where appropriate, total number of pregnancies. Results Among females, higher VNOS (range 0–18) was independently associated with a greater number of unsuccessful pregnancies after adjustment for age and total number of pregnancies, with each one-point increase corresponding to a 15% higher expected number (incidence rate ratio 1.15, 95% CI 1.09–1.22). VNOS was not significantly associated with total number of pregnancies. In the full sample, higher VNOS was associated with a greater number of lifetime sexual partners (incidence rate ratio 1.05, 95% CI 1.03–1.07) and higher self-reported sexual drive (incidence rate ratio 1.06, 95% CI 1.04–1.08). Findings were consistent across complementary modeling approaches. Conclusions Anatomical variation of the human vomeronasal organ is associated with sexual behavior and reproductive outcomes, particularly unsuccessful pregnancies, independent of overall fecundity. These findings support interpretation of the VNO as an anatomical biomarker of reproductive biology rather than a nonfunctional remnant.
