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| Testosterone drives male robins to compete |
Testosterone is in the news these days. It’s an interesting hormone that gets talked about a lot, especially in terms of human differences. Speaking personally, I value testosterone as one of the things that connects our little human family to the wider family of animals. Here’s what I’ve learned about testosterone over the last 10 to 20 years.
Testosterone (T) is a steroid. It occurs in both men and women, but most men with “low testosterone” have levels that would be off the charts for women. In addition to levels being higher in adult males, they are higher in male fetuses, infants, and adolescents.
In vertebrates at least, testosterone mediates the development of a masculine phenotype, with “masculinity” meaning different things for different species. The hormone that drives a male robin to face off with other males is testosterone. In the winter, when T levels are low, male robins tolerate each others’ presence rather than fighting. For most mammals, T spurs growth and thus males are larger. For species of lizards in which males are smaller than females, T inhibits growth instead.
The default body plan for mammals is female, with male hormones diverting the embryo toward the male phenotype. An XY mammal that is insensitive to T develops as an infertile female.
In almost all mammal species, testosterone makes males bigger and stronger, on average, than females. That’s true for humans, and men’s upper body strength points to an evolutionary history of physical struggle. The dimorphism in our species, however, is small relative to other apes and nothing compared to gorillas or sea lions. Men don’t have antlers or peacock tails or other outlandish adaptations for out-competing the other males. Our species’ modest level of dimorphism suggests that male-to-male competition over mates has been low, lower than among chimpanzees or orangutans.
Testosterone levels have a large effect on human development and a smaller effect on current behavior. Exposure to T in the womb organizes the brain along male lines, especially with connections that become active when exposed to T at adolescence. In mammals, brains are wired for both male and female behaviors, with males predisposed toward but not limited to male-typical behaviors, and the same with females and female-typical behavior.
Prenatal exposure to T predisposes a child to rough-and-tumble play and probably to being sexually attracted to women.
Boys tend to be born larger than girls, especially a woman’s first son. Hormones at puberty increase the dimorphism between the typical males and females.
T promotes the sex drive in men and women.
Trans men who take testosterone often report changes to mood.
Each human is individually more or less sensitive to testosterone depending on the length of the “CAG repeats” associated with their androgen receptor genes. The gene is on the X chromosome, so women have two copies and are thus more likely to be in the “average” range of sensitivity.
Further Reading
The Female Brain and The Male Brain by Louann Brizendine. If you want to learn how hormones affect individual development and the human experience from embryogenesis to senescence.
Other Evolution-Themed Posts
Crow Scientist is Live, 2020
Not “Race” but Ancestry, 2019
Evolution Unites Us All, 2018
Reading about Religion, 2014
Creatures of Love, 2011
