Testosterone therapy masculinizes speech and gender presentation in transgender men

Testosterone therapy masculinizes speech and gender presentation in transgender men. Carolyn R. Hodges-Simeon, Graham P. O. Grail, Graham Albert, Matti D. Groll, Cara E. Stepp, Justin M. Carré & Steven A. Arnocky . Scientific Reports volume 11, Article number: 3494 (2021). https://www.nature.com/articles/s41598-021-82134-2

Abstract: Voice is one of the most noticeably dimorphic traits in humans and plays a central role in gender presentation. Transgender males seeking to align internal identity and external gender expression frequently undergo testosterone (T) therapy to masculinize their voices and other traits. We aimed to determine the importance of changes in vocal masculinity for transgender men and to determine the effectiveness of T therapy at masculinizing three speech parameters: fundamental frequency (i.e., pitch) mean and variation (fo and fo-SD) and estimated vocal tract length (VTL) derived from formant frequencies. Thirty transgender men aged 20 to 40 rated their satisfaction with traits prior to and after T therapy and contributed speech samples and salivary T. Similar-aged cisgender men and women contributed speech samples for comparison. We show that transmen viewed voice change as critical to transition success compared to other masculine traits. However, T therapy may not be sufficient to fully masculinize speech: while fo and fo-SD were largely indistinguishable from cismen, VTL was intermediate between cismen and ciswomen. fo was correlated with salivary T, and VTL associated with T therapy duration. This argues for additional approaches, such as behavior therapy and/or longer duration of hormone therapy, to improve speech transition.

Discussion

Voice masculinization is particularly important to transgender individuals undergoing a female-to-male transition; compared with eight other masculinity traits, participants indicated that they were least satisfied with their voice prior to transition and ranked it highest in priority for seeing change. Further, after T therapy (which was effective at masculinizing fo and fo -SD in our participants), transmen were most satisfied with their vocal masculinization compared with other traits. Given its importance in a female-to-male transition and the growing number of individuals undertaking this treatment46,47, the need for evidence-based research on voice masculinization is high.

Our results show that, on average, T therapy is effective at masculinizing fo and fo-SD. Transmen’s fo values (mean and range) are comparable to those of cismen and statistically significantly lower than ciswomen’s fo values. While we do not have recordings of these men prior to T therapy, we can assume that their fo was close to the average for ciswomen and that their fo has since changed by 3.5 standard deviations (or more28), which is nearly 80 Hz. Research suggests that 50% of listeners can detect shifts as low as 1.2 semitones (e.g., 7 Hz for a 100 Hz voice42); therefore, these changes likely have a strong impact on perception of gender. Overall, the current findings are consistent with previous results documenting substantial changes in fo with T therapy in transmen32,33,34,35,36 and are suggestive of putative anatomical changes resulting from the action of T on the lengthening and thickening of vocal folds, similar to those occurring during puberty in natal males19,20,21,22,23,24. To understand the nature of these structural changes, future studies should use imaging techniques to objectively quantify vocal fold length and thickness at regular intervals during T therapy.

T therapy may not be sufficient for achieving formant frequencies that are indistinguishable from cismen. Our results showed that transmen’s estimated VTL was significantly longer than ciswomen but shorter than cismen. 23% of our participants’ VTL fell outside the range of our cismen sample, suggesting that T therapy alone does not fully masculinize larynx position. Despite research indicating that both fo and VTL contribute to gendered voice perception13,14,15,16, only one other published study on transmen’s voice changes has examined VTL or formants43. This motivates development of additional treatments, such as behavioral therapy, to increase objective speech masculinity by increasing vocal tract length48. Previous studies on transmen’s speech changes have shown that most changes have occurred prior to 9 months of continuous T therapy32,33,34,35,49; however, these studies did not examine changes in estimated VTL. This study is the first, to our knowledge, to demonstrate statistical differences in VTL between samples of transmen and cisgender speakers [see Cler et al.43 for a single, detailed case study and Papp44 for an unpublished dissertation].

Incomplete masculinization of VTL (as well as fo–SD) may partly explain why 17% of our participants reported that they were ‘neutral’ to ‘extremely dissatisfied’ with changes in their vocal masculinity. This accords with previous studies showing 12–16% of patients are not fully satisfied with their vocal transition and 25% were still sometimes perceived as female on the phone32,37. Further, 31% expressed interest in further masculinizing their speech through additional treatments like behavioral voice therapy32. Despite the need for behavioral voice therapy among transmen, only one published study has examined its efficacy48. This is in contrast to transfeminine individuals where it has been shown to help individuals express their gender identity through speech, reduce gender dysphoria, and improve mental health and quality of life50,51,52,53.

Some of the acoustic properties of speech that drive gender perception were associated with features of T therapy. We found a significant inverse association between salivary T and fo; however, the results appear largely driven by 3 data points (see Fig. 3). Given the small sample size, it is unclear whether these individuals represent the normal range of variation. The association between current salivary T and fo makes theoretical sense given the longer-term association between T administration and fo change in transmen, the presence of androgen receptors on the vocal folds54,55, and the associations between T and fo during puberty18,19,20,21,22,23. In addition, several studies have found links between salivary T and masculine vocal parameters in cismen25,26,28 (cf. Arnocky et al.29), and one study of cismen showed within-individual diurnal decreases in salivary T were associated with increases in fo27. Given the strong empirical and theoretical support for an association between T and fo, it is surprising that two previous studies on female-to-male speech changes35,36 did not find an association between serum T levels and fo.

Although there was not a significant association between salivary T and estimated VTL, T therapy duration was statistically significantly associated with VTL: longer T therapy durations were associated with longer estimated VTLs. This finding may suggest a longer-term relationship between T therapy and VTL. However, an alternative explanation is that this association reflects the confounding effect of time since transition given its close association with duration of T therapy. That is, even without formal voice training, transmen may be implicitly learning how to manipulate their vocal tract over time to achieve longer VTLs. Clinical studies on the relationships among dosing regimen, biological T availability, and speech parameters among transmen are necessary.

In summary, we see two important implications of these findings. First, a voice with a low pitch is a central aspect of masculine gender presentation because it is easily observable, highly sexually dimorphic, and difficult to approximate if not an adult male. Vocal fold dimorphism is one of the largest anatomical sex differences observed in humans (approximately 5 standard deviations28,56) and greater than any other extant ape57. Cisgender men and women differ by 60% in vocal fold length30 but only 8% in height58. Because vocal sexual dimorphism is extensive and, importantly, features little overlap in gender-typical vocal ranges, it is extremely difficult to speak in a voice consistent with the opposite sex, particularly in a sustained fashion31. These facts help explain why transmen are so dissatisfied with their fo prior to T therapy. Similarly, our participants were also highly dissatisfied with body fat distribution, which is also very dimorphic58,59, easily observable, and difficult to change without hormonal therapy.

A second implication of these findings is that more research on speech changes in transgender males is necessary. The studies that have been published are limited by small sample sizes32,33,34,49, a lack of a control group for comparisons32,33,35, and a focus on only fo34,49. Additional research on T dosing regimens as well as the efficacy of behavioral voice therapy are particularly necessary. Better evidence-based treatments for transmen have health and safety repercussions. Transgender individuals are disproportionately targets of violence and being viewed as one’s gender is likely a critical component for safety60,61,62,63. Approximately 20–47% of transgender individuals have been physically or sexually assaulted and an additional 34–46% have been verbally threatened or harassed62,64.

In contrast to voice masculinization, participants did not place high importance on seeing an effect of T therapy on the non-physical trait “psychological masculinity”, highlighting participants’ dissociation between their own perception of gender and outward display of gender prior to therapy65. This incongruence is a source of extreme distress, which is associated with higher levels of depression, anxiety, substance abuse, and suicidal ideation and attempts among the transgender population—particularly those that have not begun to transition5,7,9,65,66,67,68. Receiving hormone treatment significantly improves mental health, social health, and physical health outcomes in transgender populations2,7,9,66. Vocal congruence contributes to these improvements; Watt et al.2 showed that more masculine voices significantly contributed to improved well-being and mental health in female-to-male transgender patients.

To summarize, this research was designed with several goals in mind. First, we aimed to quantify the importance of voice change—relative to other masculine traits—for transgender men undergoing testosterone therapy. No previous studies have explored this question, in spite of the strong interest in voice change among the transmasculine population. Our results show that voice masculinization is of central importance to transgender individuals undergoing the female-to-male transition compared with eight other masculine traits. Second, we asked whether T therapy was effective at masculinizing three gendered speech parameters. Our results show that, on average, T therapy is effective at masculinizing fundamental frequency mean and variation (fo and fo-SD); however, transmen’s formant-based measure of vocal tract length (VTL) was significantly shorter than cismen. This study is the first, to our knowledge, to demonstrate statistical differences in VTL between samples of transmen and cisgender speakers. Third, we examined the association between salivary testosterone and vocal parameters. We found a significant inverse association between salivary T and fundamental frequency but no association with VTL. T therapy duration, however, was statistically significantly associated with VTL. These findings point to the need for more research on speech changes in transgender males—of particular importance are transition strategies that affect formant frequencies, which have largely been ignored in previous research.

Neural Correlates of Mating System Diversity: Oxytocin and Vasopressin Receptor Distributions in Monogamous and Non-Monogamous Eulemur

Nicholas M. Grebe, Annika Sharma, Sara M. Freeman, Michelle C. Palumbo, Heather B. Patisaul, Karen L. Bales & Christine M. Drea. "Neural Correlates of Mating System Diversity: Oxytocin and Vasopressin Receptor Distributions in Monogamous and Non-Monogamous Eulemur." Scientific Reports, February 12, 2021. DOI: 10.1038/s41598-021-83342-6

Abstract: Contemporary theory that emphasizes the roles of oxytocin and vasopressin in mammalian sociality has been shaped by seminal vole research that revealed interspecific variation in neuroendocrine circuitry by mating system. However, substantial challenges exist in interpreting and translating these rodent findings to other mammalian groups, including humans, making research on nonhuman primates crucial. Both monogamous and non-monogamous species exist within Eulemur, a genus of strepsirrhine primate, offering a rare opportunity to broaden a comparative perspective on oxytocin and vasopressin neurocircuitry with increased evolutionary relevance to humans. We performed oxytocin and arginine vasopressin 1a receptor autoradiography on 12 Eulemur brains from seven closely related species to (1) characterize receptor distributions across the genus, and (2) examine differences between monogamous and non-monogamous species in regions part of putative “pair-bonding circuits”. We find some binding patterns across Eulemur reminiscent of olfactory-guided rodents, but others congruent with more visually oriented anthropoids, consistent with lemurs occupying an ‘intermediary’ evolutionary niche between haplorhine primates and other mammalian groups. We find little evidence of a “pair-bonding circuit” in Eulemur akin to those proposed in previous rodent or primate research. Mapping neuropeptide receptors in these nontraditional species questions existing assumptions and informs proposed evolutionary explanations about the biological bases of monogamy.

Discussion

As the first study to investigate neuropeptide receptor distribution in strepsirrhine primates, we document binding patterns of both oxytocin and vasopressin in members of the Eulemur clade that fall between those of classic rodent models (e.g.18,19) and those of more recently characterized haplorhine primates46,47,57. This intermediacy may have functional implications for lemurs’ evolutionary specializations, potentially reflecting the comparatively variable role of these neuropeptides in sensory ecology (e.g.13,58). As the first primate study to directly compare neuropeptide receptor binding between brain specimens from monogamous and non-monogamous species of the same genus, our findings also fill a critical gap in knowledge of how variation in neuroanatomy reflects variation in primate mating systems or sociality. Beyond simply representing another data point in the domain of comparative neurology, findings from our study of Eulemur question the universality of classic vole models and suggest a revisitation of their implications for humans.

Like rodents, lemurs show olfactory specialization59, which is prominently displayed in their use of scent to convey a wide array of reproductive and social information60,61. Some degree of similarity in the involvement of OXTRs in processing chemically encoded socio-reproductive information in these taxa is suggested by the diffuse binding of both OXTR and AVPR1a in the olfactory bulbs and olfactory tubercle, and by dense binding of AVPR1a in the CeA and BNST (across mating systems). Likewise, AVPR1a binding has been found in the olfactory bulb of platyrrhine primates (e.g. common marmosets;57) that also rely extensively on olfactory communication62; similar binding has not been reported in less olfactory-oriented catarrhine primates.

Relative to other placental mammals, vision is exceptionally well-developed in primates, but less so in strepsirrhines than in haplorhines. In catarrhines, for example, trichromacy63,64 and visual gaze are particularly important in reproductive and social communication65,66. Consistent with previous work in haplorhine primates, we found OXTR expression in V1 and the LGN across species, and AVPR1a expression in these and additional areas related to visual attention (i.e., Rtg, SC, and amygdalar nuclei); nevertheless, binding in Eulemur was less widespread than that observed in haplorhine primates (e.g.46,47). With regard to sensory pathways, therefore, our results are consistent with lemur neuroanatomy representing a bridge between odor-reliant rodents and vision-reliant haplorhines.

Intermediary patterns were also evident in other pathways. For instance, consistent with findings in some rodent species (singing mice:23; prairie and montane voles:19), but unlike findings in haplorhine primates, we observed dense AVPR1a expression in the MGN of lemurs. Because the MGN is an essential auditory relay nucleus—receiving input from the inferior colliculus and projecting to the auditory cortices—our findings potentially implicate vasopressin in another sensory modality in Eulemur; it is possible that vasopressin plays a modulatory role in the processing of vocal communication or emotionally valent sounds.

Relative to haplorhines, additional patterns of receptor binding in Eulemur show both similarities and striking reversals. In Eulemur, we observed strong AVPR1a binding, but diffuse or modest OXTR binding, in both the striatum and hippocampal regions. This striatal pattern is comparable to that seen in coppery titi monkeys47, but it contrasts with the dense OXTR expression found in both rodents67 and marmosets57. Hippocampal patterns in Eulemur are reversed from that observed in titi monkeys47. Oxytocin acting on OXTRs in the NAcc is necessary for pair-bond formation in voles4. Precise functions of oxytocin or vasopressin within the hippocampal formation remain to be identified68, but there is some evidence that they modulate the encoding and consolidation of socially relevant memories69,70. In any event, our divergent results in these regions suggest that neural mechanisms of pair bonding in lemurs may differ substantially from other mammalian groups studied thus far.

Regarding the influential hypothesis that interspecific variation in specific populations of receptors reflects variation in social organization or mating system, our results did not reveal comparable differences to the striking findings previously reported for monogamous and non-monogamous vole species. For instance, in Insel and Shapiro18, the effect size for a mating system difference in OXTR was d = 2.23 in the NAcc and d = 2.06 for the LA; in Insel et al.19, the mating system difference in AVPR1a was d = 3.66 for the LS and d = 2.81 for the BNST. In Eulemur, despite a sample size that matched these classic vole studies, differences between mating systems, for either neuropeptide, were almost uniformly non-significant (with much smaller effect sizes; all d < 0.8) in all regions of a hypothesized rodent ‘pair-bonding circuit’. Our results do not support the suggestion that OXTR/AVPR1a differences in key dopaminergic areas separate monogamous from non-monogamous species4. When expanding our comparisons across the entire brain, however, we observed some significant differences between mating systems, including in the Rtp for OXTR and the VA Thal, DR, and PFC for AVPR1a.

How should one interpret these mixed results? Regarding null findings, we note that exhausting the available bank of Eulemur brain tissue at the Duke Lemur Center nevertheless left us with limited statistical power to detect differences in individual regions as a function of mating system. While large differences comparable in magnitude to those reported in Insel and Shapiro18 and Insel et al.19 would be detectable with our sample—indeed, we matched the sample size from these classic studies—more modest differences may have been missed. Regarding exploratory positive findings, we first caution that examining numerous regions increases the potential for false-positives, and that there is a lack of information about the functional significance for many of these differences. For instance, whereas the presence of OXTR in the pontine reticular areas of Eulemur and rhesus macaques46 suggests a possible conserved function of oxytocin in this region, it is unclear how differences in this region that controls horizontal gaze and saccadic eye movement would be involved in differences in social bonding behavior. Although the ventral anterior thalamus has important functions in spatial memory and learning71, it has not been specifically implicated in pair-bonding processes. That said, other findings more readily yield potential interpretations. First, the AVPR1a difference we observed in the DR, a source of serotonin and a region involved in reward-seeking and reward-tracking behavior72, suggests that some of the effects of vasopressin on social behavior may owe to activation of the DR serotonin system73. If so, monogamous Eulemur may have developed denser populations of AVPR1a to bolster serotonergic functions of social reward behavior that foster the creation of pair bonds. Second, rather than observing OXTR binding differences in the PFC—a key area generating the reinforcing, hedonic properties of pair-bonding behavior and mating in rodents4—we instead found a difference in AVPR1a binding in this region. Perhaps some of the mechanisms mediated by oxytocin in rodents are carried out by the structurally similar vasopressin in primates—a suggestion that has been hypothesized and substantiated in several previous studies45.

Collectively, mixed findings for mating system differences, like the aforementioned binding patterns found across lemur species, are consistent with the existence of distinctive mechanisms for the formation of monogamous mating systems in Eulemur. In questioning the universality of these mechanisms across mammalian groups, our findings in this domain can also be considered within the broader context of psychological oxytocin research, which is similarly marked by interpretive challenges and heterogeneous findings (e.g.74). We suggest that expanding the toolkits available to researchers, including broadening the animal models studied, will likely continue to reveal unexpected findings that require modification to existing theory (a point echoed by behavioral ecologists; e.g.75).

Providing context to our results is the fact that numerous factors other than species identity influence an individual’s oxytocin and vasopressin neurocircuitry. Neurobiology is not static throughout the lifespan, but rather may vary seasonally, with social circumstance, and with age or life-history stage (e.g.52). Thus, while receptor distributions can differ widely between species and social systems18,19,22, they might also differ substantially within individuals of the same species or mating system. Indeed, Phelps and Young27 report intraspecific variation in AVPR1a binding among prairie voles often comparable to or greater than interspecific variation (for a recent example of experience-dependent, intraspecific OXTR patterns in a primate model, see68). Nevertheless, these same authors also report less variation in regions regulating social bonding, relative to those unrelated to social bonding—a pattern consistent with natural selection winnowing neuropeptide expression in these former regions. We also observed substantial intraspecific and within-mating system variation in Eulemur (see individual-level estimates of receptor profiles in Table S3)—given our limited sample size per species, it is unclear to what extent this might be explained by season-level, individual-level, and/or species-level differences. In Eulemur, some areas previously identified as key to social bonding—such as nuclei of the amygdala and the BNST—showed relatively small coefficients of variation within mating systems, consistent with27, even though they did not differ significantly between mating systems. Other regions that showed relatively little variation within Eulemur mating systems, such as the primary visual cortex and SC, were not the same ones identified as part of a pair-bonding circuit in rodent studies, but they are consistently identified as sites of OXTR and AVPR1a in nonhuman primate studies46,47. Perhaps neuropeptide binding in regions responsible for processing visual information are important targets of stabilizing selection in primates, regardless of the underlying mating system.

As in the classic vole studies18,19, we categorized our Eulemur species as belonging to one of two broad mating systems, based on extant information about their wild counterparts39. On the one hand, we cannot rule out the possibility that group size reductions, selective reproduction, or long-term pair housing in captivity may have contributed to ‘monogamous-like’ receptor binding profiles across species in our sample, potentially minimizing differences by mating-system category. On the other hand, one might expect such a ‘flattening’ influence to lead to similar receptor profiles across individuals and species, but this does not reflect our results, which are more accurately characterized by a large degree of within-mating system variation. More generally, we believe our results complement the recognition of substantial, natural heterogeneity in social behavior, within or between species, under the general umbrella of ‘monogamous’ or ‘non-monogamous’. Pair-living, pair-bonding, and genetic monogamy are overlapping, yet constitute distinct components of a monogamous mating system that are often conflated40,76. Different configurations of these components across ‘monogamous’ species could conceivably create different neuropeptide receptor distributions. Importantly, we note that flexibility in putative mating systems is likely the norm, rather than the exception in animal models. Even the seemingly well-characterized mating system of prairie voles contains surprises revealed only upon extensive observation in naturalistic settings26. In some cases, differences in neuropeptide receptor distributions may be detectable in spite of intraspecific (or within-mating system) social variation, but this may less common than previously assumed.