In humans, sex is determined at conception by chromosomes and becomes visible through a combination of anatomy, hormones, and physical traits that develop over time. At birth, a medical professional assigns sex based on external genitalia. But the full picture of what makes a body male or female involves genetics, hormone levels, skeletal structure, and a cascade of changes that unfold during puberty. In animals, the visual cues can be even more dramatic, from a lion’s mane to a peacock’s tail feathers.
Chromosomes Set the Blueprint
Every human cell contains 46 chromosomes, and two of them, called X and Y, play the central role in sex determination. Girls and women typically carry two X chromosomes (XX), while boys and men typically carry one X and one Y (XY). The Y chromosome contains a gene called SRY that acts like a master switch: it triggers a fetus to develop testes and blocks the formation of a uterus and fallopian tubes. Without a functioning SRY gene, a fetus develops ovaries and female reproductive structures regardless of which chromosomes are present.
This system is reliable but not absolute. In rare cases, the SRY gene can end up on an X chromosome during sperm formation, producing a person with XX chromosomes who develops male characteristics including testes. The reverse also happens: people with XY chromosomes can develop female anatomy if their SRY gene has a variant that prevents it from working. These exceptions highlight that chromosomes set the initial direction, but the gene itself does the heavy lifting.
Anatomy at Birth
The most immediate way sex is identified is by looking at a newborn’s genitalia. Males are born with a penis and testes (though the testes may not have fully descended yet), while females are born with a vulva and vaginal opening. Internally, females have ovaries, a uterus, and fallopian tubes, while males have testes, the prostate, and seminal vesicles. These are called primary sex characteristics because they’re present from birth and directly involved in reproduction.
Roughly 0.7 to 1.7 percent of people worldwide are born with differences of sex development, sometimes called intersex traits. These are a wide range of genetic, hormonal, and anatomical variations where a person’s anatomy doesn’t fit neatly into the typical male or female category. Some of these differences are visible at birth, while others aren’t discovered until puberty or later in life.
What Changes During Puberty
The physical differences between males and females become far more pronounced during puberty, when rising hormone levels reshape the body. These are called secondary sex characteristics, and they unfold over several years in a fairly predictable sequence.
Female Puberty
Breast development is usually the first visible sign, starting around age 9 or 10. Pubic hair begins growing roughly six months later, initially light and sparse, then gradually becoming coarser and darker. About two years after pubic hair appears, underarm hair starts to grow. Estrogen drives most of these changes, including fat redistribution to the hips and breasts, the growth of reproductive organs, and bone maturation.
Male Puberty
Increased testicular size is typically the first sign in boys, accompanied by pubic hair growth at the base of the penis. The penis itself grows after the testes enlarge, first in length and then in width. About two years after pubic hair appears, facial hair, chest hair, and underarm hair begin to develop. The larynx and vocal cords also enlarge during this period, which is why a boy’s voice will often crack before settling into a deeper pitch. Increased oil production in the skin and body odor from new sweat glands are also testosterone-driven changes.
Hormones Tell a Measurable Story
Testosterone and estrogen are present in everyone, but in very different amounts. In adult males ages 18 to 99, normal testosterone levels range from 193 to 824 nanograms per deciliter of blood. In adult females of the same age range, testosterone typically stays below 40 ng/dL. Most of the testosterone produced in female ovaries gets converted into estradiol, the primary form of estrogen. These hormonal differences drive and maintain the physical distinctions between male and female bodies throughout adult life, affecting muscle mass, fat distribution, bone density, and hair growth patterns.
Skeletal and Vocal Differences
Even without soft tissue, the skeleton reveals sex. The pelvis is the most telling bone structure. The female pelvis is shaped to accommodate both upright walking and childbirth, so it tends to be wider with a more open inlet. The male pelvis is typically narrower and more compact. Forensic scientists have developed classification systems for pelvic inlet shapes, though recent research shows the differences in inlet shape are not as clear-cut as older textbooks suggested. Other skeletal clues include the skull, where males tend to have more prominent brow ridges, a larger jaw, and a more angular chin.
Voice pitch is one of the most reliable external indicators of sex in adults. Male vocal folds are about 60 percent longer than female vocal folds, and the fundamental frequency of a male voice is roughly five standard deviations lower than a female voice. That’s an enormous gap in biological terms, making voice one of the strongest examples of sexual dimorphism in humans.
How It Works in Animals
Many of the same principles apply across the animal kingdom, but the visual differences between males and females can be far more extreme. In mammals, males are usually larger than females in most species. The northern fur seal represents one of the most dramatic size differences: bulls are so much bigger than cows that a male can physically lift and toss a female. In primates, canine tooth size often differs between the sexes more reliably than overall body size, reflecting competition between males.
There are notable exceptions to the “males are bigger” rule. Female baleen whales are larger than males, and the same is true for most rabbits and hares, as well as many bat species. In birds, size and appearance don’t always track together. Males in the blackbird family (Icteridae), for example, show no correlation between how much bigger they are than females and how different their plumage looks. A male bird can be nearly the same size as a female yet sport wildly different feathers, or vice versa.
For common domestic animals, the cues are often straightforward once you know where to look. Male cats tend to have broader heads and are typically larger, while female cats have a more refined build. In chickens, roosters develop larger combs, longer tail feathers, and spurs on their legs. Male dogs can be identified by external genitalia from a young age. In many fish and reptile species, color is the key: male bettas are far more vibrant than females, and male anole lizards have a bright throat fan they extend during displays.