Why do men's eyebrows get bushier with age?

Some older men's enormous eyebrows, extensive ear hair and substantial sniffer strands seem like they never stop growing. While many men lose the hair atop their heads, others' eyebrow, ear and nose hairs may look as if they've been dipped in a magical growing elixir.

In contrast, women's hair, including their eyebrows, tend to thin as they age. So why do men's various facial hairs often get bushier in old age, while women's don't?

Older men's hairiness are influenced by high hormone levels — namely, testosterone — through to old age. These hormones have greater effects on the hair follicles over time, according to Dr. Danilo Del Campo, a dermatologist at the Chicago Skin Clinic. Meanwhile, the hormone estrogen, which increases hair growth and diameter, tends to decrease in women after menopause, according to a 2020 review in the International Journal of Molecular Sciences. 

Related: Why do we have eyebrows and eyelashes?

However, where hair grows, how fast it grows, and its texture and color are based primarily on genetics, age, nutrition, health and sex, said Dr. Marie Jhin, president of Premier Dermatology in San Carlos, California.

But as people age, "hormones and the various responses to hormones are the key reason for the difference between males and females regarding hair," said Del Campo said. 

How eyebrow hairs grow

All hairs on the body grow from hair follicles. These follicles' life cycles have three phases: the growth phase (anagen), the two-week transition phase (catagen) and the months-long dormant phase (telogen). After hair follicles go dormant, the hair eventually falls out — sometimes called the exogen phase, according to the 2020 review. The process then starts over at the anagen phase.

Hairs on different parts of the body grow differently because the lengths of these phases differ. Scalp hair has the longest anagen phase, typically ranging from two to seven years, Del Campo said. "However, eyebrow hairs and other hair-bearing sites are very different," he told Live Science in an email.

The shorter a hair follicle's anagen phase is, the shorter the hairs on that part of the body are when they go dormant. "Eyebrows have a very short anagen phase of about 30 days — that's it," Del Campo said. This short growth period is followed by an extended resting phase, which explains why eyebrow hairs are typically just 0.4 inches (1 centimeter) or less. 

"If the anagen phase was longer, then people would be having to use scissors at their barber to trim their eyebrows very frequently," Del Campo said — grooming requirements that become necessary for some men as they age. 

Testosterone's role

The lengths of the different phases are influenced by the hormones circulating in the body. "The hair cycle, as well as the structure of the hair follicle, are highly affected by various hormones," Jhin told Live Science in an email. In particular, men often have high levels of androgens, especially testosterone, while women tend to have lower levels of androgens. These androgens are the reason why men's hair grows faster, she added. 

During puberty, these androgens are responsible for hair growth in places such as the face, arms and back, according to the 2020 review."These androgens work on sex-specific parts of the body such as pubic, chest, face and axillary areas [armpits] during puberty," Jhin said. "They act as stimulation to the hairs."

Related: How fast does hair grow? 

As men age, some hair follicles grow more sensitive to these hormones over time, according to Jhin. As the follicles grow more sensitive to androgens, the hormones keep them in the anagen phase longer, leading to more hair growth with age in places such as the eyebrows, nose and ears. 

Interestingly, and somewhat mysteriously, hair follicles on the scalp react differently to testosterone, actually shortening their growth cycle when testosterone levels increase, according to Jhin. So that's why some older men have hirsute eyebrows, long nose hairs and tufty ear hairs, but a balding head.

"Men's eyebrows tend to continue to grow larger until older age (and this also applies to ear and nose hair)," Del Campo said. "In women, there are lower levels of testosterone."

As women age, estrogen levels drop during menopause. Lower estrogen levels, and other hormone changes and factors in older age, are thought to lead to thinner, less dense hair, according to the 2020 review. "While men continue to produce androgens until age 70s," Jhin said, "women's hormones become depleted with menopause around age 50."

Originally published on Live Science.