Secondary sex characteristics are the physical traits that emerge during puberty and distinguish males from females but are not directly part of the reproductive system. Breast development, facial hair, voice deepening, and changes in body fat distribution all qualify. Unlike primary sex characteristics (the reproductive organs themselves, which are present from birth), secondary traits develop later under the influence of rising sex hormones and serve functions ranging from mate attraction to social signaling. The biology behind them is more layered than most people realize, with hormones, genetics, body composition, and even environmental chemicals all shaping when and how these traits appear.
What Counts as a Secondary Sex Characteristic
The distinction is straightforward in principle. Primary sex characteristics are the organs directly involved in reproduction: ovaries, testes, the uterus, the penis. These form before birth. Secondary sex characteristics are everything else that differs between the sexes physically, and they develop mainly during puberty. In humans, the most familiar examples include breasts, wider hips, and increased body fat in females; and a deeper voice, broader shoulders, and facial hair in males. Both sexes develop pubic and underarm hair, though the pattern and density differ.
A few traits fall into a gray area. Increased muscle mass in males is sometimes listed as secondary, sometimes treated as a general effect of testosterone on the body. The Adam’s apple (the visible enlargement of the larynx) is unambiguously secondary, and the larynx itself is considered a secondary sexual organ because it changes dramatically under the influence of sex hormones over a person’s lifetime.1PubMed Central. Effect of sex hormones on human voice physiology: from childhood to senescence Body odor changes also count, since the apocrine sweat glands in the armpits and groin become active at puberty under hormonal control.
What Triggers Their Development
The engine behind puberty is a hormonal cascade that begins in the brain. A cluster of neurons in the hypothalamus starts releasing pulses of a signaling molecule called GnRH (gonadotropin-releasing hormone). This “pulse generator” is kept quiet during childhood and reactivates as puberty approaches, kicked off in part by a protein called kisspeptin that acts as the initial trigger.2PubMed. Hypothalamo-Pituitary axis and puberty The GnRH pulses tell the pituitary gland to release two hormones, LH and FSH, which travel to the gonads and prompt them to produce sex steroids: testosterone in the testes, estradiol in the ovaries.3PubMed. HPG-axis hormones during puberty: a study on the association with hypothalamic and pituitary volumes
This whole axis (hypothalamus → pituitary → gonads) is actually active briefly in infancy, goes dormant during childhood, and then reignites at puberty.4PubMed Central. Pubertal development and regulation Why it stays quiet for years and what finally flips it back on is still an active area of research. Nutritional status, body fat, and even light exposure all seem to play modulating roles, but the exact “go” signal is not fully nailed down.
The Leptin Connection
One of the more interesting threads in puberty research involves leptin, a hormone produced by fat cells. The idea is that the body needs to reach a certain energy threshold before it invests in reproduction, and leptin acts as the messenger that says “there is enough fuel.” In boys, leptin levels rise by roughly 50% just before puberty begins (as defined by the initial rise in testosterone), then drop back to baseline after puberty is underway, even as body mass keeps climbing.5The Journal of Clinical Endocrinology & Metabolism. A Longitudinal Assessment of Hormonal and Physical Alterations during Normal Puberty in Boys. V. Rising Leptin Levels May Signal the Onset of Puberty That transient spike, independent of continuing weight gain, fits the profile of a trigger signal rather than just a passive reflection of body size.
In girls, the relationship is slightly different. Low leptin levels before puberty predicted larger gains in body fat during puberty, and leptin appears to serve as a permissive factor: it does not start puberty by itself, but puberty cannot proceed normally without adequate levels.6PubMed. Longitudinal study of leptin concentrations during puberty: sex differences and relationship to changes in body composition 7European Journal of Endocrinology. The relationship between leptin, gonadotropic hormones, and body composition during puberty in a Dutch children cohort This helps explain why extreme underweight or intense athletic training in young girls can delay or disrupt puberty: without enough fat tissue producing leptin, the hormonal cascade stalls.
Examples in Females
Breast development is usually the first visible sign of puberty in girls, starting on average about five months before pubic hair appears.8JAMA Pediatrics. Longitudinal Development of Secondary Sexual Characteristics in Girls and Boys Between Ages 9½ and 15½ Years That gap narrows as development progresses, and by the later stages the two traits are roughly in sync. Other secondary characteristics in females include:
- Hip widening: The pelvis broadens under the influence of estrogen, changing the waist-to-hip ratio.
- Fat redistribution: Subcutaneous fat accumulates around the hips, thighs, and buttocks, creating a body shape that differs from the more central fat distribution typical of males.
- Skin changes: Estrogen promotes softer skin texture and can affect oil production, though acne still occurs in many girls during puberty.
- Growth spurt: Girls typically hit their peak growth velocity earlier than boys, often around age 11–12, and stop growing sooner.
Menarche (the first menstrual period) is sometimes listed alongside secondary characteristics, but it is technically a primary sexual event because it represents the maturation of the reproductive system itself. It usually arrives relatively late in the pubertal sequence, after breast development is well underway.
Examples in Males
In boys, testicular enlargement is the first sign of puberty but counts as a primary rather than secondary change. The secondary traits that follow are driven primarily by testosterone and include:
- Voice deepening: Testosterone causes the larynx to grow and the vocal cords to lengthen and thicken, dropping the pitch of the voice. This is one of the most hormonally sensitive changes and continues to be shaped by sex steroids well into adulthood and even old age.1PubMed Central. Effect of sex hormones on human voice physiology: from childhood to senescence
- Facial and body hair: Testosterone stimulates hair follicles on the face, chest, abdomen, and limbs. The degree of body hair varies enormously between individuals and between ethnic groups, which is one reason it is a poor marker of testosterone levels on its own.
- Muscle mass increase: Testosterone promotes protein synthesis in skeletal muscle, leading to greater muscle bulk and upper-body strength compared to females.
- Broader shoulders: The clavicles and shoulder girdle grow under androgenic stimulation, producing a wider frame.
- Adam’s apple: The visible protrusion of the thyroid cartilage, a direct result of laryngeal growth.
Boys also experience increased oil production in the skin (making acne common), a growth spurt that peaks later but lasts longer than in girls, and changes in sweat composition that produce the distinct body odor of adult males.
When Development Does Not Follow the Typical Script
Not everyone’s secondary sex characteristics develop along textbook lines. One striking example comes from complete androgen insensitivity syndrome (CAIS), a condition in which a person has XY chromosomes and testes that produce testosterone, but the body’s cells cannot respond to androgens at all. The result is a person who looks outwardly female, develops breasts and a female body shape at puberty (because some testosterone is converted to estrogen), but has sparse or absent pubic hair and no menstrual periods. A case report described a 15-year-old with CAIS who had developing breasts consistent with mid-puberty and normal female external anatomy but a blind-ending vaginal canal and undescended testes found on imaging.9PubMed Central. Complete androgen insensitivity syndrome in a 15-year-old female with primary amenorrhea and undescended testes: a rare case report In a longer-term follow-up of women with CAIS, secondary sexual development was rated as satisfactory by both the individuals and their clinicians, despite the unusual hormonal pathway producing it.10The Journal of Clinical Endocrinology & Metabolism. Complete Androgen Insensitivity Syndrome: Long-Term Medical, Surgical, and Psychosexual Outcome
Other conditions that alter the trajectory include congenital adrenal hyperplasia (which can cause early or atypical development of body hair and other androgenic traits in girls), delayed puberty from various causes, and precocious puberty, where secondary characteristics appear unusually early. These conditions highlight that secondary sex characteristics are not simply “male” or “female” traits: they are responses to specific hormones, and whoever produces (or is given) those hormones will develop the corresponding features to some degree.
Exogenous Hormones and Secondary Characteristics
Because secondary sex characteristics are downstream effects of circulating hormones, they can be induced or modified by providing hormones externally. This is the basis of gender-affirming hormone therapy for transgender individuals: estrogen-based regimens promote breast growth, fat redistribution, and softer skin, while testosterone-based regimens promote facial hair, voice deepening, and changes in muscle mass.11PubMed Central. Gender Affirming Hormone Treatment for Trans Adolescents: A Four Principles Analysis The traits that emerge are the same secondary sex characteristics that develop endogenously during puberty; the source of the hormone is different, but the tissue response is biologically identical.
Some changes are reversible if hormones are discontinued (fat redistribution, for instance, shifts back over time), while others are permanent. Voice deepening caused by testosterone is largely irreversible because the vocal cords, once lengthened, do not shrink back. Breast tissue developed under estrogen also persists. This distinction between reversible and irreversible changes matters clinically and is one reason the timing of hormone therapy carries real consequences.
Why Secondary Sex Characteristics Exist at All
From an evolutionary standpoint, secondary sex characteristics are products of sexual selection, the branch of natural selection driven by competition for mates. Darwin distinguished two routes: competition between members of the same sex (think antlers clashing) and mate choice, where one sex preferentially selects partners with particular traits. A broad review of sexually selected traits across animals found that female mate choice appears to be the most widespread mechanism, though male-to-male competition is nearly as common in vertebrates, and male mate choice is frequent in arthropods.12Frontiers in Ecology and Evolution. Evolution of sexually selected traits across animals
One influential idea is the immunocompetence handicap hypothesis, which proposes that traits like bright plumage, large ornaments, or a deep voice are honest signals of health because the testosterone needed to develop them simultaneously suppresses the immune system. Only a genuinely healthy individual can afford the immunological cost of a full display. Experiments in scorpionflies have tested the genetic predictions of this hypothesis by breeding females with males that differ in ornament expression and then comparing the immune function of their offspring.13PubMed Central. The immunocompetence handicap hypothesis: testing the genetic predictions In humans, testosterone-influenced facial features like prominent cheekbones and a strong jaw have been associated with perceptions of attractiveness, and some researchers argue this reflects an evolved preference for honest immune-health signals.14PubMed. Human facial beauty: Averageness, symmetry, and parasite resistance
Interestingly, sexual selection does not operate only on males. Secondary sexual characters are common in females across the animal kingdom and are often associated with competition for breeding opportunities or mate attraction, much as in males.15Animal Behaviour. Sexual selection in females In birds, for example, female ornamentation can signal territory-holding ability or parental quality. The old textbook image of drab females passively choosing flashy males is increasingly recognized as incomplete.
Secondary Sex Characteristics Across the Animal Kingdom
Humans are far from the only species with dramatic pubertal changes. In fact, most vertebrates show some form of secondary sexual dimorphism, and the variety is staggering. Male birds provide some of the most studied examples: elaborate plumage and complex song are both sexually selected traits, and there is evidence that when one of these traits becomes more elaborate within a lineage, the other can actually decrease, as though there is a trade-off in signaling investment.16Evolution. Species Divergence in Sexually Selected Traits: Increase in Song Elaboration Is Related to Decrease in Plumage Ornamentation in Finches Finch species with the most complex songs tend to have relatively plain feathers, and vice versa.
In fruit flies, a tiny structure called the sex comb (a row of modified bristles on the male’s front leg) is a rapidly evolving secondary sexual trait that affects mating success.17PubMed. Sexual selection and the evolution of secondary sexual traits: sex comb evolution in Drosophila In lions, the mane is a classic secondary characteristic that may have appeared somewhere between 320,000 and 190,000 years ago; earlier lion populations were apparently maneless, and the maned form likely spread because it conferred some selective advantage.18Cambridge University Press. Evolution of the mane and group-living in the lion (Panthera leo): a review
Fish provide another window. Many species develop striking secondary sexual characteristics under hormonal control, including breeding tubercles (small raised bumps on the head or fins), color changes, enlarged fin rays, and fat pads. These traits are so reliable as hormonal indicators that regulatory toxicology now uses them as endpoints in chemical safety tests: if a fish’s secondary sex characteristics are altered after chemical exposure, it suggests the compound has endocrine-disrupting activity.19Chemosphere / Elsevier. Interpretation of sexual secondary characteristics (SSCs) in regulatory testing for endocrine activity in fish
Environmental Chemicals and Disrupted Development
That fish are used as sentinels for endocrine disruption points to a broader concern: synthetic chemicals in the environment can interfere with the hormonal pathways that produce secondary sex characteristics in many species, including humans. Endocrine-disrupting chemicals (EDCs) affect all levels of reproduction, from the development of reproductive organs to the maturation and maintenance of secondary sexual traits and adult physiology and behavior.20PubMed Central. Mate choice, sexual selection, and endocrine-disrupting chemicals
In wildlife, EDC exposure has been linked to feminized male fish in rivers contaminated with estrogenic compounds, thinned eggshells in raptors exposed to DDT metabolites, and altered genital development in alligators living in polluted lakes. In humans, a growing body of epidemiological evidence associates EDC exposure with altered timing and progression of puberty.21Journal of Endocrinology. Male pubertal development: are endocrine-disrupting compounds shifting the norms? The concern is not that a single chemical at a single exposure will visibly change someone’s development, but that chronic low-level exposure to mixtures of hormone-mimicking compounds could subtly shift the timing of pubertal milestones and the degree of characteristic expression at a population level. The evidence is stronger in animals than in humans, but the direction is consistent enough to drive ongoing regulatory attention.
What Happens to Secondary Sex Characteristics With Age
Secondary sex characteristics do not remain static after puberty ends. They continue to be maintained, and gradually altered, by circulating hormone levels across the lifespan. In men, testosterone levels decline gradually from around age 30 onward, a process sometimes called andropause. This decline is associated with reduced muscle mass, decreased bone density, changes in body fat distribution, and thinning of body hair.22PubMed. Testosterone deficiency and mood in aging men: pathogenic and therapeutic interactions The voice can also shift with age as the laryngeal tissues lose elasticity and hormonal support wanes.1PubMed Central. Effect of sex hormones on human voice physiology: from childhood to senescence
In women, menopause brings a sharp drop in estrogen and progesterone. The downstream effects on secondary characteristics include loss of breast tissue density, redistribution of fat toward a more central (abdominal) pattern, thinning of skin, and reduced pubic and axillary hair. Some women also notice coarsening or thinning of scalp hair and, paradoxically, increased facial hair growth as the balance shifts and residual androgens from the adrenal glands face less estrogenic opposition.
These age-related changes underscore that secondary sex characteristics are not one-time developmental events. They are ongoing hormone-dependent states, which is why they respond to hormone replacement therapy in older adults and to medical conditions that alter hormone production at any age. A tumor producing excess testosterone in a woman, for instance, can trigger male-pattern hair growth and voice changes regardless of her age.
The Physiological Machinery Behind Tissue Growth
At a tissue level, secondary sex characteristics develop through two basic processes: cells multiplying (hyperplasia) and cells getting larger (hypertrophy). The distinction matters for understanding why some traits are permanent and others reversible. Breast development involves both new ductal tissue and fat deposition; the ductal tissue is largely permanent. Muscle growth from testosterone is partly hypertrophy of existing fibers, which can reverse when hormone levels drop. The organization-activation framework in endocrinology suggests that hormones can have permanent “organizational” effects during critical periods (like puberty) and temporary “activational” effects in adulthood. Thinking about the actual cellular growth mechanisms provides an additional way to understand why different secondary traits respond differently to hormone changes over time.23The American Naturalist. Vertebrate Secondary Sexual Characteristics-Physiological Mechanisms and Evolutionary Patterns
This framework also explains evolutionary patterns in sexual dimorphism. Traits that evolve through hyperplasia (permanent new tissue) tend to be more “locked in” once they arise in a lineage, while traits that depend on ongoing hormonal activation are more evolutionarily labile, appearing and disappearing more readily across related species. The lion mane, for instance, appears to have evolved relatively recently in evolutionary terms and is maintained by ongoing testosterone levels in adult males, which is consistent with it being an activational rather than deeply organizational trait.