374 - The evolutionary biology of testosterone: male development & sex-based behavioral differences

KEYPOINTS

• 01

  Sex is defined by gamete type; chromosomes determine it in mammals but don't define it across nature.

  Hooven, a Harvard-trained evolutionary biologist who studied chimpanzees in Uganda, notes that birds use a different chromosome system and some reptiles use temperature. Across sexually reproducing species, sex means being organised around making sperm or eggs. That reproductive role coordinates body, hormones and average behaviour.

• 02

  XX and XY embryos are nearly identical until about weeks 5–6, when SRY switches the shared gonad toward testes.

  The undifferentiated gonad can become either organ—evolution's way of not building two systems. Without SRY it becomes ovaries, though that is an active genetic process, not a passive 'default.' Testes then make testosterone, which keeps the Wolffian (male) ducts, and anti-Müllerian hormone, which removes the Müllerian (female) ducts.

• 03

  DHT gives a strong local androgen signal without flooding the fetus.

  5α-reductase concentrated in genital tissue turns testosterone into DHT, roughly 2–5× more potent. DHT drives penis, scrotum and prostate development. In 5α-reductase deficiency, XY individuals may look female at birth but masculinise at puberty with normal testosterone, without facial hair or male-pattern baldness. Hooven says DHT isn't needed for male-typical muscle, which matters in sports-eligibility disputes.

• 04

  About 20% of genes on the 'silenced' second X escape inactivation, and that matters.

  That partial double dose helps explain Turner syndrome (a single X): short stature, other features and usually sterility. The 70–100 Y-chromosome genes may also act in the brain before testosterone does.

• 05

  Prenatal testosterone shapes the brain; childhood testosterone barely differs.

  Male fetal testosterone begins around week 8 and peaks at weeks 15–20, perhaps 400–600 ng/dL—approaching pubertal levels. 'Mini-puberty' follows, peaking around three months and fading by about six, and is linked to boys' later activity levels. Five-year-old boys and girls have almost no testosterone. So Attia's sons' rough-and-tumble play reflects how their brains were organised, not current levels.

• 06

  Above a male threshold there's little dose-response; in females small differences matter.

  Men carry roughly 10–20× women's testosterone. Variation within the normal male range predicts little about sex drive or aggression, and Hooven warns you can't judge anyone by their current level. Different brain regions may have separate critical periods for sexual versus aggressive behaviour.

• 07

  Eggs are costly and sperm cheap, which drives different reproductive strategies.

  A hunter-gatherer woman's lifetime output might be 8–10 children, each requiring pregnancy and nursing under energy scarcity. Men can gain from competing for status and more partners, and can 'live fast, die young' successfully; women benefit from caution and longevity. Hooven offers only a speculative idea for why eggs are made upfront and winnowed—perhaps selection.

• 08

  Rough play is adaptive and teaches boys hierarchy, limits and how to back down.

  Dominance hierarchies reduce overall fighting. Male chimps play rougher, and subordinate chimps may cup a dominant's testicles to signal trust. Boys' insult-based intimacy follows a similar logic. Hooven worries phones are replacing physical play. Girls tend toward indirect reputational aggression and, she argues, lack equivalent quick conflict-resolution rituals.

• 09

  Testosterone is the ultimate cause of the male skew in violence; culture sets the rate.

  About 95% of murders and roughly 98% of sexual assaults are committed by men everywhere. But Canada's lower murder rate than the US isn't a testosterone difference. Laws, religion and norms change rates while the sex gap persists. A biological tendency is not destiny.

• 10

  Involved fatherhood lowers testosterone, and that's adaptive.

  Pair-bonding and hands-on care of young dependent children suppress testosterone in humans and birds. Raising it in birds makes males neglect offspring. Hooven suggests exogenous testosterone may override these social signals, with behavioural effects users don't consider.

• 11

  Estrogen does not masculinise the human brain; androgen-receptor function does.

  In rodents, testosterone converted to estrogen inside the brain masculinises behaviour, but human alpha-fetoprotein doesn't sequester estrogen the same way. Aromatase-deficient men are behaviourally typical. In complete androgen insensitivity syndrome, XY people with testes and high testosterone but non-functional receptors develop as typical women, often learning the diagnosis when periods don't start.